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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