I need to mount a directional antenna vertically polarized on a mast.
The boom is about 9.0 feet long and the elements measure almost 6.0
feet across. The antenna was originally designed to be mounted from
the rear on a Rohn type of tower with two U bolts using two vertical
supports of the tower. This job site has no tower so we had planned to
chiminey mount this antenna on a single single 10.0 foot length of 1
11/16 OD galvanized rigid pipe. The problem is that the antenna, if
mounted this way would be so front heavy it would make this method
impractical.

In light of this, the manufacturer has reoriented the elements on the
boom so that we can now center mount this antenna and achieve a better
balance, however there is now a special consideration.

In this arrangement the metal mast, (pipe) will now come up very close
to the driven element and the feed point and may effect the units'
impedance. The antenna has been pre tuned at the factory for the
operating frequency of 72.45MHZ and although the tuning slug can be
field adjusted it would not be technically appropriate to do this with
the antenna assembled on the ground, and it would be impossible to
accomplish with the antenna mounted on the roof. The final SWR reading
would tell the tale and at that point if the metal mast had detuned
the antenna, the whole thing would have to be taken down and tweaked
and be re installed, how many times is anyone's guess. This could be
quite laborious and something I would rather not get involved in.

To address this possibility of detuning, the manufacturer has
suggested that I obtain about a 5 or 6 foot piece of some sort of non
metallic mast. Ideally its OD should be equal to the ID of the metal
mast. The non metallic pipe would mount to the antenna and the other
end would fit into the rigid metal pipe about a foot or so and be
secured with two .25 inch bolts drilled through the mast. As an
alternative, in the case of a non metallic pipe having a different OD
I would then U bolt it to the steel mast.

It has been suggested that I consider using schedule 80 PVC pipe for
this application. However I feel that this would be much too flexible,
and besides that I don't think that it would stand up to a cold New
Hampshire Winter very well either. A friend also offered that I might
use the PVC with a wooden dowel inside to stiffen it up, the whole
thing sealed up with silicone. That seems like it may be a bit
stronger however I'm still quite apprehensive about it as well. This
is a 350.00 antenna, (my cost), PVC is very brittle in the cold and I
definitely do not want to be eating the cost of a replacement antenna
and doing this job again after a snow or ice load takes it down, in
January.

The system transmitter puts out a 1.0 watt AM signal and the receiver
is located almost ten miles away. The terrain is hilly and
mountainous. There are other installations close by which use
directional antennas aimed at this receiver site that are successful.
This new antenna is replacing the poorly performing omnidirectional
one presently installed. The ERP of this directional antenna is
approximately 6.0 watts.

Has anyone ever dealt with anything like this? I would really
appreciate any advice, on mounting arrangements or perhaps a referral
to a more appropriate News Group . Thanks very much, Lenny.