Faculty

Engineering

Module Code

ECE 444

Module Title

Antenna Theory & Design

Instructor Name

Dr.Deena Salem

TA’s Name

Eng.Amira Ali

Student ID

093679

093051

Student Name

Remon Ibrhim

Mohamed Ahmed Sayed

TABLE OF CONTENTS

1 ABSTRACT...............................................................................3

2 INTRODUCTIN .......................................................................4

3 COMBINED CO-LINEAR ARRAY STUDIES3.1 Formulas...............................................................................5

3.2 constructin notes ..................................................................5

3.3 graphes and it explain ..........................................................7

3.4 collinear gain ........................................................................9

3.5 examples of collinear .........................................................11

3.6azimuthchart……………………………………..…………12

4 CONCLUSION .....................................................................14

5 Reference ...............................................................................152

Abstract

In telecommunications, a collinear (or co-linear) antenna array is an array of dipole antennas mounted in such a manner that the corresponding elements of each antenna are parallel and collinear, that is they are located along a common line or axis.

A collinear array is usually mounted vertically, in order to increase overall gain and directivity in the horizontal direction. Theoretically, when stacking idealised lossless dipole antennas in such a fashion, doubling their number will, with proper phasing, produce double the gain, which an increase of 3.01 dB. In practice, the gain realised will be below this due to losses.

The active impedance and current distribution on a cylindrical antenna in a uniform, infinite, planar or collinear array are examined. A Fourier series for the antenna current is derived by relating the electric field variation in the gaps between adjacent collinear elements to the current. The electric field in the gaps is expanded in a series of Chebyshev polynomials whose coefficients are found by solving the set of algebraic equations obtained by enforcing at a number of points the boundary condition that the current vanish in the gaps between the ends of adjacent elements. The analysis is suitable for any combination of element length and scan angle, provided that the distance between the ends of adjacent collinear elements is comparable to or less than the element length. Results are compared with those based upon sinusoidally distributed currents and discrepancies are noted for arrays with closely spaced collinear elements and for wide H-plane scan angles of planar arrays.

Inroduction

A collinear (or co-linear) antenna array is an array of dipole antennas mounted in such a manner that every element of each antenna is in an extension, with respect to its long axis, of its counterparts in the other antennas in the array. It's usually mounted vertically, in order to increase overall gain and directivity in the horizontal direction. When stacking dipole antennas in such a fashion, doubling their number will, with proper phasing, produce a 3 dB increase in directive gain. The radiation pattern is omni-directional.

As discussed in another page about the Phelps-Dodge "Station Master" antenna having some electrical problems, a new replacement innards can replace the old, bad antenna. By pulling out the old collinear antenna and replacing with a new, home-brew version, using the existing outer (fiber glass) shell you can have a good working repeater antenna for the remote site. Commercial antennas coming out of service normally have wear on them. This can be damaged fiber glass surfaces, from improper mounting. Also, the elements (no pun intended) can cause the resin to break down. This is observed by the surface being "fuzzy" with the glass sticking out. You can re coat the surfaces; just be aware and (later) verify the resin you use won't have an effect on the RF signals. Antennas developed for this article were built for the Amateur 1 1/4-meter band

4

Combined co-linear array studies

Formulas:

The antenna consist of 1/4 wave sections of coax. Different types of coax have different electrical wavelengths, therefore will affect the physical length you cut them for. Two or three formulas works with most folks, either use the 492 divided by frequency, then half that, then multiply by 12 to get a 1/4 wave length in inches. Or use the 300 divided by frequency, half that for the metric version. A third way is take 2757 divided by the frequency to get 1/4 wave length in inches. One more consideration that most teflon type coax have a high velocity constant (K factor) while the cheaper non-foam have a lower K. The former is around 80% while the latter is around 66%. Therefore multiply the K by the first number before calculating the rest. For example, if using non-foam (66%) take the 492 x .66 = 324.72, then divide the frequency (in MHz) to get the 1/2 wave, then divide that by 2 for the 1/4, then multiply that times 12 to get it in inches. For this type of coax 223 MHz comes out to 8.736" for each section. The Author prefers this type of coax which takes up less room (length) inside a housing. This might be a factor when building a 2-meter version for an existing (discarded) old fiber glass carcass.

Construction Notes:

Starting with the bottom feed, make up the matching section, then solder (electrical) half-wave sections, alternating the center and shield for the ends. The image will better illustrate this method.

The top section is made from #12 or #10 copper wire. Solder a loop on the top end for hanging, later.

The bottom "balun stub" is made from a piece of #14 wire or equivalent. Cut pieces utilizing the above formulas, plus 2-1/4". Cut 1-1/8" of black outer insulation from each end. Then tin copper braiding with solder.

Using a sharp cutting tool or tubing cutter, cut 5/8" of tined braiding off each section end. Cut 1/2" of insulation from center conductor, leaving 1/8" between center conductor and braiding.

The more 1/2 wave sections you use, the more gain you get; about 6dB gain with 8 half wave sections and 9dB with 16 half wave sections. For down tilt, cut antenna sections 2% shorter than the calculated length and adjust VSWR (return loss) with the balun. For additional strength, the Author wraps each joint with thin wire,

before the final soldering of such joint. If you have not done so, install a termination on the end of the "pigtail". The Author prefers type "N" male on the end of the tail.

After the performing check, finish antenna with the appropriate sealing against the

environmental elements such as rain and snow.

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Graphes and it explain:

When you get one or more finished you can install them inside old commercial "shell"s. Take apart the old guts of the commercial antenna you got and put the new antenna you build in its place. Taping pieces of foam will stabilize the elements nicely inside and keep them from banging around loosely in the wind. Check to verify the entire length of the antenna, including the pigtail (feed point) will match the shell. With the top whip near the top of the shell the pigtail should extend out the bottom from 12"~24".

Before sealing you might want to re check the return loss. 10 db is an absolute minimum; SRG standards prefers a minimum of 15. The Author's building of antennas, if carefully performed typically produces ones with a R.L. of 20 db at the intended frequency. The images can be enlarged by clicking on them. The left shows a prototype built for the Amateur 1 1/4-meter band in April of 2009.

The RL shows 16 db, which is acceptable, however, being resonant at 218.200 MHz is cut way too long. The good news it's fairly easy to short up the elements, taking a hour or two, plus moving the bottom matching stub up a little. By a little, typically, 5 MHz represents about 1/4" at this frequency range so be careful. Some of this depends on the VC (velocity constant) of the coax you are using for this project. If you cut them too short you may have to start all over again with new

coax. The right shows this antenna is resonant at the bottom of the Amateur band, so the cuts are going in the right direction. Also note the RL is excellent, being 29 db, which is way better than expected for this design. If this was cut just a little more on each element it would have been a perfect antenna match.

element placement gain.

First we have the end-to-end or collinear

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Collinear Gain:

The "Relative Spacing In Wavelengths" in the graph (fig 5-22) above is the current-maximum spacing of the elements, not element "tip" or "center" spacing.

Gain for two collinear 1/2λ (or shorter) dipole elements peaks with ~0.9λ (wavelength) spacing.

Radiation is caused by current, and so areas of maximum current are where radiation primarily occurs. With a 1/2λ dipole in each element,

the dipole center spacing below would be 0.9 - .25 -.25 = 0.4λ.

The overall array length would be .9 + .25 + .25 = 1.4λ

With two dipoles placed end-to-end at the center with almost no end spacing, spacing of current maximums (S) would be .25+.25=.5λ.

Overall length would be twice that of a single dipole, or 1λ. Maximum theoretical gain for this spacing is found on the graph above, at the crossing of the vertical 0.5 RELATIVE SPACING IN WAVELENGTHS and intersection of "curve 2" (the two-element curve), as just under 2dB over a single element.

Gain would be less than 2dB over a single element in ANY collinear antenna using 1/2λ or shorter elements with very small element end spacings (D).

Despite what is commonly claimed, doubling the number of elements and doubling array length does not increase gain 3 dB.

A collinear two-element antenna, using half-wave dipole elements, has 3dB gain over a single dipole when dipole current maxima spacing (S) is .75λ. This is .25λ D, or 1.25λ overall L in the case of 1/2λ dipole elements, for 3 dBd gain. Keep in mind 3dB is not the maximum obtainable gain for two elements.

Maximum theoretical gain is about 3.25 dBd, and for 1/2λ dipole elements, occurs at about 0.9λ S current maxima spacing, or 1.45λ L overall length.   

Doubling gain again (3dB more gain, for a total of ~6dB), over the two-element case, requires a minimum of four collinear elements. The array would be 2.75λ long, or 5.5 times the overall length of the initial dipole.

10

The array will produce over 6dB gain by using more than .75λ current maxima spacing. If the array has .95λ element current maxima spacing, the array of four elements would have optimum gain of 6.7dB with an overall physical length of 3.35λ.

The more elements the array has, the further individual elements must be spaced for optimum gain. Doubling physical size while doubling the number of elements will not double the gain.

EXAMPLE OF COLLINEAR

Using EZNEC, we see the gain of a lossless dipole in freespace is 2.14 dBi.

Adding a second collinear element with close end-to-end spacing, which doubles antenna size, we have:

Adding a second collinear element with close end-to-end spacing, which doubles antenna size, we have:

Adding a second collinear element with close end-to-end spacing, which doubles antenna size, we have:

Azimuth chart

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Conclusion

The principle of complementarity is reviewed with particular reference to its application to the study of slot transmission lines and slot aerials. The conventional techniques for making measurements on transmission lines are shown to be applicable to a two-slot line that is used to centre-drive a slot aerial. The complementary normalized impedance of such an aerial as a function of its half length, and the distributions of the electric and magnetic field along it, have been measured and are described. The experimental results for the slot aerial are compared with the approximately complementary ones derived theoretically for a centre-driven cylindrical aerial. The effect of the finite thickness of the earth screen in which the slot line and aerial are cut is discussed. A similar study is described for a threeelement collinear array of slot aerials.

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References

1) International Journal of Theoretical and Applied Computer SciencesVolume 1 Number 1 (2006) pp. 63–70.

2) The Collinear Antenna Array with a Section of Two-Wire L.ine as Coupling Element by Charles C. H. Tang Cruft Laboratory, Harvard University Cambridge, Masscchusetts.

3) http://www.srgclub.org/CollinearAnt-HomeBrew.html, Collinear antenna building, by Karl Shoemaker, AK2O

4) http://www.w8ji.com/stacking_broadside_collinear.htm, stacking broadside collinear.