[Towertalk] Ref: Fiberglass Mast

[hasan schiers]

----- Original Message -----
From: "Jon Ogden" <na9d@speakeasy.net>
To: "Michael Tope" <W4EF@dellroy.com>; <Aidehua@aol.com>;
<towertalk@contesting.com>
Sent: Friday, September 06, 2002 6:44 AM
Subject: Re: [Towertalk] Ref: Fiberglass Mast


> 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.

I have studied the material referred to and I find this "summary" way too
generalized. Simply mounting the antenna in X fashion DOES NOT ensure
minimal pattern distortion. The study indicated two other variables, one of
which is CRITICAL to the metal boom being acceptable and that is the
mounting position of the antenna (front to back). The actual position of the
mounting bracket in terms of which elements are now placed closest to the
metal boom is important. IIRC, the positioning of the coax as it leaves the
feed and the size of the mounting bracket became concerns as well, but I'm
not certain of these variables.

While there is a case to be made that a metal boom can be used with
circularly polarized 70cm antennas, several factors, not just one, must be
accounted for and dealt with correctly if you want to keep gain, circularity
and match intact.

There is little doubt that using metal booms can work, but it requires a lot
of care to do it right and simply mounting the antennas in the "X"
configuration is NOT sufficient. Also, keep in mind that the work presented
was for one type of antenna on one band. The approach may or may not apply
on different bands with different antennas. I still much prefer the
non-conductive mast approach, myself.

The typical problem is vertically polarized yagis on a vertical mast. This
is almost always destructive, even with non-conductive masts if the feedline
is run up the non-conductive mast, along the boom and to the feed, because
the feedline acts just like the mast. The best approach is to use a cross
boom. If you don't have another antenna to put on the opposite side of the
cross boom, use a counter weight. Done this way the mast and feedline are
always orthogonal to the plane of the elements.

73,

hasan, N0AN