From: HWells.El_Segundo@xerox.COM
Newsgroups: rec.radio.amateur.misc
Subject: Coaxial Dipole Construction (HELP!)
Date: 27 Jan 93 18:37:39 GMT
Organization: UCSD Usenet Gateway
NNTP-Posting-Host: ucsd.edu
Originator: daemon@ucsd.edu

Mark, WO7T asked about the dimensions for building a coaxial antenna.

I've built quite a number of them for both mobile and fixed applications.  The
construction is primarily a mechanical problem rather than electrical.
Mechanical dimensions appeared to be non-critical except for the 1/4 wave
length elements.  Although the feed point impedance is 72 ohms, I've used 52
ohm coax without any noticable difference in performance.

My coaxial antenna construction followed this plan:

1.  Attach a female flange style coax connector (SO-239 or equiv) on the end of
the selected coax cable.  Center conductor soldered to the center pin, braid
soldered to the flange.
2.  Slide the cable down through a small diameter metal tube (pipe) which is
used for mounting the antenna to a structure.
3.  Mount/solder the coax connector to the end of the tube.
4.  Select a slightly larger metal tube than the mounting tube, just slightly
over 1/4 wavelength long at the desired frequency.
5.  Solder a cap over one end and drill a hole in the center of the cap that
will be tight fitting over the coax connector.
6.  Slide the outer tube over the smaller such that the coax connector
protrudes through the jhole in the outer tube's cap.  Make an electrical
connection between the outer tube cap and the coax connector with solder or
screws.
7.  The outer tube becomes a coaxial skirt.  The bottom open end of the skirt
will require plastic or fiber supports between the two tubes.  I've used three
small plastic blocks as spacers with each held with a sheet metal screw that
just protrudes into the plastic.
8.  Cut a 1/4 wavelength metal rod (1/4 wave above the top of the skirt) as a
top section and insert it into the center of the coax connector.  Bracing the
rod mechanically has always been a problem for me when the antenna was used for
low bands.  VHF and UHF allows the rod to be mounted into a male coax connector
where the assembly was held together with epoxy.

For a six meter application, instead of using a coax connector at the top of
the support tube, I fitted a fiber board plug into the top of the outer tube
(skirt).  Drilled a hole in the center of the fiber plug to fit snugly over a
vertical center rod which had been threaded.  Two jam nuts (one on either side
of the plug) were used to secure the center rod in place.  The coax was
attached between the center rod and the top ot the skirt.  For support, a short
fiber tube was fitted onto the end of a support tube.  With the coax threaded
down through it, the support tube was slipped up into the skirt.  Two fiber
ring segments were forced up into the open end of the skirt and screwed into
place to provide a very stiff mechanical support.  Upon completion, RTV was
used to seal the top of the skirt and bottom of the center rod to prevent water
penetration from getting to the open end of the coax.  This particular antenna
has been in operation for several years without any observable problems.

Hope this helps,
73

Hugh Wells,   W6WTU
Newsgroups: rec.radio.amateur.misc