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RADIO ASTRONOMY

Journal of the Society of Amateur Radio Astronomers November- December 2014

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Radio WavesPresident’s Page 3 Editor’s Notes 4 News Mark Your Calendar 5 2015 SARA Western Regional Conference 7 Call for Papers: 2015 SARA Annual Conference 8 Feature Articles Modeling and Measuring the Creative Design CLP5130‐2N Log Periodic Antenna‐ Whitham Reeve 9 ‐ Francisco Reyes 25 MOOCs Offer a Wide Variety of Free, On‐Line Courses in Astronomy and Related Fields‐ Lee Scheppmann 35 Epoch of Reionization‐ Judd D. Bowman 37 ‐ John G. Younger 43 Book Review‐ Title: Cosmic Discovery 49 Space Place Partner’s Article: What Is a Satellite Galaxy? Space Place Partner’s Article: What Is a Satellite Galaxy? 52 Membership New Members 55 SARA Membership Dues and Promotions 55 AdministrativeOfficers, directors, and additional SARA contacts 58 Resources Great Projects to Get Started in Radio Astronomy 59 Education Links 61 Online Resources 62 For Sale, Trade, and Wanted Sara Polo Shirts 64 For sale 64

Ken Redcap SARA President Kathryn Hagen Editor Whitham D. Reeve Contributing Editor Christian Monstein Contributing Editor Stan Nelson Contributing Editor Lee Scheppmann Technical Editor Radio Astronomy is published bimonthly as the official journal of the Society of Amateur Radio Astronomers. Duplication of uncopyrighted material for educational purposes is permitted but credit shall be given to SARA and to the specific author. Copyrighted materials may not be copied without written permission from the copyright owner. Radio Astronomy is available for download only by SARA members from the SARA web site and may not be posted anywhere else. It is the mission of the Society of Amateur Radio Astronomers (SARA) to: Facilitate the flow of information pertinent to the field of Radio As‐tronomy among our members; Promote members to mentor newcomers to our hobby and share the excitement of radio astronomy with other interested persons and organizations; Promote individual and multi station observing programs; Encourage programs that enhance the technical abilities of our members to monitor cosmic radio signals, as well as to share and analyze such signals; Encourage educational programs within SARA and educational outreach initiatives. Founded in 1981, the Society of Amateur Radio Astronomers, Inc. is a membership supported, non‐profit [501(c) (3)], educational and scientific corporation. Copyright © 2014 by the Society of Amateur Radio Astronomers, Inc. All rights reserved.

One the cover: Muriel Hykes, SETI League,with reconstruction of Jansky's Bruce array at Green Bank. He built this antenna to track downstatic plaguing trans‐Atlantictelephone service. Theinterference was naturally occurring from galactic center.

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Radio Waves President’s Page It may seem a long way off, but we need to be thinking about officers and directors nominations. If you are interested in serving as secretary, treasurer, director or director‐at‐large, let me know. Also, take a minute to look at the responsibilities and duties of these positions at http://www.radio‐astronomy.org/pdf/operating‐procedures.pdf. Dates have been set for the 2014 Western Conference to be held at Stanford University, Palo Alto, California March 20 and 22. Keith Payea and David Westman are soliciting papers for presentation at the conference. More information is in this Journal as well as on‐line at http://www.radio‐astronomy.org/meetings. The Eastern Conference is set for June 21 to June 24, 2014 at the National Radio Astronomy Observatory in Green Bank, West Virginia. More details will be made available on‐line at http://www.radio‐astronomy.org/meetings and in upcoming Journals. The editorial staff of the Journal is working very hard to publish a quality publication for our members. They welcome articles about observations, member projects, designing equipment, software used for observing, book reviews and analysing data. Please think about taking some time to write and tell us about what you are doing. This will enhance the Journal for all of our readers.

May your noise figure be low, Ken Redcap KR5ARA

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Editor’s Notes We are always looking for basic radio astronomy articles, radio astronomy tutorials, theoretical articles, application and construction articles, news pertinent to radio astronomy, profiles and interviews with amateur and professional radio astronomers, book reviews, puzzles (including word challenges, riddles, and crossword puzzles), anecdotes, expository on “bad astronomy,” articles on radio astronomy observations, suggestions for reprint of articles from past journals, book reviews and other publications, and announcements of radio astronomy star parties, meetings, and outreach activities. If you would like to write an article for Radio Astronomy, please follow the Author’s Guide on the SARA web site: http://www.radio‐astronomy.org/publicat/RA‐JSARA_Author’s_ Guide.pdf. You can also open a template to write your article http://www.radio‐astronomy.org/publicat/RA‐JSARA_Article_Template.doc Let us know if you have questions; we are glad to assist authors with their articles and papers and will not hesitate to work with you. You may contact your editors any time via email here: editor@radio‐astronomy.org. I will acknowledge that I have received your submission within two days. If I don’t, assume I didn’t receive it and please try again.

Please consider submitting your radio astronomy observations for publication: any object, any wavelength. Strip charts, spectrograms, magnetograms, meteor scatter records, space radar records, photographs; examples of radio frequency interference (RFI) are also welcome. Guidelines for submitting observations may be found here: http://www.radio‐astronomy.org/publicat/RA‐JSARA_Observation_Submission_Guide.pdf

Tentative Radio Astronomy due dates and distribution schedule

Issue Articles Radio Waves Review Distribution Jan – Feb February 12 February 20 February 23 February 28 Mar – Apr April 12 April 20 April 25 April 30 May – Jun June 12 June 20 June 25 June 30 Jul – Aug August 12 August 20 August 25 August 31 Sep – Oct October 12 October 20 October 25 October 31 Nov – Dec December 12 December 15 December 20 December 31

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News Mark Your Calendar 2015 Annual Conference Keynote Speaker Announced

Vice President Tom Hagen announced that Duncan Lorimer from West Virginia University Department of Physics and Astronomy has agreed to be the Keynote Speaker at the 2015 Annual SARA Conference to be held June 20 to 24 at the National Radio Astronomy Observatory (NRAO) in Green Bank, WV. The following excerpt is from WVU website:

I’m an astronomer interested in compact objects (black holes, neutron stars and white dwarfs) which I study using radio pulsars: rapidly spinning, highly magnetized neutron stars. Pulsars are great fun to study and have lead to a lot of exciting adventures over the years. A nice

behind‐the‐scenes article describing how this work is carried out can be found here .

I arrived at WVU in May 2006 from the Jodrell Bank Pulsar Group where I worked as a Royal Society Research Fellow. Before that I was at Arecibo Observatory (1998‐2001) and at the MPIfR in Bonn (1995‐1998). My research revolves around surveys for radio pulsars and what they tell us about the population of neutron stars. This work is carried out with many collaborators and uses some of the classic radio telescopes around the world. Of particular interest are young, energetic pulsars and binary systems where the orbiting companion is a white dwarf, a main sequence star, another neutron star, and (perhaps soon!) a stellar‐mass black hole.

February 13‐15, 2015 Hamcation Orlando, Florida http://www.hamcation.com/ March 20‐22, 2015 SARA Western Conference at Stanford University, Palo Alto, California http://www.radio‐astronomy.org/node/177 May 15‐17, 2015 Hamvention Dayton, Ohio http://www.hamvention.org/index.php June 21‐24, 2015 SARA Annual Conference at National Radio Astronomy Observatory in Green Bank, West Virginia www.radio‐astronomy.org/meetings Do you have an event to share with SARA members? Send information to editor@radio‐astronomy.org to be included in the next issue.

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L to R: Carl Lyster, David Cohen, James Thompson and Charles Osborne doing noise figure testing in the Drake Lounge.

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2015 SARA Western Regional Conference Palo Alto, California, USA on 20 ‐ 22 March 2015 The 2015 SARA Western Regional Conference will be held at Stanford University in Palo Alto, California on Friday, Saturday and Sunday, 20 ‐ 22 March 2015. The meeting will include a visit the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). Call for papers: Papers are welcome on subjects directly related to radio astronomy including hardware, software, education and tutorials, research strategies, observations and data collection and philosophy. If you wish to present a paper please email a letter of intent, including a proposed title and abstract to the conference coordinator at westernconference@radio‐astronomy.org no later than 31 December 2014. Be sure to include your full name, affiliation, postal address, and email address, and indicate your willingness to attend the conference to present your paper. Submitters will receive an email response, typically within one week. Presentations and proceedings: In addition to presentations by SARA members, we plan to have speakers from the Stanford University faculty, and possibly KIPAC. Papers and presentations on radio astronomy hardware, software, education, research strategies, philosophy, and observing efforts and methods are welcome. Formal proceedings will be published for this conference. If presenters want to submit a paper or a copy of their presentation, we will make them available to attendees on CD. Basic schedule: Our first day will include a visit to the KIPAC facilities at Stanford Linear Accelerator Center (SLAC). The next two days' meetings will take place on the Stanford University campus and will include presentations by members and guest speakers. Getting there: Fly into San Jose or San Francisco airports and rent a car to drive to Palo Alto. Registration: Registration for the 2015 Western Regional Conference is just US$55.00. This includes breakfast and lunch on Saturday and Sunday. Payment can be made through PayPal, www.paypal.com by sending payment to treasurer@radio‐astronomy.org. Please include in comments that the payment is for the 2015 Western Regional Conference. You also can mail a check payable to SARA, 2189 Redwood Ave, Washington, IA 52353, USA. Please include an e‐mail address so a confirmation can be sent to you when we receive your payment. Hotel reservations: TBA

What to wear: Our conference settings are casual. Saturday night dinner: We will make a group dinner reservation at a local restaurant for Saturday night. Additional Information: Additional details will be published online at www.radio‐astronomy.org/meetings and in the SARA journal, Radio Astronomy, as we get closer to the conference date. Please contact conference coordinator David Westman if you have any questions or if you would like to help with the conference: westernconference@radio‐astronomy.org.

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Call for Papers: 2015 SARA Annual Conference The Society of Amateur Radio Astronomers (SARA) solicits papers for presentation at its 2015 Annual Conference to be held 21 June ‐ 24 June 2015. Sunday 21 June, will start with an introduction to Radio Astronomy at the Science Center classroom, followed by learning to operate the forty foot radio telescope (1,420 MHz (21 cm). Presentations by SARA members and guests are scheduled on Monday and Tuesday. A High Tech tour of the NRAO facility will be conducted on Tuesday 23 June. Papers are welcome on subjects directly related to radio astronomy including hardware, software, education and tutorials, research strategies, observations and data collection and philosophy. SARA members and supporters wishing to present a paper should email a letter of intent, including a proposed title and abstract to the conference coordinator at vicepres@radio‐astronomy.org no later than 6 April 2015. Draft of papers are due 20 April and final versions of the papers due no later than 4 May. Be sure to include your full name, affiliation, postal address, and email address, and indicate your willingness to attend the conference to present your paper. Submitters will receive an email response, typically within one week. Guidelines for presenter papers are located at:http://radio‐astronomy.org/pdf/guidelines‐submitting‐papers.pdf Formal printed Proceedings will be published for this conference and all presentations can be made available on CD

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Feature Articles

Modeling and Measuring the Creative Design CLP5130‐2N Log Periodic Antenna

Whitham D. Reeve

1. Introduction A log periodic antenna is an array of dipoles with mathematically related lengths and spacings. A more formal name is logarithmic periodic dipole array (LPDA). The Creative Design CLP5130‐2N log periodic antenna described here is a popular antenna in solar radio spectrometers such as those used in e‐CALLISTO eCallisto because it is sturdy, inexpensive and covers a broad frequency range, 105 to 1300 MHz (figure 1). Another very similar and popular antenna is the CLP5130‐1N, which also is used in e‐CALLISTO and it has a frequency range of 50 to 1300 MHz Creative. This paper describes an electromagnetic model of the ‐2N antenna along with field measurements for comparison. The model is based on the Numerical Electromagnetic Code (NEC) implemented in the EZNEC+ v.5.0 software application EZNEC.The purpose of this work is to verify the manufacturer’s claims of gain, beamwidth and voltage standing wave ratio, VSWR (table 1).

Figure 1‐1 ~ Creative Design CLP5130‐2N log periodic antenna against a blue Anchorage sky. The antenna shown has a “dragonfly” mount (see text). Log periodic antennas are recognized by their obvious apex (point at which imaginary lines along the element tips meet at the front of the antenna) and progressively shorter elements with progressively smaller spacing.

2. Design and construction

Note: Links in braces and references in brackets [ ] are provided in section 7.

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A log periodic antenna is a type of broadband antenna with good directivity. It can be described by two geometric parameters, a scale factor τ that specifies the relative lengths and a spacing factor σ that specifies the relative spacings of the antenna elements (for additional detail, see [Carrel], [Hutira] and [Isbell]) (figure 2‐1). A third parameter, α, is one‐half the apex angle and is derived from τ and σ. The following values were measured for the CLP5130‐2N: α = 27.3°, τ = 0.84, σ = 0.193.The individual antenna elements of the log periodic antenna are interconnected by a transmission line, which often also serves as a structural boom (as in the CLP5130‐1N and ‐2N).

Table 1‐1 ~ CLP5130‐2N – manufacturer’s data. Source: [Creative]

Parameter Value Frequency range 105 to 1300 MHz Number of elements 17 Forward gain 11 to 13 dBi Front‐to‐Back ratio 15 dB Beamwidth – E‐plane 70 to 60 degrees Beamwidth – H‐plane 130 to 110 degrees Impedance 50 ohms VSWR ≤ 2.0:1 Boom length 1.4 m Maximum element length 1.3 m Weight 3 kg

Figure 2‐1 ~ Antenna photograph annotated with dimensional characteristics. Note: Actual measured length of the longest element differs from the manufacturer’s datasheet.

ApexAngle

Element Spacing

Element Length

ElementDiameter

Structural Transmission Line

Antenna RF Connector

Mounting Clamp

1.45 m

1.4 m

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The highest directivity is along the longitudinal axis in the direction of the shortest elements. The longest dipole determines the low frequency and the shortest determines the high frequency. Usually, the low frequency dipole is about 7% longer than 1/2‐wavelength at the low design frequency and the high frequency dipole is about 30 to 35% shorter than 1/2‐wavelength at the high design frequency. These criteria apparently were used for the CLP5130‐2N, whose uncompensated dipole lengths correspond to a frequency range of 98 to 1760 MHz (105 to 1300 MHz working range). When the antenna is illuminated by an electromagnetic field, currents flow in each dipole. Dipoles with lengths closest to 1/2‐wavelength carry the highest currents (figure 2‐2). The currents in this active region combine on the transmission line according to their phase and work together to form the antenna radiation pattern characteristics. Log periodic antennas are fed from the end nearest the apex (front).

Figure 2‐2 ~ Log periodic antenna elements (green) and the current magnitudes in each element (thin violet lines) when the antenna is illuminated with a single‐frequency 245 MHz radio wave. Elements 6 and 7 (from the rear) carry the highest currents. Element 6 is 584 mm long and very close to the length of a 1/2‐wave dipole at 245 MHz (582 mm). The shorter dipoles carry lower currents, which decrease as their lengths decrease. Dipoles longer than 1/2‐wave at 245 MHz carry very little current. The currents in each dipole add or subtract on the transmission line according to their phase to form a total current that depends on the frequency and direction of the incoming radio wave. This image was produced by EZNEC. An important attribute of log periodic antennas is the use of a transmission line to interconnect the dipole elements. This line can be made from ordinary wires, in which case the dipole elements need to be supported by a separate insulating structure, or the line itself can be made from structural components such as round or square tubes or channels. The CLP5130‐2N uses rectangular aluminum channels for the transmission line and support. Half‐elements are attached to alternating channels; for example, one‐half of element number E2 is attached to the upper channel and the other half is connected to the lower channel. This type of connection provides the phase reversal at adjacent dipoles that is needed for a log periodic antenna. The transmission line often extends passed the rear (longest) dipole element a distance of 1/4 wavelength at the low frequency. This line extension is terminated with a shorting bar or coil. The bar or coil provides a dc short for lightning and static build‐up protection and also is required to provide an electrical termination (termination is discussed briefly in the next section). The CLP5130‐2N line extends only 33 mm beyond the longest element and uses a coil (inductor) termination that is electrically equivalent to a length extension. At the front of the antenna, the transmission line terminates in an N‐type RF connector for connection to 50 ohm coaxial cable.

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The CLP5130‐2N is made entirely from aluminum except for stainless steel fasteners and plastic brackets that hold the longer elements (figure 2‐3). The antenna is broken down for shipment and is assembled on‐site using common tools in about 30 minutes.

Figure 2‐3 ~ Left: Photograph of the CLP5130‐2N showing transmission line channels and plastic brackets that hold the longer elements (black object on lower element). A stainless steel self‐threading screw through the channel bonds each half‐element to the transmission line. Also seen is a stepped element consisting of two tube diameters and non‐stepped elements. Right: Transmission line termination coil at the rear of the antenna.

3. NEC Model This section describes the model, its limitations and the compromises and simplifications that were necessary during its development. In numerical electromagnetic codes, an antenna or any arbitrary metal structure is modeled as a collection of thin wires. Each wire is divided into a number of segments. The NEC software solves the mathematical equations that describe the impedances and induced currents in these wire segments. Except for a very simple wire dipole antenna, an NEC antenna model is a compromise between physical reality and computational expediency. Several versions of the NEC exist, and EZNEC+ v.5.0 uses version 2 (NEC‐2), which is in the public domain. The EZNEC main window shows a model summary (figure 3‐1). Log periodic antennas that are built like the CLP5130‐2N are difficult to simulate or model because of the stepped dipole elements and structural boom transmission line. A stepped element consists of two or more tubing diameters along its length. EZNEC has the ability to handle stepped elements but only if the entire antenna uses them. In the case of the CLP5130‐2N, only seven of the 17 elements are stepped. The stepped elements consist of two tube diameters, 10 mm and 7 mm. All non‐stepped elements are 4.5 mm diameter except the shortest one, which is 4.0 mm diameter.

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Figure 3‐1 ~ EZNEC main window showing a summary of the CLP5130‐2N antenna model. This particular setup modeled the antenna in free space but it is simple to change to any height above ground.

To resolve the problem of stepped elements, I used EZNEC to separately calculate an equivalent length and single (non‐stepped) diameter for each stepped element and then substituted the equivalent dipoles in the log periodic antenna model. EZNEC uses the calculation method described by [Leeson] for the stepped element conversion (or correction). The equivalent dipole has the same resonance and Q as the original stepped dipole. Its length is slightly less than the original and its diameter falls between the two original diameters (figure 3‐2).

Figure 3‐2 ~ Example of one‐half of a stepped element (upper) converted to electrically equivalent nonstepped element. The conversion is done on the entire dipole but only one‐half is shown here.

As a practical matter, the structural transmission line cannot be physically modeled in EZNEC. Therefore, the dipole elements in the model were interconnected with an ideal transmission line setup to provide the phase reversal between the elements that is required for log periodic antennas. Because the actual CLP5130‐2N transmission line is made from channels (figure3‐3), determining its characteristic impedance is a problem in electromagnetics. With the help of a colleague and fellow Dane, Kurt Poulsen, I found an arbitrary transmission line calculator ATLC2 that could handle the calculations. This calculator yielded 111 ohms characteristic impedance. The actual transmission line impedance used in the model has no effect on the antenna radiation pattern but does affect the calculated impedance and VSWR. Substituting an ideal transmission line for a real physical structure leads to idealistic results that most probably deviate from the antenna’s actual performance. A question arises as to the amount of deviation, so I briefly address this in the next section. It is an interesting characteristic of many commercial log periodic antennas that no provision is made for impedance matching or balanced‐to‐unbalanced conversion between the antenna transmission line (111 ohms), which is inherently balanced, and the unbalanced coaxial cable feedline (50 ohms). Some antennas do use a so‐called “choke balun”

300 mm mm426

7 09 .268 mm

Original Stepped Element

Equivalent Element

1 0 mm diameter mm diameter7

8.188 mm diameter

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but none is provided or called for with the CLP5130‐2N. Nevertheless, as seen in section 4, the voltage standing wave ratio (VSWR) does not seem to suffer by this crime of technicality.

Figure 3‐3 ~ Left: Dimensions of transmission line channel cross‐sections. The larger set of channels (B1) is about 1 m long. Nested at the high frequency end of the larger channels is the set of smaller channels (B2) that extend another 0.4 m. Right: Electric field around the channels as calculated by the ATLC2 software tool.

The CLP5130‐2N can be obtained with a “standard” or “dragonfly” mounting arrangement. The standard mount is a U‐shaped aluminum bracket below the transmission line boom that is clamped to a mast. This mount usually is used with a vertical mast to hold the antenna in a horizontal configuration. The dragonfly mount includes an aluminum tube section that projects slightly more than 1 m behind the antenna. This tube is clamped to a mast and allows both horizontal and vertical configurations. These mounting arrangements probably affect the antenna patterns and impedances, but I made no attempt to include the antenna mounting structure in the antenna model. Programs that use numerical electromagnetic codes require wires to be divided into short straight segments. These segments are required to comply with certain dimensional rules in relation to the wavelength. A conservative segmentation requires at least 20 segments per wavelength at the highest design frequency (segment length ≤ λ/20). In the case of the CLP5130‐2N, the highest frequency is 1300 MHz and the corresponding wavelength is 231 mm. This results in a minimum segment length of 11.5 mm. I reduced this slightly and used segment length ≤ 10 mm, leading to a total segment count of 888 for the whole antenna. I experimented with different segment lengths and found that 10 mm is conservative. More segments require longer computation times. The longer times were readily apparent when the antenna was analyzed over a wide frequency range with high resolution. For example, analyzing the impedance characteristics with 1 MHz resolution over a range of 50 to 1000 MHz required several minutes on my Windows 7 x64 laptop. Running the same analysis with 50 MHz resolution required a few seconds. In the next section I discuss some model calculations at 50 MHz, below the antenna design frequency range. The segmentation that I used is adequate for the range 100 to 1000 MHz; however, at 50 MHz EZNEC issued a segmentation warning indicating the model calculation accuracy could be compromised. Since I investigated 50 MHz out of curiosity, I made no attempt to adjust segmentation. Another check available for NEC models is the average gain test, and I used this to check the model over its design frequency range. The average gain is the ratio of total power in the far field to the power delivered to the antenna by the sources. A very good model has a value of 1 ± 0.05. For the model described here, the value is 0.995 from 100 to 1000 MHz but falls to 0.942 at 50 MHz. Although the value at 50 MHz is not particularly bad, it could indicate compromised accuracy of the patterns and gain at that frequency (but the antenna is not designed for this frequency, anyway).

B oom B 1 Length: 9 mm 58

Boom B2Length: 40 mm6

22

1312

27

17

16

49

B oom Transmission Line Cross-Section

Thickness: 2.4 mm Thickness: 2.0 mm

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The termination at the rear of the antenna significantly affects its low frequency characteristics, and it must be placed in the NEC model as a load to yield useful results. The actual CLP5130‐2N uses a 6‐turn, 23 mm diameter coil with a calculated inductance of 0.276 µH. I placed the inductive load in the model at the actual physical distance (with zero dc resistance), but I found that varying the value of the inductance had little effect.

4. Modeling results and measurements General considerations: This section describes the EZNEC model simulation results over a frequency range of 50 to 1000 MHz, the lower limit being below the lower design frequency of 105 MHz and the upper limit not reaching the upper design frequency of 1300 MHz. I used a lower frequency of 50 MHz to investigate performance below the antenna’s design frequency range out of curiosity, and I limited the upper frequency because I was not interested in antenna performance above 1000 MHz. Model simulations and measurements have two main components: Antenna radiation patterns and impedance. Antenna gains, beamwidths and front‐to‐back ratios are derived from the patterns. The maximum available gain, beamwidth and front‐to‐back ratio at each frequency as calculated by EZNEC are tabulated for comparison (table 4‐1) and explained below. Impedance includes VSWR for a 50 ohm reference and is shown separately. As already mentioned, the NEC model is a simplified electrical model of physical reality. The two aluminum channels that make up the transmission line boom have physical dimensions that are significant fractions of the wavelengths at the high end of the antenna’s design frequency range. There is little question electrical currents on the antenna elements and boom interact with each other, and this probably is the reason the dipole elements at the high frequency end are much shorter than 1/2‐wavelength at 1300 MHz. On the other hand, effects on the impedance up to 1000 MHz appear to be very small because field measurements and the model calculations agree very well. Radiation patterns: Antenna patterns for a receive antenna indicate its response in azimuth and elevation and are used to determine the directions from which reception is the best or sidelobe response is the lowest. All patterns in this article are based on a horizontally polarized antenna, thus the E‐plane (electric) corresponds to azimuth and H‐plane (magnetic) to elevation. The antenna elevation angle is 0° in all simulations. Antenna patterns depend on the height of the antenna above ground surface and the electrical characteristics of the earth in the vicinity of the antenna. The elevation patterns for an antenna above ground and in free space are different (figure 4‐1), but the azimuth patterns are similar (figure 4‐2). Gains: The antenna gain is relative to an isotropic antenna (an isotropic antenna has equal response in all directions and often is used as a reference in antenna engineering work). When the antenna is above ground, the maximum gains calculated by EZNEC are a few degrees above the horizon, and when in free space the maximum gains are at 0° elevation angle. Antenna gains usually are given in datasheets as maximum values. In this regard, the modeled antenna shows good agreement with the manufacturer’s data only when the model includes the influence of ground reflections. The peak gains for the antenna above ground are approximately 5 to 6 dB higher than for free space. When the antenna is above ground, the patterns are affected by ground reflections that add or subtract depending on the electromagnetic field phase and amplitude at each dipole element. The resulting peaks and nulls are obvious in the elevation patterns and, as the antenna height changes, the peaks and nulls change. However, in free space,

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the antenna elevation patterns are symmetrical about an elevation angle of 0° and there are no reflections and no peaks or nulls in the patterns. A limitation of the model is that if an earth ground is present, the simulation shows no downward response in the vertical antenna pattern. Even if the antenna is modeled at a height of 100 000 km, the patterns show no downward response (that is, no response at negative elevation angles). However, practical experience shows there is, in fact, downward response and that as height is increased, peak gains decrease to free space values and the pattern peaks and nulls smooth out. Therefore, it is appropriate to use the lower gains calculated for free space when the antenna is, say, 10 wavelengths or more above ground (height > 10λ). For UHF, this height is 10 m or more.

Table 4‐1 ~ CLP5130‐2N antenna pattern characteristics for free space from the EZNEC simulation Frequency Peak Gain 3 dB Horizontal 3 dB Vertical Front/Back Front/Sidelobe (MHz) (dBi) Beamwidth (°) (note 1) Beamwidth (°) (note 2) Ratio (dB) Ratio (dB) 100 4.93 72.6 170.4 6.94 6.94 150 6.39 69.2 135.6 19.77 17.59 200 6.50 70.6 140.6 17.67 17.67 250 7.45 59.2 100.4 17.89 17.89 300 7.35 55.8 110.8 16.33 13.89 350 6.91 62.4 117.4 28.66 14.88 400 7.08 63.6 115.4 25.14 18.19

450 7.26 69.6 88.4 20.87 9.40 500 7.53 55.0 101.2 21.64 9.51 550 6.97 67.6 111.6 19.99 14.74 600 7.51 53.8 100.4 21.64 9.05 650 7.08 66.0 114.2 23.33 11.26 700 7.33 58.6 101.6 27.94 9.60

750 7.57 59.0 105.4 28.59 10.00 800 7.25 64.0 113.6 29.02 10.97

850 7.30 61.0 106.0 27.91 8.47 900 7.11 62.6 111.4 27.37 10.81

950 7.18 63.0 117.8 27.72 10.06 1000 7.08 59.8 105.6 27.82 9.73

Table notes: 1. Horizontal beamwidth at 0° elevation angle. 2. Vertical beamwidth at 0° azimuth angle.

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Figure 4‐1 ~ Upper: Modeled free space elevation patterns at 50 MHz intervals from 50 to 1000 MHz for horizontally polarized CLP5130‐2N antenna. Lower: Same antenna mounted 3 m above ground level showing that ground reflections add and subtract to the radio waves directly received by the antenna elements. The patterns are based on “average” ground characteristics, conductivity = 0.005 S/m and relative dielectric constant = 13.

Elevation patterns for antenna above ground

Elevation patterns for antenna in free space

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Figure 4‐2 ~ Upper: Modeled CLP5130‐2N azimuth patterns for installation in free space. Lower: Azimuth pattern at 5 degrees elevation for the same antenna mounted 3 m above ground level. The wide variations in gains are due to reflections from the ground. Front‐to‐back ratios: Front‐to‐back ratio is the ratio of forward gain to rearward gain (180° azimuth). An antenna with high front‐to‐back ratio is useful for limiting interference that comes from behind the antenna. However, often the interference comes from other directions. A more useful indication of antenna response from directions other than directly in front of the antenna is the front‐to‐sidelobe ratio. Front‐to‐sidelobe ratio is the ratio of the peak mainlobe gain to the gain of the lobe with the second highest gain. Sidelobes are easily seen by examining the patterns. Field measurements: The ideal results from antenna modeling can be verified only by field measurements. However, because of trees and buildings, the site I used for pattern measurements has too much multipath

Azimuth patterns for antenna above ground

Azimuth patterns for antenna in free space

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interference and foliage attenuation to be of any real use for measurement purposes (figure 4‐3). Nevertheless, I made the measurements anyway to get an idea of how bad the site really is. For comparison, I also measured the pattern of a similar antenna (the CLP5130‐1N) that is much higher and the results were much better. The CLP5130‐1N model and measurements will be reported in a future paper. For the pattern measurements described here, I used FM and television broadcast stations and cellular base stations as far‐field sources in the 105 to 870 MHz frequency range. All power measurements were made with a Callisto solar radio spectrometer and the NF software tool (this tool was developed by Christian Monstein and I use it during manufacturer of Callisto instruments). I measured the received power at 30° intervals for a fixed elevation angle of 0° (figure 4‐4).

Figure 4‐3 ~ Antenna pattern measurement setup at an inadequate test site having a lot of foliage loss and multipath interference. The antenna was installed on a tripod and mast 5.3 m above ground level, and a compass was used to point the antenna to the station. The tripod was held down by a 23 kg lead weight. The mast was then rotated in 30° increments determined from a paper scale taped to the mast (inset at upper‐left).

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Figure 4‐4 ~ Modeled and measured antenna patterns in the frequency range 105.7 to 870 MHz. The red dots are the measurement points. In all plots, the antenna azimuth is normalized to the direction of the radio source. The results are useless because of multipath and foliage loss.

VSWR: Voltage standing wave ratio indicates the degree of matching to a 50 ohm reference impedance. In practical work, a well‐matched antenna provides 2:1 VSWR or better. VSWR measurements are easier than pattern measurements, and for these I used a vector network analyzer (VNA). The VNA was calibrated with a short interconnecting cable in‐place to remove its effects from the measurements, and I pointed the antenna away from interference sources to reduce their influence on the VNA. The VSWR calculated by the model followed very closely the measurements and also the VSWR chart provided in the antenna manufacturer’s datasheet (figure 4‐5). As with the patterns shown previously, the VSWR was calculated at 50 MHz to see how it looked. It is seen that the VSWR increases sharply at 50 MHz.

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Figure 4‐5 ~ Modeled (upper), measured (middle) and datasheet (lower) antenna VSWR relative to 50 ohms impedance from 50 to 1000 MHz. The model indicates the antenna VSWR ≤ 1.7:1. A DG8SAQ VNWA‐3E vector network analyzer was used for VSWR measurements with the antenna mounted 5.3 m above ground level. The vertical scale on the VNWA‐3E measurement plot is interpreted as follows: The reference position for 1:1 VSWR is shown on the right at the 3rd division from the bottom, and the 4th division is 2:1 VSWR. Markers tabulated at the top of the plot show the VSWR readings at various frequencies.

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5. Conclusions Log periodic antennas that use a structural boom transmission line and stepped elements are difficult to model and any model is a compromise. For the CLP5130‐2N antenna described here, the structural transmission line is replaced by an ideal transmission line for modeling purposes. The model uses actual physical dimensions for the non‐stepped dipole elements and adjusted (“corrected”) dimensions for the stepped elements. The modeled VSWR, and thus the impedance, shows reasonably good agreement with measurements and manufacturer’s data. On the other hand, because of severe multipath and foliage at the test site, the measured antenna patterns show only casual agreement with the modeled patterns, but this is the fault of the test site and not the model. The modeled patterns, including front‐to‐back ratios, generally agree with the manufacturer’s data. The antenna gains given in the manufacturer’s specifications apparently are peak values for an antenna within the influence of ground reflections. For heights greater than about 10λ, free space gain values about 5 to 6 dB lower are more appropriate but not given in the manufacturer’s data.

6. Ordering information The CLP5130‐1N (50 to 1300 MHz) and CLP5130‐2N (105 to 1300 MHz) may be ordered at Order. .

7. References [ ] and internet links [Carrel] Carrel, R, Analysis and Design of the Log‐Periodic Dipole Antenna, Armed Services Technical

Information Agency, Antenna Technical Laboratory Report No. 52, 1961 [Creative] CLP5130‐2 Assembly Manual, AM 871‐930‐21, Rev. 2, 1994‐7, Creative Design Corp. [Hutira] Hutira, F., Bezek, J., and Bilik, V., Design and Investigation of Log‐Periodic Antenna for DCS,

PCS and UMTS Mobile Communications Bands, date and source unknown (found via web search)

[Isbell] Isbell, D.E., Log Periodic Dipole Arrays, IRE Transactions on Antennas and Propagation, May, 1960, pg 260~267

[Leeson] Leeson, D., Physical Design of Yagi Antennas, ARRL, 1992 (ISBN: 978‐0872593817)

ATLC2 http://www.hdtvprimer.com/KQ6QV/atlc2.html

Creative Design http://www.cd‐corp.com/english/ eCallisto http://www.e‐callisto.org/ EZNEC http://www.eznec.com/ Order

http://www.reeve.com/Solar/e‐CALLISTO/e‐callistoOrderInfo.htm

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Author ‐ Whitham Reeve has lived in Anchorage, Alaska his entire life. He worked as an engineer and engineering firm owner/operator in the airline and telecommunications industries for more than 40 years. He has been a director of SARA and is a contributing editor for the SARA journal.

Astronomy Joke 3: It is reported that Copernicus' parents said the following to him at the age of twelve: "Copernicus, young man, when are you going to come to terms with the fact that the world does not revolve around you." ‐‐ http://www.jupiterscientific.org/sciinfo/jokes/astronomyjokes.html

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OwensValleyRadioObservatory Big Pine, California

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Observations and Studies of Pulsars at 26.3 and 45 MHz using Two Large Arrays

Francisco Reyes Department of Astronomy, University of Florida, P.O. Box 112055, Gainesville, FL 32611

1. Introduction Detecting pulsars at low radio frequencies present several difficulties and challenges. Some of

the complications arise from the turn-over of the spectral index which decreases the flux density of the pulses toward low frequencies, the increase of the galactic background noise temperature and the increase of the dispersion of the pulses. The dispersion of the pulses at low frequency introduced by the electron content of the interstellar medium forces the use of narrow band. In order to overcome the low S/N ratio, it is necessary the use of antennas with large effective area.

Several techniques have been used to detect pulsars a low frequency. One of the techniques is the pulse stacking using one narrow band channel and averaging the pulses by folding the pulses with the observed pulsar period to increase the S/N ratio. In this technique, the dispersion in the narrow band channel is not corrected. A second technique called incoherent dedispersion consist in using many narrow band channels (spectrum analyzer) and averaging the pulses from the different channel previously shifting them (dedispersing the different channels) by a certain delay given by the dispersion of the pulsar. This technique does not dedisperse the pulses inside each channel. Another technique is the coherent dedispersion in which several narrow band channels are used and the dispersion inside each narrow band channel is corrected before combining the different channels with the appropriate delay.

In order to process the data, accurate timing pulses for digitizing the signal are needed. In order to fold the pulses to increase the S/N ratio, the correct and accurate observed pulse period, corrected by the Doppler shift due to the velocity of the Earth respect the heliocentre and the period derivative of the pulsar must be used.

Studies of pulsars at low radio frequencies are important. They provide information on the location and extent of the emitting regions and they are used to study the spatial structure, variability and turbulence of the electron content of the interstellar medium (ISM).

This paper is a description and discussion of the method used in the observations and data processing in the 80’s and 90’s at low frequencies, in particular those used at Univ. of Florida and Univ. of Chile radio observatories. Some of the restrictions and limitations are still valid today and they are directly connected to the basic physics of the problem. But advances in the instrumentation may provide easy access to cheaper and faster instruments and also provide access to data reduction processes that may facilitate and simplify reducing the data.

2. Observations and Instrumentation

In the 1980’s two large low frequency arrays, the Univ. of Florida 640 dipoles array at 26.3 MHz and the Univ. of Chile 528 dipoles at 45 MHz were used for this study. Most observations were made using one or two narrow band channels. Pulse stacking techniques were used to rescue the pulsar signal from the galactic background noise. For some pulsars it was possible to do wide band observations. The wide band signal was processed with a 200 channel spectrum analyzer. Four pulsars were detected at 26.3 MHz and six at 45 MHz.

Pulsars PSR 0031-07, 0628-28, 0826-34, 0834+06, 0950+08, 2045-16 were detected at 45 MHz. Pulsars 0834+06, 0950+08, 1508+55 and 1919+21 were detected at 26.3 MHz.

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In order to avoid important effect due to the dispersion within the bandwidth of the receiver, all these pulsars were chosen for their relative long period, between 0.25 to 1.96 seconds and their rather small Dispersion Measure (DM), between 2.9 to 45 cm-3pc.

The dispersion delay equation is given

where f1 = lower frequency

f2 = higher frequency DM = Dispersion Measure e = electron charge m = electron mass c = speed of light

The observations at 45MHz were made using a one or two-channel receiver each with a 10 kHz

bandwidth and separated by 140 kHz to 1.5 MHz depending on the pulsar DM. The analog signals from the one or two channels were recorded in an Ampex FR-600 7-channel instrumental tape recorder. In a third channel was recorded a timing signal. The timing signal was obtained from a Systron Donner time code generator (IRIGB) driven by a Rhode and Schwarz quartz frequency standard. The data was digitized on playback .The timing signal was used to trigger the sampling in the A/D converter and also to synchronize the start and stopping of the digitization. Using the recorded time signal makes the sampling independent of the speed variations of the tape recorder.

A similar system was used for processing the 26.3 MHz signal except that only one channel was used with a bandwidth of 8 kHz and the signal was recorded in a Magnecord tape recorder. The timing signal was derived from a Traicor quartz frequency standard.

3. Data reduction

The flux density calibrations were provided by a special built pulsar simulator. The simulator consists in two independent noise generators, one to simulate the galactic background temperature around the position of the pulsar and the second to simulate the pulsar’s pulses. The second noise generator was pulsed to simulate the period and width of the observed pulsar. The period can be simulated within ± 0.5 microsecond of the apparent observed pulsar period. The signal from the two noise generators were combined in a power combiner. The signal from the pulsar simulator were digitized and averaged in the same way as a pulsar, for each of several noise temperature steps of the simulated pulses. The pulsar flux density is then deduced by interpolation from the simulated pulsar data.

The data reduction for one single narrow band channel was done by digitizing the signal on playback with an analog-to-digital converter (A/D) and the samples were stored in a computer disk. The sampling of the A/D was triggered by pulses generated by dividing the signal from the second channel of the tape recorded to match the sampling frequency. Sampling frequencies of 50 or 100 samples/sec were used at 45 MHz and 31 samples/sec at 26.3 MHz. At 45 MHz the sampling rate depended on the pulsar period. At that time (1970’s and 80’s) the limitation for the data sampling rate was the access to a “fast” A/D converter and the slow transfer rate of the data to a computer.

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The integrated pulse profiles were obtained by folding the data at the apparent pulsar period. The apparent pulsar period was computed using the position, period and period derivative and epoch of the pulsar. The period was also corrected by Doppler shift due to the velocity of the telescope respect to the heliocentre. The program used for these corrections was DOPVEL, developed by R.N. Manchester and M.A. Gordon at NRAO in the 70’s. This is a very important correction. If the wrong pulsar period is used, the pulses are washed out.

4. Analysis and Results The parameters measured for some of the pulsars were the pulse flux density and energy, the

pulse shape and width and the dispersion measure. For some pulsars it was possible to improve the determination of the flux density spectrum toward lower frequencies.

4.1 Integrated profiles The Figure 1a-f shows examples of the integrated pulses of six pulsars detected at 45 MHz. The

intensity of the pulses has been standardized to the unit for the peak of the pulse. The origin of the longitude axis in degrees was chosen at the peak of the pulse. Due to the low sampling rate, the pulse profile has been smooth out and some of the sub pulses present in these pulsars are not resolved in the main pulse.

Figure 1. Integrated 45 MHz pulse profiles: a) PSR0031-07 integrated over 2270 period. Pulse height 6.7 st. dev., b) PSR 0629-28 integrated over 9140 periods. Pulse height 20.7 st.dev. c) PSR 0826-34 integrated over 1590 periods. Pulse height 4.1 st. dev. d) PSR0834+06 integrated over 2310 periods.

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Pulse height 12.9 st. dev. e) PSR 0950+08 integrated over 17755 periods. Pulse height 22.9 st. dev. f) PSR 2045-16 integrated over 740 periods. Pulse height 6.8 st dev.

An example of the details of the structure of the pulse gained by sampling faster and using

narrower bandwidth is shown in the Figure 2. Pulsar PSR0834+06 wide band recording was processed with a 200-channel Saicor spectrum analyzer. Figure 2a shows the incoherent dedispersed pulse obtained by averaging the 200 channels after shifting each channel by a delay to compensate for the dispersion between channels. Figure 2 b shows the same pulse drifting across the 200 kHz bandwidth. The two sub pulses characteristic of this pulsar is clearly visible in the upper figure.

Figure 2 a) Pulsar PSR0834+06 integrated profile over 200 channels of the Saicor spectrum analyzer. b) PSR0834+06 pulse drifting across the pass band of the 200 channel spectrum analyzer. The noise outside the pulse is due to the galactic background emission.

4.2 Flux densities and energies The values of flux density and energy of the pulses were obtained for five of the six pulsars. The

results are presented in Table 1. The values are average obtained over periods of about 15 minutes (durations of one observation).

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4.3 Pulse width at half intensity The pulse width at half intensity was obtained for five pulsars at 45 MHz. The values are listed

in Table 1. The observed width must be corrected for dispersion and time constant. In order to obtain the corrected width, it was assumed that the profile had a triangular shape and accordingly corrected for dispersion and time constant. A correction using a Gaussian shape was attempted but it gave width values larger than the actual ones. In order to test the validity of the triangular and Gaussian correction, the profile of PSR0834+06 shown in Figure 2 was used as a reference. This profile was obtained using a narrow bandwidth and very short time constant and it gives a value of the width close to the intrinsic width. No such data was available for the other pulsars.

Figure 3 shows an example of the spectral pulse energy and the fitting of a curve (polynomial) giving an approximation of the spectral index for pulsars 0628-28, 0834+06 and 0950+08. The filled triangle is the value of pulse energy obtained at 45 MHz. The values at other frequencies were taken from the published values in the literature. The energy spectrum of PSR0834+06 and 0950+08 seems to show a turnover of the spectra at low frequencies. PSR0834+06 show a turnover around 55 MHz. PSR0950+08 shows a turnover around 105 MHz. The energy of the pulses of PRS 0628-28 continues to increase towards low frequencies and no turnover of the spectra could be identified at the time of this study.

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Figure 3. Pulse energy spectrum of pulsars a) PSR0628-28, b) PSR0834+06 and c) PSR0950+08. Filled triangles are the data at 45 MHz.

5. Conclusions Using these two large arrays it was possible to observe and measure important parameters of a

few pulsars at low frequencies. At 45 MHz it was possible to observe pulsars during daytime. Sometimes the interference was strong and contaminated the data to appoint in which it has to be discarded. At 26.3 MHz the interference was too strong during day time and only night time

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observations were made. The parameters measured at 26.3 and 45 MHz contributed to the scarce data available at that time at the extreme low frequencies (< 50 MHz). Most of the observations around 25 MHz came from the Karkov observatory using the UTR-2 radio telescope. Some observations were made a 34.5 MHz at the Gauribidanur radio telescope in India.

In the early 1990’s observations around 25 MHz from the Arecibo radio telescope made by A. Phillips with a high S/N ratio and good time resolution provided excellent results. Some observations obtained previously at 25 MHz with the UTR-2 telescope with a much lower S/N ratio and time resolution showed the existence of interpulses and a superdispersion delay at low frequencies for some pulsars. The superdispersion was an extra delay of the pulses at low frequency added to the normal delay due to dispersion in the ISM. The 45 MHz observations reported here could not confirm or denied of any of these two effects due to the relative low S/N ratio. For a couple of pulsars there was an indication of sporadic interpulses at 45 MHz but their intensity was to low and they were buried in the noise.

The superdispersion was attributed to a twisting of the magnetic field at larger distance from the star where the low frequencies emissions originate. The Arecibo observations having a high S/N ratio did not show any interpulses or superdispersion.

6. Discussion

Several restrictions and limitations for observing pulsars at low frequencies are imposed by the low S/N ratio of the pulses and the dispersion of the pulses in the ISM. In order to process the data, good timing for sampling the pulsar signal and an accurate calculation of the apparent pulsar period are needed. The following is a short discussion on how to improve and overcome some of these limitations.

6.1 Effective area of the antenna In order to overcome the low S/N ratio, antennas with a large effective area are needed. The 45

MHz antenna used here had an effective area of about 9,650 m2. The 26.3 MHz array had an effective area about 16,000 m2. The shorter integration times used at 26.3 MHz to be able to detect pulses just about 3-4 standard deviation from the noise for PSR0834+06 required at least about 8 minutes of integration for a receiver with a pre detection bandwidth of 8 kHz.

Small antennas like a 7-element Yagi antenna have an effective area around 27 MHz of 100 m2. The ratio of effective area respect to the 26.3 MHz is 160. Assuming that one uses the same pre detection bandwidth, it will require an integration time of about 1280 minutes or about 21 hours! This time is longer than the time the pulsar remains in the beam of the antenna and even longer than the pulsar remains in the sky for one day. This estimate is assuming that the pulsar is emitting a steady signal. The pulses from several pulsars are received sporadically. They may emit strong pulses for a few minutes and then the emission intensity falls and they may become undetectable for minutes, hours or even days. A classic example of this effect is PSR1919+21, the first pulsar to be discovered.

6.2 Galactic background temperature The galactic background temperature also presents more problems at low frequencies. The

galactic background spectral index has a positive value. The temperature T increases with decreasing frequencies (or longer wavelengths).

The dependence is given by the equation

where λ = wavelength α = spectral index The values of the spectral index depend on the position in the sky and the range of frequencies. The values at low frequency can range from 2.5 to 3.7 around 30-40 MHz.

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As a comparison, the galactic background temperature for PSR0834+06 at 45 MHz is about 6,000 K. The galactic background temperature for the same pulsar at 26.3 MHz is about 24,000 K.

For observations of pulsars around the frequency normally used for the Radio JOVE system of 20.1 MHz, the conditions are even worse. The galactic background temperature is higher. The effective area of the antennas available at that frequency is small. The two-half wave dipole array antenna used by Radio JOVE has an effective area of about 130 m2.

6.3 Dispersion of pulses Another aspect of the problem is the dispersion of the pulses in the ISM. Lower frequencies are

delayed respect to higher frequencies. This causes the pulses to drift across the bandwidth resulting in them staying longer within the band. This results in a lack of time resolution of the pulses. The pulses appear wider and smoother. The dispersion can be corrected using a incoherent or coherent methods. The incoherent method corrects for the dispersion of the pulses between channels in a multichannel system but doesn’t correct for dispersion within each channel. The coherent dispersion correct for the dispersion inside each channel. This method gives the best time resolution and provides the true pulse shape and it is good for pulsar timing.

The dispersion also restricts the detection of short period pulsars at low frequencies. For a given bandwidth ∆f, if the period of the pulsar P is short, less than the ∆t, the time for the pulse to sweep across the bandwidth, one pulse has not left the low frequency end of the bandwidth when the next pulse enters the high frequency end of the bandwidth. The result is having two or more pulses within the bandwidth at the same time. The emission of the pulsar appears like a continuum emission.

The bandwidth of the RJ receiver is about 5 kHz but the conversion to base band causes a sweeping signal to cross the bandwidth from high to low frequency and then it appears to reverse, sweeping from low to high. This causes the signal to be present within the bandwidth twice the time.

6.4 Accuracy of timing pulses and apparent pulsar period Another important aspect is the use of the correct apparent period of the pulsar for folding the

data and the use of the timing signal derived from a frequency or time standard to generate the pulses for sampling the data. In order to detect a pulse at low frequencies and using an antenna of small effective area, it will be necessary to observe and reduce the data over periods of hours, maintaining a precise and constant timing for the sampling pulses. The Figure 4 shows the results of a test done in the early stages of the project showing de effect in the height of the pulse if the data is folded with period longer or shorter that the actual observed period. An error of a few hundred microseconds in the period can wash out the pulse.

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Figure 4. Reduction of the height of the integrated pulses of a pulsar obtained by folding the data with period departing from the actual apparent observed period P in steps larger or smaller than the period P.

Some of the limitations for detecting pulsars at low frequencies are still valid but new development of instrumentation and data processing may make it easy to overcome some of the limitations. New and cheaper spectrograph allows the use of many narrow band channels that can improve the S/N ratio. Faster computer with large memory and faster A/D converters can digitize the output of a spectrograph, digitizing and dedispersing and averaging the pulses on line. Large memory allows storing large amounts of raw data for later processing off line.

A look to the parameters in the basic equation of the minimum detectable temperature provides an easy way to determine which parameters can be modified to improve the ∆Smin.

The minimum detectable temperature is given by the equation

=

where ∆Smin = minimum detectable flux density k = Boltzmann’s constant

Ae = effective aperture of the antenna Ks = sensitivity constant, order of unit Tsys = System noise temperature

∆f = predetection bandwidth t = post detection time constant n = number of records averaged

The parameter Tsys cannot be improved at a given frequency. Its value is determined by the

temperature of the galactic background around the pulsar. The contribution of the noise temperature of the receiver or preamplifier to Tsys is usually low at low frequencies and just a small fraction of the galactic background temperature.

The effective area of the antenna Ae can be increased by using a large antenna arrays. The predetection bandwidth of the receiver ∆f cannot be increased too much because of

problems with the dispersion of the pulses, unless some dedispersion is applied to each of the receiver channels. The post detection time constant cannot be made to long due to the short duration of the pulses and it has to be keep to a small fraction of the pulse width. The number of records averaged n can be increased by using the method of folding of the pulses and also by increasing the number of channels and averaging the dedispersed pulses. These three parameters ∆f, t and n can be increased in order to improve the ∆Smin but their contribution to the improvement (square root of their values) is slower than the contribution from Ae.

References

Bruck, Yu.M, et al. 1986, Sov. Astron. Lett. 12(6) Nov-Dec. Deshpande, A. A. and Radhakrisnan, 1994, J. Astrophys. Astr. 15, 329-341 Lyne, A. G. and Graham-Smith, F., 1990 “Pulsar Astronomy”, Cambridge University Press Manchester, R. N. and Taylor, J.H. 1977, “Pulsars”, W.H. Freeman and Co. Phillips, J. A., 1991, Dissertation, Cornell University, Pulsar Timing at Meter and Decameter Wavelengths. Phillips J.A and Wolszczan, A. 1989, Astrophys. J. Lett. 344, L69. Reyes, F., 1989, Dissertation, University of Florida, Observation of Pulsars at Low Radio Frequency. Reyes, F. Aparici, J., and Olmos, F. 1995, Astron. Astrophys, 301,182-186.

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Shitov, Yu. P., 1985, Sov. Astron. 29(1) Jan-Feb. Smith, F.G., “Pulsars”, 1977, Cambridge University Press. FRANCISCO J. REYES, Ph.D.

He is an Associate Scientist in the Dept. of Astronomy of Univ. of Florida. Director of the Univ. of Florida Teaching Observatories including the Rosemary Hill Observatory and the Campus Teaching Observatory and the Director of the Univ. of Florida Radio Observatory. He got his Ph. D. in Astronomy from Univ. of Florida in 1989 under the supervision of Prof. Thomas Carr and an Electrical Engineer degree from Univ. of Chile in 1977. His areas of expertise are low frequency radio astronomy, Jupiter decametric emission, low frequency pulsar emission and studies, radio astronomical instrumentation, computer control of telescopes and astronomical instrumentation. More recently he has been involved in the testing of Mid IR astronomical instrumentation and the observation of

transiting exoplanets. Member of the International Astronomical Union and the American Astronomical Society. Team member of the NASA Radio JOVE educational and public outreach project.

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MOOCs Offer a Wide Variety of Free, On‐Line Courses in Astronomy and Related Fields

Lee Scheppmann Abstract: Currently a number of free/low cost, high quality, undergraduate courses in Astronomy and related fields are being offered on-line. These courses originate from major universities around the world and provide an efficient means for gaining new knowledge and skills. The last few years has seen the emergence of mature, high quality, undergraduate, Massive, Open, On-Line Courses (MOOCs) coming from universities and organizations. While MIT has had its courses available on-line for years, what differentiates the new MOOCs from the pioneering MIT videos is the sophisticated level of user/student/instructor integration. I’ll get more into that later. Wikipedia [http://en.wikipedia.org/wiki/Massive_open_online_course] has an excellent article on the topic of MOOCs and goes into great detail. It’s interesting reading, but not essential to taking advantage of, what for me, has been an amazing learning experience. Several organizations and companies currently offer free courses and you can check out their offerings at their websites. Here are some of the more prominent providers: Coursera www.coursera.org edX https://www.edx.org/ Udacity https://www.udacity.com/nanodegrees As of late December 2014, Coursera is offering Galaxies and Cosmology from Caltech, Astrobiology and the Search for Extraterrestrial Life from the University of Edinburgh, Astronomy: Exploring Time and Space from University of Arizona, Origins – Formation of the Universe, Solar System, Earth and Life from University of Copenhagen, Science of the Solar System from Caltech, Introduction to Astronomy from Duke University, Imaging Other Earths from Princeton University, Analyzing the Universe from Rutgers University. You will have to check the schedule for dates and availability. edX courses include Alien Worlds: The Science of Exoplanet Discovery and Characterization, Relativity and Astrophysics, Super-Earths and Life, The Evolving Universe, Cosmology, and The Violent Universe. Udacity courses seem to focus primarily on Computer Science. What can you expect when taking these courses? My experience is a bit dated, being two years old, but I can only imagine that most of it still applies. Each course should have an introductory video where you meet the professor while they explain the course and set your expectations. My courses, (which included image processing, math, and Gamification), all involved several hours of lecture per week, regular homework, exercises, examinations, and some pop quizzes! I was spending 6-8 hours a week per class. An undergraduate course from a top university is not going to be a walk in the park, no matter how benign the title. I found that out quickly. This is where the integration comes in. My classes all had very active, on-line discussion groups moderated by Teaching Assistants. If my question was not answered by the group, the TA would jump in and provide one within 24 hours or so. The user interface, i.e. what I saw on the screen varied from course to course. Some were better than others. The Image processing course was impressive. The instructor, as a talking head, was in a box on the right of my screen and his white board was on the left. He could write code, run it, and I could see the results in the sample image. Then the whiteboard would be available to me, (remember, the lecture was pre-recorded), and I could write code, run it, and see the results. This all happened during the lecture portion. After the lecture, during homework, I could go back to a whiteboard to write and run code. It was a great experience. While this format worked well for a software course, other topics would require something different.

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At the end of the course there is a final exam, and if you meet all the course criteria, you are awarded a certificate. These days, there may be a nominal cost for the certificate. If you are not interested in officially completing the course, you can simply listen to the lectures and basically audit with no pressure and no cost. In addition to the Astronomy courses that I’ve listed, there is a deep offering of related Math, Science and Computer courses available which could enhance the study of Radio Astronomy. See you in class! Postscript: Different from the MOOCs that I’ve discussed, Khan Academy https://www.khanacademy.org/ is a very useful learning site and tool for brushing up on math skills that may be rusty.

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Epoch of Reionization Judd D. Bowman Assistant Professor School of Earth and Space Exploration (SESE) Arizona State University (ASU)

Despite the amazing progress of observational astrophysics and cosmology over the last decade, a large gap remains in the history of the Universe for which there are currently no direct probes. The period between 370,000 and 1 billion years after the Big Bang (1), referred to as the cosmological Dark Ages, is completely invisible to contemporary observations. Information from this period is not explicitly contained in the cosmic microwave background (CMB) (2) because baryonic matter and radiation have already decoupled and CMB photons stream freely through the intergalactic medium (IGM) during this epoch. Yet, there has been insufficient time for significant numbers of stars, galaxies, and quasars to form and produce emission that could be detected with the telescopes available today. Not coincidentally (and almost by definition), the existing capabilities to detect high-redshift galaxies and quasars reach only to the very end of the Dark Ages.

Probing this epoch is at the forefront of modern astrophysics and cosmology. It is a daunting challenge to study the sources of the first light, but several options do exist. According to theory, the first generation of stars could form as early as 100 to 200 million years after the Big Bang. These stars will be extremely massive, of order 100 solar masses, and therefore very short-lived. In principle, their deaths should produce prodigious numbers of gamma-ray bursts that may eventually be detected. Additionally, extremely over-dense regions in the Universe will collapse particularly early due to gravitational instability, and thus rare, but massive galaxies and quasars may exist, even during early times. The James Webb Space Telescope (JWST) (3), scheduled for launch in 2013, and next-generation 30 m ground-based telescopes (4) should be able to detect these objects out to very high redshifts (z < 15). Finally, there is a third major potential probe of the Dark Ages. The bulk of the baryonic matter in the Universe during this period is in the form of neutral hydrogen gas in the IGM. Rather than target observations at the galaxies and quasars that are the rare, early products of gravitational collapse, it should be possible to detect directly the presence of the ubiquitous hydrogen gas. The most promising method of achieving this detection is to search for signatures of the (highly redshifted) 21 cm hyperfine transition line of neutral hydrogen in the radio spectrum. These observations will be one of the key science goals of the Square Kilometer Array (SKA) (5) when it is built around 2020, but precursor experiments should be able to make significant progress in the near future.

The transition period at the end of the Dark Ages is known as the epoch of reionization (EOR). During this epoch, radiation from the very first luminous sources--early stars, galaxies, and quasars--succeeded in ionizing the neutral hydrogen gas that had filled the Universe since the recombination event that occurred as the Universe cooled following the Big Bang. Reionization marks a significant shift in the evolution of the Universe. For the first time, gravitationally-collapsed objects exerted substantial feedback on their environments through electromagnetic radiation, initiating processes that have dominated the evolution of the visible baryonic Universe ever since. The epoch of reionization, therefore, can be considered a dividing line, of sorts, when the relatively simple evolution of the early Universe gave way to more complicated and more interconnected processes. This is the period that

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will be probed most thoroughly by the next generation of optical, infrared, and radio observatories.

Redshifted 21 cm Background

New experiments are aimed at detecting redshifted 21 cm signatures against the CMB. This signal from the Dark Ages appears as a faint, diffuse background in radio frequencies below 200 MHz (for redshifts above z = 6). Measuring the brightness temperature of the redshifted 21 cm background could yield information about both the global and local properties of the IGM. Determining the average brightness temperature over a large solid angle as a function of redshift would eliminate any dependence on local density perturbations and constrain the evolution of the product xHI (1 - TCMB/TS), where xHI is the global neutral fraction of hydrogen, TCMB is the CMB temperature, and TS is the spin temperature of neutral hydrogen in the IGM. During the reionization epoch, it is, in general, a good approximation to assume that the spin temperature of neutral hydrogen is much greater than the CMB temperature (TS>> TCMB) and, therefore, that the brightness temperature is proportional directly to xHI . Global constraints on the brightness temperature of the redshifted 21 cm line during the EOR, therefore would directly constrain the neutral fraction of hydrogen in the IGM. This would yield significant improvements in estimates of the optical depth to CMB photons and, thus, would help to break existing degeneracies in CMB measurements between the optical depth and properties of the primordial matter density power spectrum [Tegmark et al., 2006]. They would also provide a basic foundation for understanding the astrophysics of reionization by setting bounds on the duration of the epoch, as well as identifying unique features in the ionization history (for example if reionization occurred in two phases or all at once).

Results of a simulation illustrating fluctuations in the observable brightness temperature due to redshifted 21 cm emission near the end of the reionization epoch when the ionized fraction of gas is only xi=0.3. Data courtesy of A. Mesinger& S. Furlanetto, 2007.

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On small scales, local perturbations in the density, ionization fraction, or spin temperature may produce significant deviations from the typical, global averages of the observed brightness temperature. Characterizing these fluctuations would be a powerful approach to exploiting the information in the redshifted 21 cm background. As primordial hydrogen cools following recombination and later reheats, density contrasts in the baryonic matter distribution should be revealed as fluctuations in the brightness temperature of the redshifted 21 cm line [Sunyaev and Zeldovich, 1972, Hogan and Rees, 1979, Scott and Rees, 1990, Iliev et al., 2002, 2003, Loeb and Zaldarriaga, 2004, Barkana and Loeb, 2005b]. At high redshifts prior to reionization, fluctuations in the redshifted 21 cm background are expected to follow closely the matter density fluctuations -- at a time when baryon perturbations were still substantially in the linear regime and should contain information regarding the fundamental cosmological model. Redshifted 21 cm observations may help constrain the geometry of the high redshift universe between recombination and reionization [Barkana and Loeb, 2005c]. In particular, at very small spatial scales, where neither CMB anisotropy measurements nor large-scale structure surveys are able to directly probe the matter power spectrum, redshifted 21 cm measurements may dramatically improve constraints on alternatives to the standard inflationary model. Loeb and Zaldarriaga [2004] calculate that the number of independent modes accessible through redshifted 21 cm measurements of the matter power spectrum is up to nine orders of magnitude greater than for CMB measurements.

During the reionization epoch (z < 15), a unique pattern will be imprinted in the redshifted 21 cm signal that reflects the processes responsible for the ionizing photons and that evolves with redshift as reionization progresses. As the first luminous sources ionize their surroundings, voids are expected to appear in the fluctuating emission [Madau et al., 1997, Tozzi et al., 2000, Ciardi and Madau, 2003, Zaldarriaga et al., 2004, Furlanetto et al., 2004b]. In principle, these features may be studied through direct imaging or through the determination of the power spectrum (and higher-order statistics) of the spatial fluctuations. The fluctuations may be probed along a single line-of-sight (resulting in spectral features that are similar to the Lya forest and that are dubbed the 21 cm forest), angularly in the plane of the sky (yielding maps of the fluctuations like those produced by WMAP for the CMB), or three-dimensionally in an observed volume of space. The last method gives rise to 21 cm tomography.

The properties of the three-dimensional power spectrum of the spatial fluctuations in the redshifted 21 cm brightness temperature are expected to be dominated by the characteristics of the reionized voids in the background emission due to the first luminous objects [Zaldarriaga et al., 2004, Furlanetto et al., 2004a]. Measurements of the power spectrum from this period would provide insight into many of the poorly understood processes responsible for reionization and structure formation, such as the radiative feedback mechanisms in star-forming regions, the physics of the first (Population III) stars, and the role of quasars. Tracing the power spectrum as a function of redshift during this epoch will chart the history of the formation of structures. Directly imaging voids in redshifted 21 cm brightness temperature from individual HII regions surrounding quasars during this epoch would probe quasar physics [Wyithe and Loeb, 2004a,c, Kohler et al., 2005] and could provide guides in searches for high-redshift galaxies [Wyithe et al., 2005]. Measurements of the redshifted 21 cm background during the EOR may also be useful for constraining cosmological models. Ali et al. [2005] and Barkana and Loeb [2005a] have shown that differences in the line-of-sight versus angular components of the observed redshifted 21 cm power spectrum can be used to separate primordial density perturbations from features caused by the radiative processes responsible for reionization, and Barkana [2006] has discussed the application of the Alcock-Paczynski (AP) test [Alcock and Paczynski, 1979] to

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redshifted 21 cm measurements. Additionally, Barkana and Loeb [2005b] consider the effects of the earliest galaxies on the redshifted 21 cm fluctuations and Naoz and Barkana [2005] discuss using redshifted 21 cm observations to study the thermal history of hydrogen gas by detecting a small-scale cutoff in the power spectrum due to thermal broadening of the hyperfine line.

Experimental Approaches

There are two broad approaches to using the redshifted 21 cm signal to probe the Dark Ages. The first approach is to constrain the global evolution of the average redshifted 21 cm differential brightness temperature with redshift, and the second is to characterize the local fluctuations in the background. Both types of observations would provide important information about the Dark Ages and the epoch of reionization. In principle, the easier approach to observing the redshifted 21 cm background is to chart the evolution of the global differential brightness temperature with redshift. Since the goal in this case is to average over a large solid angle at multiple frequencies, global signature experiments do not need necessarily to image the sky. Furthermore, since the redshifted 21 cm signal is visible in all directions, the signal will fill the primary beam of any antenna. This provides a significant simplification and means that there is no loss in sensitivity by increasing the field of view (as would be typical if one were observing a point source). Thus, global signature experiments are able, in principle, to use very simple antennas, such as individual dipoles.

Several small experiments are underway that are designed to detect distinct features in the global redshifted 21 cm background [Shaver et al., 1999, Gnedin and Shaver, 2004, Furlanetto, 2006], such as a sharp step transition in the all-sky spectrum that would be present if reionization occurred very rapidly. Amazingly, these modest experiments could have been performed easily (for the most part) anytime in the past few decades, but were not conceived until significant attention was turned to understanding the reionization epoch. Two primary efforts in this category are the Compact Reionization Experiment (CORE) led by Dr. Ron Ekers at the Australian Telescope National Facility, and our Experiment to Detect the Global EOR Signature (EDGES) (6), a collaboration with Dr. Alan Rogers at the MIT Haystack Observatory.

First deployment of the EDGES system at Mileura Station in western Australia in December, 2006. The antenna and

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groundscreen are seen in the foreground, while the receiver and analog-to-digital conversion unit is visible in the background. The two components are connected by a 50 meter transmission line and several control cables. Alan Rogers stands ready to adjust the configuration.

The second approach discussed above to observing the redshifted 21 cm background is to characterize the fluctuations in the signal. Unlike the global signature efforts, experiments using this approach are required to provide information about the sky on small angular scales. The ideal outcome of observations for this approach would be true maps of the redshifted 21 cm background. However, due to the extremely intense synchrotron radiation from our own galaxy, directly imaging the fluctuations and voids in the redshifted 21 cm background with the desired arc-minute or better resolution will require the sensitivity of the planned Square Kilometer Array [Furlanetto and Briggs, 2004], and thus will not be feasible until at least 2020. Statistical observations of the fluctuation power spectrum, on the other hand, should be obtainable with much smaller radio telescope arrays since statistical measurements allow a greater degree of information compression, thus increasing the effective signal to noise ratio in the measurements. Characterizing the power spectrum and its evolution would provide a wealth of information about structure formation and the fundamental astrophysics behind reionization. In large part because of this promise of opening the Dark Ages to scrutiny, as well as because of new enabling technologies, several radio-frequency experiments are underway that hope to detect the redshifted 21 cm background produced by neutral hydrogen above z = 6 and constrain its statistical properties.

Referenced Links

(1) http://map.gsfc.nasa.gov/m_uni/uni_101bb1.html

(2) http://map.gsfc.nasa.gov/m_uni/uni_101bbtest3.html

(3) http://www.jwst.nasa.gov/

(4) http://www.gsmt.noao.edu/

(5) http://www.skatelescope.org/

(6) http://www.haystack.mit.edu/ast/arrays/Edges

Reprinted from the 2011 Western Conference Proceedings held at Embry‐Riddle in Prescott, Arizona

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Construction of a Partial Sky Map at 50 MHz John G. Younger North Michael Road Radio Observatory Ann Arbor, MI INTRODUCTION: The 6m band is a commonly used wavelength range in amateur radio, and a large selection of equipment, including receivers and antennas, is commercially available for this application. From the perspective of amateur astronomy, 50 MHz is on the low end of frequencies at which large scale astronomical structures can be practically observed from a single location, in part because of the antenna beam geometry of practical antennas. In this report, I describe the design and use of a 50 MHz system to map the galactic plane in a large region of earth’s northern hemisphere. The report begins with a description of the design and assembly of the antenna. Subsequently, methods for acquiring and summarizing data are discussed, followed by a brief presentation of some recently made observations. TELESCOPE DESIGN AND ASSEMBLY: The site. The North Michael Radio Observatory is located in a housing subdivision on the western edge of Ann Arbor, Michigan (42o15.9’ N, 83o48.4’ W). There is extensive tree cover. The installation site for the 50 MHz antenna is a north‐south oriented patch that permits a boom length of no more than 30 feet. Additionally, a need to easily service the antenna and mast, local covenants, and overhead trees limit mast height to 20 feet. Lastly, the available space, while allowing the use of an elevation rotor, does not permit azimuthal directional motion – the antenna is confined to operating in a north‐south oriented vertical plane. The distance from the antenna assembly to the receiver shack is 120 feet. Antenna selection and simulation. The initial step in building the system was selecting a reasonable antenna. Given site constraints, a 7‐element Yagi antenna (6m7JHV) was selected from M2 Antenna Systems (M2inc.com). When positioned horizontally, it had a manufacturer‐rated gain of 10.9 dB, a front‐to‐back directionality of 25 dB, and a horizontal beam width of 40o. Once the antenna had been selected, a series of simulations were performed using EZNEC antenna modeling software. Because the antenna ultimately would be used for drift scanning at varying antenna elevations, simulations were used to evaluate two important design issues. First, would antenna performance suffer from placing the antenna on such a short mast? Second, what would the impact on beam geometry be of elevating the antenna off of a horizontal position? EZNEC software was used for all simulations. To study beam geometry at different antenna orientations, a short software routine was written to read the coordinates of the 7 antenna elements, connecting wires, etc., from an EZNEC antenna file, rotate and translate those elements arbitrarily in space, then re‐save them in the antenna file. For each antenna orientation, EZNEC was used to estimate the beam pattern as well as the VSWR. Example results are shown in Figure 1. Importantly for the current work, neither the horizontal width of the beam pattern nor the VSWR were significantly impacted by tilting the antenna off of its horizontal position. Somewhat surprisingly, this included the elevation limit of the final design (50o), wherein the rear‐most reflecting element was less than 12 inches off of the ground. While the horizontal beam geometry was unaffected by antenna elevation, the vertical beam suffered significantly. The manufacturer‐rated vertical resolution of the antenna was only achieved in the fully horizontal position; the vertical beam spread with increasing antenna elevation. None of these observations were significantly impacted in simulation by the vertical height of the entire antenna – placing the antenna assembly on a 100m mast produced essentially the same beam effects.

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Figure 1. Antenna beam simulations for a 7‐element Yagi antenna on a vertical rotor. Simulations of the

antenna at a variety of elevation angles were performed using EZNEC based on the design geometry of the M2 Systems 6m7JHV antenna. The antenna’s best vertical resolution was achieved at 0 degrees elevation. The vertical extent of the beam increased with antenna angle of elevation, with the primary lobe becoming

increasingly spherical at higher elevations. The horizontal beam width (not shown) was mostly unaffected by elevation angle.

Construction. Details of the mast and antenna assembly are shown in Figure 2 and 3. A fork built from two 4x4 lengths of treated lumber was anchored into a concrete piling. Between these two posts the main mast, a 16 foot 4x4, could be pivoted into position. At the top of the mast, a Yaesu model 550 elevation rotor was secured for holding a 12 foot fiberglass cross boom. On one end of the boom, the 6m7JHV antenna was attached, facing directly south. The other end was counter weighted to balance the mass of the antenna. Two cables exited the assembly from above the antenna – Belkin 9913F low‐loss coaxial cable from the antenna and the 6‐conductor rotor control cable. Both were secured to their attachments on the antenna assembly with careful attention to waterproofing. These lines were carried overhead to the shack.

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An ICOM R75 receiver was used for all measurements. The receiver and all other instrumentation was fed with an Astron power supply. The receiver output was interfaced to the audio input of a standard sound card on an Intel‐based Dell personal computer running Windows 2007. An RF2050 S calibrated noise source was interposed between the antenna feed line and the receiver. DATA COLLECTION AND ANALYSIS Observations. Measurements were made during the early spring of 2011. Recordings were taken between 49.95 and 49.98 MHz and varied daily to minimize local radio interference. The lower side band was used, and the devices automatic gain control was disabled. Analog to digital conversion was made at a 1 Hz sampling rate using the built in sound card and associated clock of a standard Dell personal computer running Radio SkyPipe II data acquisition software. Clock calibrations to the atomic clock at Fort Collins were made before each set of observations. Noise output from the receiver was converted to temperature in Kelvin using an RF 2050 S calibrated noise source. The device, originally intended as a noise calibrator for the 20 MHz Radio Jove receiver, was specifically recalibrated for use at 50 MHz and spanned 27,000 to 880,000 Kelvin. To develop a noise standard curve in the range of sky measurements encountered at 50 MHz, the source was used in conjunction with 3‐, 6‐, and 9‐dB attenuators. Raw data from each observation were typically extensive (86,400 data points per 24 hour session) and were saved, along with corresponding right ascension coordinates, as text files for later assembly into temperature maps.

Figure 2. Schematic of the antenna assembly.

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Figure 3. Configuration of the mast, rotor, cross‐boom, and antenna. The mast was a 16‐foot length of pressure treated 4x4 lumber anchored to a concrete piling via a hinged fork. The 6m7JHV antenna was joined to a Yaesu 550 elevation rotor via a 12’ fiberglass counter‐weighted cross boom. The feed line and rotor control cables

exited the assembly from above and were taken overhead back to the shack. An example tracing is shown in Figure 4. Some terrestrial interference was encountered on every day of measurement. For the most part, this interference was significantly less than the galactic signal.

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Map Assembly. To construct the 50 MHz map, serial drift scan observations were taken at 9 equally spaced antenna elevations ranging from 0 to 50o, such that 777, 600 ‘pixels’ (9 elevations x 86,400 seconds/24hr) were observed. Twenty‐two days of measurements were taken altogether and therefore, for any pixel, between 2 and 5 observations were available. For each pixel, all available measurements were averaged. These were then displayed as a temperature map using the contour() function in the statistical programming language R (http://cran.r‐project.org/). No data smoothing beyond averaging over multiple observations was employed. The final map is shown in Figure 5. Two prominent features are noted. First is the band of the galactic plane, located between 15 and 20 hr RA in this region of the sky. There is an evident tilt associated with the band, as should be the case. The second significant feature is at approximately 18 hr RA, ‐26o declination, the location of Sagittarius A*, the galactic center. Also evident in the plot is the significant amount of persistent noise in the data set which, although containing several million observed points, is still susceptible to terrestrial noise in any given drift scan. Future Directions. The telescope as built has performed reliably over the winter and spring. Going forward, there are two areas of investigation under consideration. The first is computational, and involves improving the quality (and accuracy) of the sky map by correcting for the change in beam geometry over the range of antenna elevations used. This is not a trivial mathematics problem, and involves deconvoluting the raw sky map with EZNEC beam geometry simulations. The second potential application is converting the antenna into a 50 MHz polarimeter. This would involve placing a second, matching antenna on the currently unoccupied end of the rig’s cross boom and placing the two antennas in 90o cross polarization. Adding a second receiver would allow mapping both sky temperature and the angle of incoming radio polarization. The polarization angle contains additional information that permits separating terrestrial from extraterrestrial signal and would potentially provide a significantly improved signal:noise ratio of the system. Acknowledgments. I’m deeply indebted to Richard Flagg for his assistance and enthusiasm in essentially every step of this project.

Figure 4. Example 24‐hour tracing of sky temperature at approximately ‐26o declination. The primary peak corresponds to the galactic plane and, more specifically at this declination, Sagittarius A* and the galactic center.

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Figure 5. Temperature map of the sky available to the antenna. The map consists of > 700,000 pixels of drift‐scan measurements averaged over 22 days of observations. Temperatures vary from 6,000 K (the minimum detectable temperature of the instrument) to 10,000 K. Off‐scale high measurements are depicted in white, and areas for which no observations were captured are shown in black. The region at 18 hr RA, ‐26o Dec corresponds to Sagittarius A and the galactic center. The hot vertical band corresponds to the galactic plane.

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Book Review Title: Cosmic Discovery Author: M. Harwit Publisher: The MIT Press ISBN:0‐262‐58068‐3 Date published: 1984 Length: 334 pages, 17 page index Status: Out of print Availability: Hardbound and paperbound copies may be obtained from used book sellers for a couple bucks (or less) Reviewer: Whitham D. Reeve Cosmic Discovery is the first book I have read that systematically describes the mechanics of scientific discovery. But that is only a small part of the story. It also describes the many important accidental discoveries that resulted from work unrelated to the discovery itself. This book makes it clear that the majority of important astronomical discoveries were made by outsiders who were neither trained as astronomers nor looking for astronomical phenomena. It also makes clear that advances in astrophysics did not come about as a result of theoretical insight and that further advances do not require the building of bigger and bigger optical telescopes (as is commonly claimed by many astrophysicists). The first and second radio astronomers ever, Karl Jansky and Grote Reber, were not astronomers or even trained in astronomy – they were radio men. Cosmic microwave background radiation was accidentally discovered by terrestrial radio communications researchers trying to find excess noise in an antenna system and they did it before a group that was purposefully looking for it. There are many more examples given in the book, and not all of them are discoveries made through radio astronomy. Indeed, before World War II there was only one radio astronomy discovery – the “Electrical Disturbances Apparently of Extraterrestrial Origin” first detected by Karl Jansky in 1931 [Jansky]. Cosmology is the study of the origin and evolution of the universe. At the time this book was written and continuing through today, most progress in cosmology resulted from the use of improved technology. With few exceptions, cosmological theories are driven by surprises in measurements and most of these are made with new equipment. At first, all the new equipment came from systems that were developed during World War II and then surplused when it ended. Later, new equipment was purpose‐built and most recently much of it has been specifically designed for and flown on spacecraft. The technology used in research has changed and improved immensely since Cosmic Discovery was written and presumably a lot more is understood about the universe but many questions remain unanswered. So, no, we are not there yet. Cosmic Discovery gives both the big picture and many of the little pictures. The book has only five chapters but each is comprehensive: 1 – The Search; 2 – Discoveries; 3 – Observation; 4 – Detection, Recognition and Classification of Cosmic Phenomena; and 5 – The Fringe of Legitimacy – The Need for Enlightened Planning. It also has two appendices – The Number of Undetected Species and Information, Capacity, and Information Rates. At the end, just before the Index, are 12 pages of references listed by preface, chapter and appendix. The index is quite unique in that it actually is a glossary + index in which many words and phrases point to a page and also are defined. I found myself reading the index.

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This book is not an artificial account written for the casual reader; it is serious and well researched, but it is easy to read and well‐illustrated with charts and images (all black‐white), and it certainly is well within reach of the casual reader. Chapter 2 – Discoveries is about 100 pages long and is especially enlightening and interesting. There are 43 sections in chapter 2, one for each type of major discovery from 1798 up to about 1984 including who, what, when, where, how and why. It is interesting to note that none of these discoveries followed the same or even similar path and there almost always was a major detachment in time between the discovery and the theory explaining it. The discoveries are summarized in tabular form in Chapter 5 – The Fringes of Legitimacy. Chapter 5 also tries to teach us that we have much to learn from past successes and failures. It touches on the military and commercial influences as well as the contributions by non‐astronomers, all of which are important ingredients to discovery. Without them we would not even be close to where we are now – in contrast to the more recent but revisionist A Single Sky written by Munns and reviewed by me in the January/February 2014 issue of Radio Astronomy [Reeve]. The author likes to list things out including seven common traits in the major discoveries, and these are worth repeating here:

1. The most important observational discoveries result from substantial technological innovation in observational astronomy;

2. Once a powerful new technique is applied in astronomy, the most profound discoveries follow with little delay;

3. A novel instrument soon exhausts its capacity for discovery; 4. New cosmic phenomena frequently are discovered by physicists and engineers or by other researchers

originally trained outside astronomy; 5. Many of the discoveries of new phenomena involved use of equipment originally designed for military

use; 6. The instruments used in a the discovery of new phenomena often have been constructed by the

observer and used exclusively by him; 7. Observational discoveries of new phenomena frequently occur by chance – they combine a measure of

luck with the will to pursue and understand an unexpected finding. In the early days of scientific discovery, just about everything was learned through visible light – the light was the carrier by which the information was transported across the universe. Cosmic Discovery tells us that in 1984, there were five known carriers, or channels, of information through which we learn about the universe: Electromagnetic waves (gamma‐ and x‐rays through radio); cosmic ray particles (highly energetic electrons, protons and heavier nuclei); solid bodies (meteors and meteorites); neutrinos and antineutrinos; and gravitational waves. There is no mention in this book of so‐called dark energy or dark matter, which I suppose also could be called channels of information by their absence of direct detection and measurement. We are told in Cosmic Discovery that in the United States all astronomy research takes place at national centers. This should be no surprise; however, up to the time the book was written, not a single one of the 43 major cosmic discoveries mentioned above were originally at a national center. But apparently they are trying. All research in the United States (and the world) is government funded one way or another and scientific goals, funding agency and the scientist’s motivations are not always aligned. The author spends a fair amount of time offering his opinions on how future research should be conducted and, therefore, how taxpayer’s money should be spent. Everyone has their own ideas about this, and the author has some good ones. On the other hand, it is human nature to use lofty goals as justification for spending other people’s money even though it is a paycheck that really matters. Many of the large radio astronomy projects being undertaken as I write this review grew from seeds planted about the time Cosmic Discovery was written 30 years ago.

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Much contemporary research is centered on confirmation of the various theories of cosmology (for example, the Big Bang theory) and Einstein’s theory of relativity. In a way, each confirmation is in itself a new discovery. Except in these cases, the theory came first. Many early discoveries came first and theory came second so it is easy to conclude there is no single best method, process or path. What characterizes an astronomical observation? Cosmic Discovery provides a laundry list with seven items:

1. Type of carrier (or information channel) 2. Wavelength or energy of the carrier 3. Angular resolution of the observing instrument 4. Spectral resolution of the observing instrument 5. Time resolution of the observing instrument 6. Polarization, if any 7. Time and date and direction observed

When we, as amateur radio astronomers, undertake our own observations, we most likely are limited by our apparatus. The angular, spectral and time resolution of our observations may be extremely broad. We may be using only one linearly polarized antenna with low directivity and, thus, cannot derive any polarization or direction information at all. Also, it is well known that data time and date stamping is problematic in amateur radio astronomy. After collecting the data, all we know is the type of information channel (radio), wavelength (frequency) and relative intensity. But serious amateurs do not simply throw up their hands and complain to congress, which would be a total waste of time anyway. Instead, they start at the top of the laundry list and work their way down, improving each line item as best they can. Thus, Cosmic Discovery not only entertains and enlightens us but also helps us get our own non‐government funded observatories in order. In conclusion, this is a very good book and it will not break the bank. I first heard about it from the director of the NRAO Operations Center in Socorro, New Mexico during a talk he gave to a group of us at the Radio Astronomy at the Frontiers of Astrophysics short course in 2011. He said the book served as inspiration for him to pursue the path he followed into radio astronomy. I took that as a good recommendation and after reading the book agree that Cosmic Discovery is inspiring. Readers of this review might be interested that Physics Today magazine interviewed the author; see the October 2014 issue: http://scitation.aip.org/content/aip/magazine/physicstoday/news/10.1063/PT.5.3015?utm_medium=email&ut m_source=Physics+Today&utm_campaign=4838684_Physics+Today%3a+The+week+in+Physics+610+October&dm_i=1Y69,2VPJW,HPI212,AFJ1Q,1 [Jansky] Jansky, K., Electrical Disturbances Apparently of Extraterrestrial Origin, Proceedings of the Institute of Radio Engineers, Vol. 21, No. 10, pg 1387–1398, Oct. 1933 [Reeve] Reeve, W. Book Review of A Single Sky: How an International Community Forged the Science of Radio Astronomy, Society of Radio Amateur Astronomers, Radio Astronomy, January‐February 2014

Reviewer ‐ Whitham Reeve has been a director of SARA and presently is a contributing editor for the SARA journal, Radio Astronomy. He worked as an engineer and engineering firm owner/operator in the airline and telecommunications industries for more than 40 years and has lived in Anchorage, Alaska his entire life.

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Space Place Partner’s Article: What Is a Satellite Galaxy? Our sun is part of a massive collection of stars in the Milky Way galaxy. These hundreds of billions of stars orbit the galaxy’s center. But did you know that there are things that are even bigger orbiting the Milky Way’s center? Other galaxies orbit it too!

The Andromeda Galaxy with two satellite galaxies surrounding it. Original image credit: Boris Štromar. These less massive galaxies have their own impressive collection of stars, which all orbit their own center; but the galaxies and everything in them orbit our galaxy too. It’s as if our galaxy is the sun and those other galaxies are planets. Astronomers call them “satellite galaxies.” Where Are They and What Are They Like?The Milky Way has a number of satellite galaxies, but the biggest one is theLarge Magellanic Cloud. It is about 163,000 light‐years away and around 1/100th the size of the Milky Way. Unlike our spiral galaxy, this one lacks a clean spiral shape. Some scientists think that is because the Milky Way and other galaxies are pulling and warping it.

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In terms of distance, there are two contenders for closest satellite galaxy. One group of stars is small enough that astronomers consider it a “dwarf galaxy.” The other group is so close that they still debate whether or not it is part of our galaxy or its own dwarf galaxy. Astronomers have named the one that everyone agrees on the SagittariusDwarf Spheroidal Galaxy. It’s about 50,000 light‐years away from the Milky Way center. It orbits over the top and down below the disk of our galaxy, like a ring over a spinning top. But there is something even closer to our Milky Way—a cluster of stars named by some to be the Canis Major Dwarf Galaxy. Scientists estimate that it contains around a billion stars. It is so close to the edge of the Milky Way that it is closer to our solar system than to our galaxy’s center. It’s about 25,000 light‐years away from us. Where Does One Galaxy Start and the Other End? Some scientists don’t think the Canis Major cluster of stars is actually its own galaxy or dwarf galaxy. Instead they think it is just a dense area of faraway stars that are still part of the Milky Way. Either way, it is clear that this bunch of stars has been pulled very close to our Milky Way by our galaxy’s massive gravity. Over time, this could be the fate of other satellite galaxies in the area. They could all one day merge into an even larger Milky Way galaxy!

The Large Magellanic Cloud

If you liked this, you may like:

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Dr. Marc: http://spaceplace.nasa.gov/dr‐marc‐space/en/ What’s in space? http://spaceplace.nasa.gov/story‐whats‐in‐space/en/ Podcasts: http://spaceplace.nasa.gov/podcasts/en/

Images credit: NASA's Galaxy Evolution Explorer (GALEX) spacecraft, of Mira and its tail in UV light (top); Margarita Karovska (Harvard‐Smithsonian CfA) / NASA's Hubble Space Telescope image of Mira, with the distortions revealing the presence of a binary companion (lower left); public domain image of Orion, the Pleiades and Mira (near maximum brightness) by Brocken Inaglory of Wikimedia Commons under CC‐BY‐SA‐3.0 (lower right).

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Membership New Members Please welcome our new or returning SARA members who have joined since the last journal. If your name is missing or misspelled, please send an email to treasurer@radio‐astronomy.org. We will make sure it appears correctly in the next Journal issue. As of December 20, 2014:

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Johnny Bates Birmingham AL USA Grahame Booth Calgary Alberta Canada Wes Bunker Gig Harbor WA USA Devin Cody New Haven CT USA Richard Glassner Jefferson City MO USA N0EAX Ruben Gomez Dearborn MI USA Mary Gomez Dearborn MI USA KD8HLK Devoyon Guillaume Canejan France F8ARR Calixto Herrera Bogota Colombia Dean Knight Glen Ellen CA USA KI6FFM Ronnie P. Milione Huntington Station NY USA Chris Munson Mountain View CA USA KG6MOZ Mia M. Nasimullah Gainesville FL USA Edwin S. Olson Grand Forks ND USA John Otte Riverbank CA USA K6JRO Maria Parales Andrews AFB MD USA

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What is Radio Astronomy? This link is for a booklet explaining the basics of radio astronomy. http://www.radio‐astronomy.org/pdf/sara‐beginner‐booklet.pdf

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Administrative Officers, directors, and additional SARA contacts The Society of Amateur Radio Astronomers is an all‐volunteer organization. The best way to reach people on this page is by email with SARA in the subject line SARA Officers President: Ken Redcap, president@radio‐astronomy.org, +1 248‐630‐6810 Vice President: Tom Hagen, vicepres@radio‐astronomy.org, +1 248‐650‐8951 Secretary: Bruce Randall, secretary@radio‐astronomy.org, +1 803‐327‐3325 Treasurer: Melinda Lord, treasurer@radio‐astronomy.org, +1 319‐591‐1130 Past President: William Lord, tbd@radio‐astronomy.org, +1 319‐591‐1131 Founder Emeritus & Director: Jeffrey M. Lichtman, Jeff@radioastronomysupplies.com, +1 954‐554‐3739 Board of Directors Name Term expires Email Jim Brown 2015 starmanjb@comcast.net Chip Sufitchi 2015 ciprian@sufitchi.com Carl Lyster 2016 ctlyster@bellsouth.net Stephen Tzikas 2016 Tzikas@alum.rpi.edu David James 2016 dave@greenover.net Curt Kinghorn 2015 curtkinghorn@gmail.com Keith Payea 2016 kbpayea@bryantlabs.net Stan Nelson 2015 stannelson@cableone.net Other SARA Contacts All Officers ‐‐‐‐ officers@radio‐astronomy.org Annual Meeting Coordinator Vice President vicepres@radio‐astronomy.org All Radio Astronomy Editors ‐‐‐ editor@radio‐astronomy.org Radio Astronomy Editor Kathryn Hagen kathryn.hagen@gmail.com Radio Astronomy Contributing Editor Christian Monstein monstein@astro.phys.ethz.ch Radio Astronomy Contributing Editor Whitham D. Reeve whitreeve@gmail.com Radio Astronomy Contributing Editor Stan Nelson stannelson@cableone.net Educational Outreach Jon Wallace education@radio‐astronomy.org Grant Committee ‐‐‐‐ grants@radio‐astronomy.org International Ambassador Librarian Membership Chair Tom Crowley membership@radio‐astronomy.org Mentor Program Jon Wallace mentor@radio‐astronomy.org Navigators Tom Crowley tomcrowley@mindspring.com Technical Queries David Westman technical@radio‐astronomy.org Webmaster Ciprian (Chip) Sufitchi webmaster@radio‐astronomy.org

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Resources

Great Projects to Get Started in Radio Astronomy Radio Observing Program The Astronomical League (AL) is starting a radio astronomy observing program. If you observe one category, you get a Bronze certificate. Silver pin is two categories with one being personally built. Gold pin level is at least four categories. (Silver and Gold level require AL membership which many clubs have membership. For the bronze level, you need not be a member of AL.) Categories include 1) SID 2) Sun (aka IBT) 3) Jupiter (aka Radio Jove) 4) Meteor back‐scatter 5) Galactic radio sources This program is a collaboration between NRAO and AL. William F Bogardus is the Lead Coordinator and a SARA member. For more information: http://www.astroleague.org/programs/radio‐astronomy‐observing‐program

The Radio Jove Project monitors the storms of Jupiter, solar activity and the galactic background. The radio telescope can be purchased as a kit or you can order it assembled. They have a terrific user group you can join. http://radiojove.gsfc.nasa.gov/

The INSPIRE programuses build‐it‐yourself radio telescope kits to measure and record VLF emissions such as

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tweeks, whistlers, sferics, and chorus along with man‐made emissions. This is a very portable unit that can be easily transported to remote sites for observations. http://theinspireproject.org/default.asp?contentID=27 Sky Scan Awareness Project When a meteor passes through the Earth's atmosphere, it ionizes the atmosphere which improves its ability to reflect radio waves. This allows you to briefly hear a far away radio station that you normally couldn't detect. In this project, you can install an antenna, use an FM radio receiver, computer software, and learn to observe meteor showers using this very simple radio telescope. For more information about this project, please visit http://www.skyscan.ca/getting_started.htm . SARA/Stanford SuperSID

Stanford Solar Center and the Society of Amateur Radio Astronomers have teamed up to produce and distribute the SuperSID (Sudden Ionospheric Disturbance) monitor. The monitor utilizes a simple pre‐amp to magnify the VLF radio signals which are then fed into a high definition sound card. This design allows the user to monitor and record multiple frequencies simultaneously. The unit uses a compact 1 meter loop antenna that can be used indoors or outside. This is an ideal project for the radio astronomer that has limited space. To request a unit, send an e‐mail to supersid_at_radio‐astronomy_dot_org

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Education Links Open access to almost 1 million e‐prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics: http://arxiv.org/ The SAO/NASA Astrophysics Data System (ADS) is a Digital Library portal for researchers in Astronomy and Physics, operated by the Smithsonian Astrophysical Observatory (SAO) under a NASA grant: http://www.adsabs.harvard.edu/ The Impact of Coronagraphs: http://www.mmsend61.com/link.cfm?r=1021842096&sid=55555786&m=7251166&u=AGU_&j=22654060&s=http://onlinelibrary.wiley.com/doi/10.1002/2014EO410001/pdf STEREO Spacecraft in Trouble: http://stereo‐ssc.nascom.nasa.gov/solar_conjunction.shtml STEREO Spacecraft Enter New Phase of Operations with limited data output: http://www.nasa.gov/content/goddard/stereo‐entering‐new‐stage‐of‐operations/#.VEqMQMlwni8 Video of the massive 32 m dish removal from Radio Telescope RT32 near Ventspils, Latvia on 28 November 2014. See Christian Monstein’s article in the July‐August 2014 of Radio Astronomy for some background. Not only is the dish impressive but so is the crane that lifted it. The 2nd video is a shorter version of the 1st: http://venta.lv/2014/12/16/video‐rt32‐nocelsana‐irbene/ Mysterious Lightning 'Bolts From The Blue' Defy Physics And Make No Sense: http://www.businessinsider.com/rare‐mysterious‐bolt‐from‐blue‐lightning‐2014‐10 Lockheed Tracks Space Debris with One of the World's Largest Telescopes: http://www.pddnet.com/news/2014/11/lockheed‐tracks‐space‐debris‐one‐worlds‐largest‐telescopes?et_cid=4257235&et_rid=210447177&type=headline Effects of solar and geomagnetic activities on the sub‐ionospheric very low frequency transmitter signals received by the DEMETER micro‐satellite: http://www.annalsofgeophysics.eu/index.php/annals/article/view/5463 Building a Space Weather Radar in Antarctica: http://ciresblogs.colorado.edu/spaceweather/ Software defined radio receiver with up to 8 MHz IF bandwidth and covering the frequency range 0.1 to 380 and 430 to 2000 MHz. Driver support for Windows, Linux, Mac and Android: http://www.sdrplay.com/index.html Ever wonder about capacitor marking codes? Here is some help: http://www.radio‐electronics.com/info/data/capacitor/capacitor‐markings.php Electromagnetic Compatibility (EMC) Pocket Guide: http://www.rohde‐schwarz‐usa.com/EMC‐LIVE‐POCKET‐GUIDE.html Miniature USB spectrum analyzers by Triarchy Technologies: http://www.triarchytech.com/product.html

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Measuring Phase Noise with a Spectrum Analyzer: http://www.radio‐electronics.com/info/t_and_m/spectrum_analyser/measuring‐phase‐noise‐measurements.php Selecting PCB Materials for RF and Microwave Applications: http://mwrf.com/materials/selecting‐pcb‐materials‐rfmw Online RF Calculators and Conversions: http://www.pasternack.com/t‐rf‐microwave‐calculators‐and‐conversions.aspx RF Tools Apps for iPhone: http://www.hubersuhner.com/en/SpecialPages/App‐RF‐Tools Coaxial cable and connector technical resources: http://www.rfindustries.com/white‐papers.html AR RF/Microwave Instrumentation: Application Guide to RF Connectors and Cables: http://www.arworld.us/pdfs/appNotes/AppNote51.pdf Wi‐Fi Standards Poster: http://mwrf.com/test‐measurement‐analyzers/new‐wi‐fi‐standards‐poster Coaxial Connector Frequency Chart: http://mwrf.com/datasheet/coaxial‐connector‐frequencies‐pdf‐download Worldwide Frequency Allocation Chart: http://mwrf.com/test‐measurement/poster‐worldwide‐spectrum‐allocations Frequency Nomenclature Chart: http://mwrf.com/datasheet/frequency‐nomenclature‐pdf‐download Fundamentals of USB Audio: http://www.edn.com/design/consumer/4376143/Fundamentals‐of‐USB‐Audio

Online Resources British Astronomical Association – Radio Astronomy Group http://www.britastro.org/baa/

Radio Astronomy Supplies http://www.radioastronomysupplies.com

CALLISTO Receiver & e‐CALLISTO http://www.reeve.com/Solar/e‐CALLISTO/e‐callisto.htm CALLISTO data archive: http://e‐callisto.org

Radio Sky Publishing http://www.radiosky.com

Deep Space Exploration Society http://dses.org/index.shtml

RF Associates Richard Flagg, rf@hawaii.rr.com 1721‐I Young Street, Honolulu, HI 96826

European Radio Astronomy Club http://www.eracnet.org

RFSpace, Inc. http://www.rfspace.com

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GNU Radio http://www.gnu.org/licenses/gpl.html

Shirleys Bay Radio Astronomy Consortium marcus@propulsionpolymers.com

Inspire Project http://theinspireproject.org

Simple Aurora Monitor Magnetometer http://www.reeve.com/SAMDescription.htm

NASA Radio JOVE Project http://radiojove.gsfc.nasa.gov Archive: http://radiojove.org/archive.html

SETI League http://www.setileague.org SkyScan Science Awareness (Meteor Detection) http://www.skyscan.ca/getting_started.htm

National Radio Astronomy Observatory http://www.nrao.edu

Stanford Solar Center http://solar‐center.stanford.edu/SID/

NRAO Essential Radio Astronomy Course http://www.cv.nrao.edu/course/astr534/ERA.shtml

UK Radio Astronomy Association http://www.ukraa.com/www/

Pisgah Astronomical Research Institute http://www.pari.edu

SARA Facebook page https://www.facebook.com/pages/Society‐of‐Amateur‐Radio‐Astronomers/128085007262843

SARA Web Site http://radio‐astronomy.org

SARA Twitter feed https://twitter.com/RadioAstronomy1

SARA Email Forum and Discussion Group http://groups.google.com/group/sara‐list

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For Sale, Trade, and Wanted Sara Polo Shirts SARA has polo shirts with the new SARA logo embroidered. (No pocket) These are 50% cotton and 50% polyester, machine washable. Currently in stock:

Price is $15 with free shipping in the USA. Additional cost for shipping outside the USA. Other colors and sizes available, contact

SARA Treasurer, Melinda Lord, at treasurer@radio‐astronomy.org. There is no charge to place an ad in Radio Astronomy; but, you must be a current SARA member. Ads must be pertinent to radio astronomy and are subject to the editor’s approval and alteration for brevity. Please send your “For Sale,” “Trade,” or “Wanted” ads to editor@radio‐astronomy.org. Please include email and/or telephone contact information. Please keep your ad text to a reasonable length. Ads run for one bimonthly issue unless you request otherwise. For sale

Items listed below. Send request to SARA by email to supersid@radio‐astronomy.org. For more information:http://www.radio‐astronomy.org/pdf/sid‐brochure.pdf. Description, items for sale by SARA Price (US$) SuperSID VLF receiver (assembled) $48.00 PCI soundcard, 96 kHz sample rate $40.00 Antenna wire 24 AWG (120 m) $23.00 Coaxial cable, Belden RG58U (9 m) $14.00 Shipping (United States) $10.00 Shipping (Canada, Mexico) $25.00 Shipping (all other) $40.00

Size Color Small Navy, Royal Blue Medium Navy, Dark Green, Royal Blue Large Maroon, Black, Navy, Royal Blue X‐Large Maroon, Black, Navy, Royal Blue XX‐Large Maroon, Black, Navy, Dark Green, Royal Blue XXX‐Large Black, Navy, Dark Green, Royal Blue