[amsat-bb] Re: Helix Groundplane?
Sil - ZL2CIA
zl2cia at paradise.net.nz
Tue Dec 11 23:59:18 PST 2007
But since you are looking at the signal from the other side (the back)
of the helix, the circular polarisation appears reversed. Reflecting it
changes this so that it is in phase.
Think of a clock, built from invisible materials except for the hands.
If you stand in front of it, the hands rotate clockwise.
Stand behind it and they appear to be turn anticlockwise. Look at the
rear that is reflected in a mirror and they moving clockwise again.
Now, imagine that radio wave coming towards you....
So, positioned correctly, a reflector behind a helix will add to it's
gain and reduce the beamwidth, won't it?
Robert Bruninga wrote:
>>Helix antennas being circulary polorized have
>>rejection to the opposite rotation generaly
>>caused by reflections provided that both...
> Wow, interesting. I just realized then, that the "reflector
> screen" at the back of a Helix is not adding any gain then.
> Because it is true, that any reflected energy will be the
> opposite circularity! Thus it cannot add in phase to the direct
> I guess all the ground plane is doing is providing a
> counterpoise for the feed system. Oh, and possibly a degree of
> attenuation to ground noise... Both well worth the metal, I
> guess. Comments?
> Bob, WB4APR
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