At 04:30 PM 10/10/2006, Julio Peralta wrote:
>Am I correct in my thinking that when you stack 2 antennas the E-plane
>beamwidth is halved.
Assuming you've got vertically polarized antennas, (i.e. you're
stacking in the E-plane), the beamwidth will be reduced, but not
necessarily by half.
As a first approximation, your pattern would be the individual
antenna pattern multiplied by the pattern of two isotropic antennas
separated by the stacking distance. If you stacked the antennas,
say, 20 wavelengths apart, you'd have very narrow "interferometer"
lobes a few degrees wide and a few degrees apart, with the outer
lobes reduced by the antenna pattern.
As an example, two isotropes one wavelength apart have a beamwidth of
about 29 degrees.
Combining that pattern with a notional 35 degree wide pattern (I used
cos^3 as an approximation), I got an overall 3dB beamwidth of about 25 degrees.
stacking about 1.5 wavelengths apart gives you an overall beamwidth
of around 17-18 degrees (2 isotropic elements 3dB beamwidth is about
19.5 degrees). Of course you'll get a side lobe at about 30 degrees
off boresight that's about 7-8 dB down from the main lobe.
Stacking at 1 lambda keeps the sidelobe at about -17dB relative to
the boresight, at around 40 degrees either side of boresight.
>In my case I'm going to stack 2 VHF beams for 2M
>that have a 35 degree E-plan beamwidth I think it will be 17.5 degrees
>after stacking them one above the other.
> I also think the H-plane
>beamwidth will not change.
>
>Am I right or wrong?
>
>Julio, W4HY
>
>
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