Fiberglass booms are also used a lot by satellite folks who have circularly
polarized yagis. A ham who is an antenna and microwave expert did some
studies and modeling into whether or not the fiberglass really makes much
difference. If the antennas are mounted in an X fashion, it doesn't make
much difference at all. The amount of pattern distortion is minimal.
Now, in this case, I'd tend to agree with Mike that the feedlines running up
the mast do introduce that conductive element into the vertical mix. How
much is the pattern distortion? Probably minimal.
I see a number of problems with the original poster's approach too. At 20
feet, fiberglass is going to be HEAVY. You will need a solid fiberglass
pole and that is not only expensive but heavy. It will also be able to flex
quite a bit as well. Are you planning on guying the mast? You will
certainly need it.
If I were you, I would try to do it differently. Mount the antennas side by
side on a cross boom instead of stacked on top of each other or something.
If you must do it the way you'd suggest, I'd look at using something like a
carbon fiber mast. It will be a lot lighter than fiberglass and a lot
stronger. It's also more expensive still.....
on 9/6/02 3:07 AM, Michael Tope at W4EF@dellroy.com wrote:
> Don't forget that if you run feedlines up the fiberglass mast past your
> lower antennas to get to the higher antennas, then you will to some
> extent defeat the purpose of having a nonconductive mast as the
> antennas won't be able to tell the difference between a metal cable
> and a metal mast. Fiberglass masts are generally used for stacking
> horizontally polarized antennas side by side. In this case the feedlines
> are dropped off the back of the antennas vertically for some distance
> so as not to interfere with the pattern of the horizontally polarized
> antennas. The fiberglass prevents the cross boom from looking like
> a horizontally polarized antenna element.
NA9D (ex: KE9NA)
Life Member: ARRL, NRA
Member: AMSAT, DXCC
"A life lived in fear is a life half lived."