[TowerTalk] mounting a dipole to the side of a tower

[Wes Stewart]

It doesn't sound to me like the mounting plate would have a high enough shunt 
capacitance or loss resistance to bring about the effects you are seeing.

Nevertheless, if you wanted to eliminate the possibility, you might consider 
better mounting insulators:

http://www.stauffusa.com/pipe-clamps/

or a non-metallic plate:

http://www.eplastics.com/fiberglass-frp-channel

Those guys seem to sell material from here: 
http://www.strongwell.com/products/structural-shapes-and-plate/

I have a couple of 20', 2 inch square lengths of Strongwell tubing bought for an 
antenna project that never materialized that have been laying in the desert sun 
for ten years and show no sign of degradation.

Wes  N7WS



On 3/31/2018 1:45 PM, Dave Sublette wrote:
> I have a Rohn 55 tower that rotates.  I recently built a rotary dipole for
> 40 meters.  It is full size.  The center section tubing is 1.625 inches in
> diameter. It is a split construction.  I feed it with an RF choke and 50
> ohm coax.
>
> Hoisting the dipole up to 100 feet or so and leaving it dangle, the antenna
> shows an almost perfect SWR curve. It has a 1.1:1 SWR at the bottom of the
> band and rises gently to 1.5:1 at 7300.  Since I operate mostly CW, I
> didn't bother to shorten it.
>
> However, after I have bolted it to the mounting plate, the SWR goes bad,
> raising to 5:1 at the band edges and being at 3:1 at about 7200 Khz. It
> still works great and has a good pattern.(Nulls off the ends).
>
> My mount is made from two aluminum plates bolted to two pieces of Angle
> steel, one on the top edge and one on the bottom edge.  They run the entire
> width of the tower face.  The edges of the angle are within an inch of the
> dipole, so that makes it practically enclosed on three sides.  I'm guessing
> the stray capacitance is killing me.
>
> I propose to redesign and get rid of the angle pieces, leaving a flat
> plate.  I presently insulate the dipole with a full length (tower face)
> piece of schedule 40 PVC that is split and slides between the U-bolts and
> the saddle clamps.
>
> The diameter of the tubing in the center is 1.625 inches.  If I were to
> replace the saddle clamps with insulated type, is there a product available
> that does
> this, or should I just go ahead and make them?  If so, What sort of
> material? I am also thinking of ditching the metal plate and using Marine
> plywood.
>
> The antenna uses dacron support ropes out about 25 feet from the center so
> a lot of the stress is relieved by doing this.
>
> This thing works great.  The change in SWR drives me nuts.  So I'm going to
> try to fix it.
>
> Building stuff is not a problem for me, but I don't have a lot of
> experience with tough insulating material.  Any advice is welcome.
>
> Thanks and 73,
>
> Dave, K4TO
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