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!
Sincerely,
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.
(12.34
cm) (c) 
do you want more Hor or Az directivity ? I like
vertical mount. This way, the hor. beam is not too
sharp.