----- Original Message -----
From: "StellarCAT" <RXDesign@ssvecnet.com>
To: "tower" <towertalk@contesting.com>
Sent: Monday, December 26, 2005 9:07 PM
Subject: [TowerTalk] Holy SteppIR!
> adjusting the vertical tilt of an antenna does nothing to raise
> (or even change) the radiation angle... or at least this was the
> conclusion W2PV came to in his book.
>
> g.
>
That is not always true, otherwise there would be no point in
putting elevation rotators on VHF/UHF antennas arrays
used for space communications. Granted you can't ignore the
ground reflections, but if you point a high-gain array high
enough above the horizon, at some point the main lobe of the
array's free space vertical pattern will no longer illuminate the
ground effectively (at the image point) and the dominant effects
of the ground reflection will go away (another poster alluded to
this earlier).
I think what PV was talking about were modest yagi's at
modest heights. In that case, tilting the boom won't do much
to the overall vertical pattern until the tilt angle gets very high.
Eventually the "shadow" of the first null of the free space
vertical pattern will fall on the ground image reflection point,
but for a low gain antenna with a very broad free-space vertical
lobe, the antenna may be pointed darn near straight up before
this happens.
The antenna array in question (6 yagi H-frame), has enough
gain and is mounted high enough that it is probably starting to
look more like a high-gain VHF/UHF array than a traditional
stack of HF yagis.
73, Mike W4EF.......................................
_______________________________________________
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