Introduction to DSTV dish...

Introduction to DSTV dish observations

Aletha de Witt AVN/Newton-fund 2017

Observational & Technical Training HartRAO

Theory: Radio Telescope Antennas

• A “classic” radio telescope for use in the microwave band has a circular parabolic reflector with a feed horn at the focus to collect the incoming microwaves and pass them to transistor amplifiers in the receiver.

• A DSTV satellite dish also works in this way. It can be used as a mini radio telescope by replacing the DSTV decoder with a radiometer for measuring the signal strength.


• Facts about the DSTV dish.

- This is a 12 GHz radio telescope and is 50cm in diameter. - It can detect frequencies in the range of 11.7 to 12.2 GHz. - Satellite dishes have a smooth solid surface in order to reflect incoming waves with

high efficiency- It is not a radio telescope system that can be used for serious sky surveys. - It can detect the Sun. - It can detect blackbody radiation such as 300 K trees, buildings, people, when

viewed against blank sky. - It can be used to demonstrate the basic concepts of a radio telescope.- It can be used for student training and outreach .

* Radiometer: measures the strength of the radio signal coming from the receiver on the dish. Build using spares from the 26m. * Supplies the 15V DC needed by amplifier on the dish.

* Use a “square-law” detector => output voltage proportional to input voltage.

* Output voltage is displayed on a meter (arbitrary scale). * Output voltage is fed to a loudspeaker (audio output).

* Hiss of varying levels of intensity => hiss is “white noise” radio equivalent of white light we see with our eyes.


Capacitor to block DC power to LNB

Attenuatorfor signal level adjustment

1000-1500 MHz amplifier

Diode detector Vout proportional Pin

Low-pass filter

Op-amp

Loudspeaker Voltmeter

Radiometer components:Power for LNB15V

Microwave feed horn at focus of dish

Radio frequency (RF) Low-Noise Amplifier (LNA) for 11700 - 12200 MHz

MixerIntermediate frequency (IF) outputIF = RF=LO

Local Oscillator

10700 MHz

Satellite dish - paraboloid with offset focus

LO

Path of radio waves

Satellite dish receiver head = Low-Noise Block converter (LNB)

Block diagram of 12 GHZ Satellite Antenna and Radiometer

• `` The diffraction pattern produced by a circular

focusing lens or reflectorA focusing lens or

reflector with a circular aperture

“Ideal” antenna would produce a beam that captures 100% of the incoming energy in the main beam and have no sidelobes =>

main beam efficiency = 1.0 (usually between 0.6 - 0.8)

DSTV dish => 0.75 based on measurements at HartRAO


Angle from Beam Axis [wavelength/diameter] (radians)

Rel

ativ

e G

ain

- lin

ear S

cale

The beam cross-section through the beam pattern of an ideal antenna and a practically realisable antenna, shown with a linear

vertical scale (unblocked aperture).

FWHM = ~ 1.2λ/D [radians] => the “beamwidth” of the antenna

BWFN => first min or null in pattern ~ 2.4λ/D [radians]

• The telescope beam size depends on the frequency in use and the diameter of the telescope:

Theory: Angular Sizes

FWHM = 1.2�/D [radians]

• Outside of the main beam, the telescope is still weakly sensitive to radiation coming from other directions, in what are known as its “sidelobes”

• Man-made radio signals (RFI) become a problem if they can be detected in these sidelobes

• A radio telescope looks at a sky which has a continuous, varying, distribution of radio emission.

• There are objects of large angular extent and objects of small angular extent

• There are objects of large angular extent: hot gas clouds in the Milky Way objects of small angular extent: distant galaxies, masers and pulsars.

• Some of the these radio sources will be of larger angular size than the main beam of the telescope and some will be smaller. What the telescope “sees” is the actual radio brightness distribution in the sky convolved with (“smeared out” by) the beam of the telescope. The bigger the beam, the more it smears out.


• For a spherical object whose actual diameter equals d and where D is the distance to the centre of the sphere, the angular diameter can be found by the formula:

• For very distant or stellar objects, the small angle approximation can be used:


physical radius and angular radius

r2 > r1, θ1 = θ2 r4 > r3, θ4 < θ3

Angular diameter = arcsin( d2D )

Angular diameter = dD Angular radius = d

2D

• Angular area is called a solid angle and the units are radians^2 or steradians (sr)



⌦ = 2⇡(1� cos✓)

⌦A = 1.133(FWHM)2 [sr]

• Angular area is called a solid angle and the units are radians^2 or steradians (sr)


✓s = d/D [radians]

⌦ = 2⇡(1� cos✓)

for small ✓, ⌦s = ⇡✓2 [sr]

Parabolic dish antenna

Offset parabolic dish antenna

• A “classic” radio telescope for use in the microwave band has a circular parabolic reflector with a feed horn at the focus to collect the incoming microwaves and pass them to transistor amplifiers in the receiver


Factors reducing the aperture efficiency (0.80, 0.75, 0.64)

Pointing accuracy


Theory: TB and TA

Blackbody radiation from solid objects of the same angular size, at different temperatures.

Brightness as a function of frequency.

Visible light

Satellite TV transmission

Rayleigh-Jeans Law:The brightness B and hence the

power measured by a radio telescope is proportional to the

temperature T of the emitting source

B = 2h⌫3

c21

eh⌫/kT�1

[W m�2 Hz�1 sr�1]

For a black body radiator, the Brightness B is given by;

B = 2kT�2h⌫ << kT ,

[W m�2 Hz�1 sr�1]

B = 2kT�2h⌫ << kT ,

Rayleigh-Jeans Law holds all the way through the radio regime for any

reasonable temperature.

In the Rayleigh-Jeans limit a black body has a temperature given as;

TB = B�2/2k [K]

For some astronomical objects TB measured by a radio telescope is

meaningful as a physical temperature. Radiation mechanisms are often

non-thermal => effective temperature that a black body would need to have.

- “Blank” sky ~ 2.73 K (thermal big bang BB radiation)- Sun at 300 MHz = 500000 K (mostly non-thermal) - Orion Nebula at 300 GHz ~ 10-100 K (“warm” thermal molecular clouds) - Quasars at 5 GHz ~ 10^12 K (non-thermal synchrotron)

Theory: TB and TA

• The “antenna temperature” TA of a source is the increase in in temperature (receiver output) measured when the antenna is pointed at a radio emitting source.

• NB: The antenna temperature has nothing to do with the physical temperature of the antenna.

• The antenna temperature will be less than the brightness temperature if the source does not fill the whole beam of the telescope. Must also correct for the aperture efficiency.

• By pointing the antenna at objects of known temperature that completely fill the beam we can calibrate the output signal in units of absolute temperature (Kelvins). One can think of a radio telescope as a remote-sensing thermometer.

TB = ⌦ATA⌦s✏m

[K]

Theory: TB and TA

• When the telescope looks at a radio source in the sky, the receiver output is the sum of radio waves received from several different sources:

Theory: Detecting Radio Emission

Tsys = TBcmb + TA + Tat + Twv + Tg + TR [K]

CMB radiation coming from every direction in space.

~ 2.7 K at 1.4 or 4 GHz, reducing to 2.5 K at 12 GHz

The emission from the radio source we want to measure, which

produces the antenna temperature.

Radiation from the dry atmosphere. Adds about 1 K.

Radiation from the water vapour in the atmosphere.

At 12 GHz adds 1 - 2 K, depending on the humidity.

The radiation the feed receives through the antenna sidelobes from

the (warm ~ 290 K) ground. Adds 5 - 15 K pointing straight up at zenith, and increases when pointing

close to the horizon.

The amplifiers in the antenna produce their own electronic noise, receiver

noise temperature.

The sum of these parts is called the system temperature Sky temperature Tsky ~ 10 K

Experimental Procedure:

• Turn the DSTV dish to blank sky. Listen to the speaker or look at the meter.

• Now turn the DSTV dish towards the ground and see/hear the difference. REMEMBER The noise level depends on the temperature of the object.

• Sky – shows lowest signal level. Note that when aimed at different parts of the sky the signal level hardly changes. This means that it is not sensitive enough to detect stars.

• Remember that blank sky is about 10 K while the ground is about 300 K !

• Turn the DSTV dish towards the Sun.


• Projecting the Sun through a pinhole


• Calibrating the radio telescope:

• Why isn’t the Sun, with all its enormous energy (temperature of 6000 K), pinning the meter?

• It turns out that the DSTV dish has a beam width of 3.4° while the Sun appears to be only 0.5° in our sky. Thus the area of the dish occupied by the sun is small and the signal appears weaker than the ground at 300K.

• FWHM (beamwidth) => ~ 1.2λ/D (λ = 12 GHz = 2.5 cm, D = 50 cm).

• Measuring the diameter of the Sun => θ = d/2D (diameter = 0.5°).

• We could fit ~66 Suns into the beam of the dish (ratio of the angular size of the source to the angular size of the Sun).

⌦A = 1.133(FWHM)2 [sr]

⌦s = ⇡✓2 [sr]


• Calibrating the radio telescope:

• We need to establish a scale of Kelvins per radiometer output unit. We do this by using the sky at zenith as a “cold load” and the ground as a “hot load”

• If V1 and V2 are the two meter readings on the sky and ground and c is a constant of proportionality (kelvins per meter reading);cV1 = TR + Tsky (TR + Tsky = Tsys => system temperature)cV2 = TR + Tground (TR => electronic noise generated by receivers)

• Some typical values: V1 = 10, V2 = 30, Tsky = 10 K, Tground = 300 K

• Tsky = TBCMB + Tat + Twv + Tg ~ 10 K

• c = 14.5 kelvins per meter division, TR = 135 K


• Measuring the brightness temperature of the Sun

• If V3 is the meter reading for the Sun; cV3 = TR + Tsky + TA

• Some typical values: V1 = 10, V2 = 30, Tsky = 10 K, Tground = 300 K, TSun = 24

• c = 14.5 kelvins per meter division, TR = 135 K

• TA = 203 K

• TB = 203 K x 66 = 13400 K (only a fraction of beam filled by source)

• Correcting for the efficiency of the dish 13400/0.75 = 18000 K at 12 GHz

• How does your result compare to the temperature usually quoted for the Sun’s photosphere (light emitting surface) ?

TB = ⌦ATA⌦s✏m

[K]



1 radian = 57.29577 degrees Equations

1sr = ( 180⇡ )2 square degrees

TB = ⌦ATA⌦s�m

[K]

nr of suns/beam=

⌦beam⌦s

TB = suns/beam⇥ TA [K]

1� = radians⇥ 180⇡

Equations

TR + Tsky = cV 1 [K]TR

+ Tground

= cV 2 [K]

Tsky = TBcmb + Tat + Twv + Tg ⇠ 15 K

Tsys = Tsky + TA + TR [K]

TR + Tsky + TAsun = cV 3 [K]

✓s =d2D [rad]

�s = ⇡✓s2 [sr]

1 radian = 57.29577 degrees

HPBW/FWHM ⇠ 1.22⇥ �D [rad]

⌦A = 1.133(FWHM)2 [sr]

• Body temperature detection.

• Nearly anything with a temperature can be detected with a radio telescope and people are no exception. Having a temperature of 300K (37°C), your reading will be similar to the ground if you fill the beam.

• The first musical use of this radio created music was the Theremin, played by waving your hands near antennas to vary pitch and amplitudeLook it up on the web, it’s fascinating! Léon Theremin

More Fun Activities

• Satellite detection.

• Many geo-stationary satellites are in orbit above the Earth and many transmit radio signals. Remember though that the sun is a broadband (extremely!) transmitter whereas the satellite is a very narrow beam transmitter.

• Most of these satellites orbit above the equator so figure out where your celestial equator is by taking you latitude and subtracting it from 90°. This is a rough altitude to look for satellites.

More Fun Activities

• Find the tree line and gaps between trees

• You could map the tree line using the angle of tilt of the antenna (altitude measured with an inexpensive angle finder available from hardware stores and the azimuth found with a compass).

More Fun Activities

• Measure the HPBW (FWHM) of the antenna.

• If the satellite dish is mounted on a tripod or mount so that it can be locked in position, then it is possible to carry out a “drift scan” across the Sun, as follows.

• Point the antenna to get the maximum signal from the Sun. Lock the antenna’s position. Immediately write down the time (minutes and seconds) and the voltage on the meter recording the signal strength, and repeat every ten seconds.

• The drift scan will give a cross-section of half the antenna beam pattern.The time for the signal to go from maximum to halfway down to minimum is equal to half of the HPBW, in seconds. Units of time are converted to angle by noting that the Sun moves through 1 degree in 4 min/cos (Sun’s DEC).

More Fun Activities

• Measure the HPBW (FWHM) of the antenna.

Sign

al S

treng

ht

0,000

0,144

0,288

0,431

0,575

0,719

0,863

1,006

1,150

Time 0,00 7,50 15,00 22,50 30,00

Sign

al S

treng

ht

0,00

3,00

6,00

9,00

12,00

15,00

18,00

21,00

24,00

Time 0,00 7,50 15,00 22,50 30,00

More Fun Activities

Introduction to DSTV dish...

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