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antenna array - RF Cafe Forums

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 Post subject: antenna array
Posted: Wed Jan 28, 2004 1:33 pm 
Hi all, I have 2 10-db helix antennas and want to put them together. what is the distance should be if I want to get maximum gain and not having high sidelobe level. Does anyone have any ideas about it. YOur advise will be very appreciated.

 Post subject: antenna array
Posted: Wed Jan 28, 2004 2:10 pm 
Welcome to forum,

First off, I need to know the following: Frequency band (it would be nice if you could provide center frequency (Fc), RF power level, applications (specific application) and transmission and receiving radius. Is that an Yagi type antenna? Is that for Ham or commercial radio applications?

Well, let me give you some basic example to illustrate the (as yet unwritten) points above. And helix that shows excellent overall performance:

L=48", N=30, Gain=18.8dBic. First sidelobe level -22.4dB.This is much better than the typical uniformly wound long helix, which typically shows 8-10dB sidelobes.

With a clean pattern like this, the array design process is selecting the element spacing to balance between maximum gain (maximum spacing) and minimum sidelobes (minimum spacing). The two goals trade off directly (high gain means higher sidelobes, low sidelobes means less gain).

Here is a low-sidelobe approach:

The array has 20.6dBic gain (only 1.8dB more than a single element!) but the sidelobes are <-30dB all the way out past 45 degrees (this is better sidelobe performance than the individual element!). This happens because the array factor null is moved out to hit the element pattern just past the -20dB point, thereby suppressing the overall radiation pattern well below 20dB. The array factor takes care of business in close to the main lobe; the cleanness of the element does the rest past 45 degrees.

Here is the maximum-gain approach:

This array has 24.1dBic gain (!), a 5.2dB improvement over a single element. In exchange, you get a -15dB sidelobe right next to the main lobe. By increasing the spacing, you move the array factor null in closer to the main beam (thus reducing the overall half-power beamwidth, and therefore the gain), but the closer you move the null to the beam peak, the more of the main lobe "leaks" out behind the null, producing a sidelobe in the overall pattern. One nice thing about this pattern is if it is aimed in space much above about 25 degrees in elevation, that sidelobe points into cold space (rather than at the warm earth) and G/T is greatly improved.

How to decide? The goal here is to maximize G/T. We see that in the selection of array spacing, we can change the G term by 3.5dB; the question is to determine how much the T term changes between the two approaches.

Or...who says the array has to be square? One compromise approach is to pick the maximum gain spacing in the horizontal direction (azimuthal plane), and the minimum sidelobe spacing in the vertical direction (elevation plane).

I thought this might help you. Good luck!


John Pereira

Note: You may want to check these books and authors below:

The "definitive" measurement campaign for helical elements, oft-cited, is: King and Wong, "Characteristics of 1 to 8 wavelength Uniform Helical Antennas", IEEE Transactions on Antennas and Propagation, vol, AP-27, January 1979, pp. 72-78.

Jasik and Johnson, Antenna Engineering Handbook, 2nd ed., Chapter 13. An excellent NEC-2 modelling program was reported by Emerson, "The Gain of an Axial-Mode Helix Antenna", ARRL Antenna Compendium Vol. 4, pp. 64-68.

 Post subject: antenna array
Posted: Wed Jan 28, 2004 2:33 pm 
Keep in mind that this type of antenna can work for both, directional and omnidirectional pattern and well into Microwave region. I have prototypes for both in 2.4 GHz frequency range and work very well.


John Pereira

 Post subject: antenna array
Posted: Thu Jan 29, 2004 12:37 pm 
thanks a lot John.

Let forget the sidelobe for a moment. the gain of the antenna is maximum at certain distance between the two elements. and if the distance too far, the gain begins to drop. the question is: what should the distance be??

thanks in advanced

H. Le

 Post subject: antenna array
Posted: Thu Jan 29, 2004 6:26 pm 
For my own prototype in 2.4 GHz (ISM band), I have used as following and it is working very well. Now, I don't know your applications, frequency band, gain, sidelobes desired…etc. It would be nice if you could provide more information.

I think it is wise to go futher with explanation, thus you will get the results you are asking for: For frequencies in the range 2 - 7 GHz this design is very easy to do, practical and not difficult at all. This contribution describes how to produce a helix antenna for frequencies around 2.4 GHz which can be used for an example; high speed packet radio. Developments in WLAN equipment result in easy possibilities for high speed wireless internet access using the 802.11b (as know as WiFi) standard.

The helix antenna can be considered as a spring with N turns with a reflector. The circumference (C) of a turn is approximately one wavelength (l), and, the distance (d) between the turns is approx. 0.25C. The size of the reflector (R) is equal to C or l, and can be a circle or a square. The design yields circular polarization (CP), which can be either 'right hand' or 'left hand' (RHCP or LHCP respectively), depending upon how the helix is wound. To have maximum transfer of energy, both ends of the link must use the same polarization, unless you use a (passive) reflector in the radio path.

The gain (G) of the antenna, relative to an isotropic (dBi), can be estimated by:

G = 11.8 + 10 * log {(C/l)^2 * N * d} dBi (a)

The characteristic impedance (Z) of the resulting 'transmission line' empirically seems to be:

Z = 140 * (C/l) (b) Ohm

l = (0.3/2.43) = 0.1234567 m ;-)(12.34 cm) (c)

The diameter (D) of one turn = (l/pi) = 39.3 mm (d)

Standard PVC sewer pipe with an outer diameter of 40 mm is perfect for the job and can be obtained easily from a (do it yourself) shop or a plumber. PVC isolation and a 1.5 mm thick copper core. Winding it around the PVC pipe will result in D = ca. 42 mm, due to the thickness of the isolation.

With D = 42 mm, C = 42*pi = 132 mm (which is 1.07 l) (e)

Now d = 0.25C = 0.25*132 = 33 mm (f)

For distances ranging from 100 m - 2.5 km with Line Of Sight (LOS), 12 turns (N = 12) are sufficient. The length of the PVC pipe therefore will be 40 cm (3.24 l).

I hope this will help!

John Pereira

 Post subject: antenna array
Posted: Thu Jan 29, 2004 9:23 pm 
John, I think you misunderstand my question. The antennas are made, and what I want to do now is to use these 2 antennas as a 2-element array(uniformly excited). So, how far apart should I place them; in terms of wavelength (lamda).

BTW, thanks for your information. that helps a lot... :) :)

H. Le

 Post subject: antenna array
Posted: Mon Feb 02, 2004 1:33 pm 
In order to give you an exact answer, you must provide the following:

Frequency band [it would be nice if you could provide center frequency (Fc)], RF power level, Vertical or Horizontal pattern, radio applications (specific application) and transmission and receiving radius, VSWR desired, Line of Sight details, type of transmission line used, Antenna mount (mounting top or roof top)…etc.


John Pereira

 Post subject:
Posted: Sat May 08, 2004 12:44 am 
:lol: do you want more Hor or Az directivity ? I like vertical mount. This way, the hor. beam is not too sharp.

Best distance you can find out with spectrum analyzer. The match is not straight forward. gl

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