For tough insulating material, this stuff is good:
http://lists.contesting.com/archives//html/Towertalk/2006-09/msg00158.html
Attached is a picture of what it looks like on one of my
antennas. Maybe the reflector strips attachments, but you will
get it by direct email.
Dave Hachadorian, K6LL
Yuma, AZ
-----Original Message-----
From: Dave Sublette
Sent: Saturday, March 31, 2018 4:03 PM
To: Ron Young W8RJL ; kj6y--- via TowerTalk
Subject: Re: [TowerTalk] mounting a dipole to the side of a tower
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@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@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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