The "distortion" you're seeing is the sum of the fields from the two
antennas, which vectorally add to produce a total pattern. This is a
"phased array". The pattern of a phased array depends on the relative
phase and magnitude of currents in the elements (in your case, the
antennas), the spacing between elements, and their relative
orientations. So if you don't want to change antenna positions, the only
alternative is to alter the relative magnitude and/or phase of the
currents in the two. One source of information about this topic is
Chapter 8 of the _ARRL Antenna Book_.

Roy Lewallen, W7EL

Rohit wrote:
Hi all,
I have placed 2 similar antennas (3 dB beamwidth = 90deg,
Polarization= LHC) on a PEC box with a small spacing between the 2.
Both antenna's are tilted and the first one is at 45deg and other at
135 deg from the horizontal so that both can cover 180 deg.

With this setup, in the simulation result i am observing heavy
distortion (about 10dB ripple or more) at 90 deg. What is the reason
for this heavy distortion. Is there any way i can remove the
distortion in the pattern without changing the antenna's position??

Thanks in advance,
Rohit

Hi all,
With the same setup as i have stated before, I have observed the
pattern by changing the spacing between the 2 antennas. For every 1
wavelength increase in spacing i am abserving one lobe around 90deg.
As the spacing becomes lage multiple of wavelength, the ripples
magnitude increase to greater extent. Can someone explain this. With
the experiment i've done, it indicates that for the spacing of 1
wavelength i get the best pattern without ripples.

Rohit wrote:
Hi all,
With the same setup as i have stated before, I have observed the
pattern by changing the spacing between the 2 antennas. For every 1
wavelength increase in spacing i am abserving one lobe around 90deg.
As the spacing becomes lage multiple of wavelength, the ripples
magnitude increase to greater extent. Can someone explain this. With
the experiment i've done, it indicates that for the spacing of 1
wavelength i get the best pattern without ripples.



It is the constructive and destructive mixing of the rf being emitted by
your antenna's. Your results are exactly correct. It is the reason that
twin cb radio antenna's don't really work on an 8 foot wide truck. At
that spacing they are only a fraction of a wave length apart and act as
one antenna.

Dave WD9BDZ

( I am working in GHz range and so the spacing of multiples of
wavelength means it will be only few cms.)
One more observation i have made is that i have placed my antennas on
a rectangular plate and then on a curved surface. The former setup
gives me around -20db gain at 270deg, i.e back side of the rectangular
plate whereas in the latter case there is no such diffraction effect.
Why is it so?

Also i want to know how is the minimum spacing between antennas
decided ?

On 15 Feb 2007 03:37:43 -0800, Rohit wrote:
Also i want to know how is the minimum spacing between antennas
decided ?

It's *never* "decided". It is constantly _argued._ Just hang around here.