Ted Leaf wrote:
>Hi guys,
>
>I have a fixed 10 element VHF Yagi pointing at Ohau, which is 160 miles
>across open ocean. I can usually hit the repeaters there, but many times
>the propagation changes in seconds, as indicated by my S meter which
>wavers.
>
>I now have a tower with a rotor and would like to put a second identical
>Yagi above the HF beam. My intention is to have a VHF beam that is
>rotatable, as well as, to get some extra power and phase separation when
>they are pointing in the same direction.
>
>Here is the situation: The beams will be about 20 feet apart in
>distance and 10 feet in height. The coax lengths will be different and
>each has a different velocity factor.
>
>Cushcraft has said to use their stacking harness at the rig end of the
>coax and my system should work fine. I question this arrangement.
You're right to! All that the standard Cushcraft phasing harness can do
is to match the impedances - but the changes in pattern and gain will be
very hard to predct - a real mess.
You can only get gain in the wanted direction if radiation from both
antennas is in phase. Usually this is done by having both antennas in
the same vertical plane, and feeding them in-phase... see the
information on my web site, although thatt is mainly aimed toward
stacking of horizontally-polarized yagis for VHF DX.
However, you don't absolutely have to mount the two yagis in the same
plane. You can move either antenna forward or backward ("along its own
boom") and change the electrical lengths of the feeders to compensate,
so long as you preserve the in-phase relationship as measured at a
distant point... but that is tricky to do, and it does affect the
radiation pattern.
Another problem with your proposal for the second yagi is that mounting
a vertically-polarized yagi on a mast will spoil the performance of the
antenna. Expect 3dB decrease in forward gain, or maybe as low as 1dB if
you do the full computer predictions or are very, very lucky. The web
pages from F/G8MBI (hosted at my site) tell more than you ever wanted to
know about this subject.
If you want a better signal into one fixed location, the way to do it is
to stack two identical beams on a cross-arm, either side of the tower.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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