Re the vertically-polarized 2m yagi:
If there is anything metallic running through the plane of the yagi
elements there will be a loss in performance. This can range from
moderate to severe, but the only practical way to quantify it is by
modeling.
So, the objective is to have >nothing< metallic in the same plane as the
yagi elements, unless it's behind the yagi.
>However, for optimum performance of the 2 meter beam, you should install a
>fiberglass mast extension out the top of your steel mast for the tribander
>to eliminate interference from the mast being in the same plane as the yagi
>on 2.
Don't forget the coaxial feedline! The fibreglass mast won't help unless
you also run the feedline out from the rear of the 2m yagi boom, which
is not very practical.
>Another good alternative would be to offset mount the 13B2, but
>you'll still have some interference from the mast if it's metal.
This is the best option for a yagi like the 13B2. If you offset the
vertical 2m yagi on a horizontal boom by one-half of the manufacturer's
recommended stacking distance for two 13B2s, the effect of the mast will
be very small. To balance the windload you can offset-mount either
another 13B2 or some other vertical VHF/UHF yagi (eg for 220 or 440) on
the opposite side of the mast.
>Yet
>another option is just mounting the yagi at the very top of the steel mast
>so the mast is only "visible" to the lower elements. This isnt ideal, but
>still should work well and requires no effort or extra money.
Sorry, that will still result in loss of performance.
Yet another option would be a much shorter vertical 2m yagi (eg a
4-element), mounted in front of the mast and clamped at the rear of its
boom. The mast-to-reflector distance is not critical, if it's more than
about 6 inches. Because this much shorter yagi can be mounted correctly,
it will probably give about the same gain as a 13B2 mounted incorrectly.
Re the horizontally-polarized 2m yagi above the tribander:
>I have a cushcraft 10 element 2 meter beam 3 feet above my A3s and the
>20 meter swr is degraded to approximately to 2:1 instead of 1.2:1.
It seems surprising that the 2m beam affects the 20m beam - the elements
are nowhere near resonant, and neither is the boom. See if the 20m VSWR
changes when you rotate the 2m beam to a 90deg heading and/or turn the
elements vertical. If it still doesn't change, the chances are that you
disturbed something when installing the 2m beam.
The other issue is that the tribander must be affecting the performance
of the 2m yagi at a spacing of only 3ft. As an absolute minimum I'd
suggest increasing the spacing to one-half of the recommended stacking
distance for two identical 2m yagis, as above, and preferably more.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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