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

[Ron Young W8RJL]

Well Dave, my questions were basic but one never knows the expertise of the person asking the question. Obviously you know what you are doing. I learn something new every day so I will be watching for your resolution of this sticky problem. GL & 73, Ron W8RJL

Sent from AOL Mobile Mail

On Saturday, March 31, 2018, Dave Sublette <k4to.dave at gmail.com> wrote:

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
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk