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

[Dave Sublette]

Very good questions, Ron.  I will attempt to clarify.  The antenna always
hangs or bolts to the tower horizontally.  To raise it, I strung a trolly
cable to a point well out from the base of the tower.  The raising angle
was less than 45 degrees.

The guys are all non-inductive. Phillystran.

Although I haven't measured with an ohmmeter, I would be very surprised to
find either half of the antenna shorted to the tower.  The saddle clamps
hold it about a half inch away from the mounting plate and each side of the
dipole is insulated with the split PVC pipe under the U-bolts between them
and the saddle clamp.
I've never noticed any change in SWR as the antenna rotates.

Dave

On Sat, Mar 31, 2018 at 4:54 PM, Ron Young W8RJL <youngron at verizon.net>
wrote:

> You said dangling. Vertical or horizontal???   Was it the same distance
> from the guy wires when dangling and bolted???  Did you check for
> resistance between either side of dipole and tower??
> Does SWR change as it is rotated???
>
> Ron W8RJL
>
> Sent from AOL Mobile Mail
>
> On Saturday, March 31, 2018, Dave Sublette <k4to.dave at gmail.com> 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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