Tom Hammond wrote:
>
>>I am installing a 2m yagi for local nets/emergency ops onto the top of
>>my stack and would like to mount it vertically for proper polarization,
>>but am wondering if the metal mast will interfere with it's properties?
>
>Guaranteed that it WILL.
>
>Years ago, I did just that very same thing... and used the 2M antenna
>(a 10L yagi) for about 4 years, but with only mediocre performance.
That's what most people find. With intensive modeling of both the yagi
and the support structure, it is possible to get the gain degradation
down to about 1dB - information from F/G8MBI (hosted at my web site)
will probably tell you much more than you ever wanted to know about
that!
It seems that the CC 13B12 has been designed specifically with that
problem in mind, which shows that it can be largely avoided. However,
with "random" positioning of the mast the degradation will usually be
quite severe, as Tom and many others have found.
Since you're thinking mainly about local or semi-local operation, there
is another option. Something like a 4-element yagi, supported at the
rear end of the boom so that it's totally in front of the metal mast,
may be good enough.
>Then, for some reason, I decided to off-center mount the yagi, to see
>if I WAS actually experiencing problems from the in-plane mast. I
>added a 3' long horizontal mast to the mast from the rotator, and then
>attached the 2M yagi about 2-1/2' out on the horizontal mast, the
>yagi was still oriented vertically. The difference in performance
>was astounding, probably 10dB OR MORE (as measured by a 2+ S-unit
>increase in received signal strengths).
That's another option, and you could mount another vertically-polarized
yagi (for a different band) on the opposite side of the cross-boom to
make the wind-load more balanced.
>Note that at the same time
>as when I moved the yagi, I also moved the COAX out of the plane of
>the yagi elements as well.
>
>>Should I try to change over to a fiberglass mast above the beam or does
>>the polarization really matter with 2m after a short distance?
Even if you eliminate the metal mast, the metal coax has a similar
effect unless you route it out the rear of the yagi, well clear of the
reflector element.
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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