Several thoughts -
Any insulator increases the capacitance between two plates, PVC has
about 3x air dielectric constant, so it is making things worse. Ditch
the full collars.
I made UV resistant Nylon split saddles for my 86' EF180C 80m dipole
center support, about 1.5" from the 6061 element OD to 6061 mast plate
16 x 4 x 3/8". Most any UV and creep resistant plastic (no UHMW or TFE)
will work. Be careful with ABS as some is foam core and will compress
over time (original EF180C insulator for example). My collars are 1.25"
thick with 3/8 thru bolts each side and a 3/8" 6061 top plate . Given
you need to span the 18" R55 face and the R55 is adding capacitance,
more element to plate distance is probably a good idea. I wouldn't use
wood, 6061 plate in small sizes is readily available from
onlinemetals.com, metalsupermarket.com or your local remnant supplier.
Stauff clamps are the store bought insulated plastic split saddles, you
might find them in the right size, check ebay and major supply houses
(Grainger/Zoro not HD). However they hold the tube fairly close to the
mounting plate so an insulating spacer would be appropriate. I thought
DX Eng carried them but can't find them now.
Grant Z1W
On 3/31/2018 13:54 PM, Ron Young W8RJL 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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