In article ,
Steve Nosko wrote:
This is interesting, Dave.
They only give dimensions for the 2M.
I assume that the #12 wire wound around the long pole is the gamma
capacitor. I also wonder what the sun & time does to the insulation on this
wire.
Correct. The length and position of the wire form the usual
gamma-match transmission line out to the correct tap position. The
insulated wire wrapped around the pole forms the gamma match capacitor
and cancels out the inductance of the transmission-line stub.
The ones I've seen made by the Legion post all use commonly-available
insulated household wire (I think it's THNN insulation). This stuff
seems to be reasonably stable in sunlight, although not infinitely so.
Using the black variety might tend to prolong its life.
Rotting of the insulation on the exposed portions of the wire ought
not to matter. Only the insulation in the gamma-match capacitor area
is performing a functional task, and this has been shielded from
sunlight by the heatshrink tubing around it.
I've never seen one of these antennas fail due to deterioration of the
insulation. Their weak points seem to be the acrylic insulator, and
the set-screw arrangement used to fasten the rods to the base. The
commonest failure mode for these antennas seems to be a combination of
a deterioration of the insulator (sunlight) and a loosening of the
set-screw fastening (possibly due to metal creep, probably due to
mechanical stresses from the rods being wobbled back and forth by wind
pressure). Eventually, the insulator cracks and the rods flop around,
and your match and SWR to all to heck.
It ought to be possible to build a more robust antenna of this sort.
Sticking with aluminum, one might use one of the aluminum-soldering
kits on the market instead of (or in addition to) the set-screws. Or,
one could use brass rods and a brass crossplate, and ordinary plumbing
solder. Even a copper-pipe variety might be possible, although the
gamma match would probably have to be significantly redesigned to cope
with the larger diameter.
In this version, the SO-239 is exposed to the WX as is the PL-259 screwed on
to it. In the more usual
"run-the-coax-up-to-the-feed-point-and-solder-or-clamp-it-on" method exposes
the coax end to the Wx - allowing seepage into the coax.
Yup. On my Copper Cactus J-pole I found it necessary to glop the end
of the coax, and all of the wiring leading to the feedpoint
connections _very_ carefully with outdoor-rated black silicone
sealant. The first time I put it up, I missed a spot on the 440 coax,
and ended up with water in my coax and moss growing in the N connector
at the bottom (as Dave Barry says, "I Am Not Making This Up!").
I am trying to figure out a good way to get the feed line completely
INTERNAL when using the "tap-the-coax-up-a-ways-on-the-matching-section"
feed method. This is so the feed line and the connector (if needed) is
not exposed to the Wx.
http://www.antennex.com/hws/ws1002/sperrtof.pdf is one possibility.
In its essence, it's a J-pole with the matching section being
implemented coaxially rather than as a stub on one side, and the
attachments are internal. The insulating disk at the top ought to be
amenable to a complete waterproofing... a bit of epoxy would do the
trick.
I'm tempted to try making one, if I can figure out how to mill/lathe
the necessary discs.
Sumptin' like this:
Drill a hole in the long pole, facing the short pole, where the feed line
will attach. However, if you try to solder the shield here and just extend
the center insulation and center conductor to the short pole, water can
enter the coax. Trying to put a SO239 or something smaller (and probably an
elbow version), inside the long pole at this point (to take advantage of its
"sealed" construction) is difficult. You could put a "thru joint" right
below that point to improve access for soldering such a connector -- that
may do it. Or you could use a pipe "T" with the lone opening facing the
short pole and do something in there to use an already sealed "something" to
go from the coax (inside the pipe) to the other side of the matching
section.
Any Ideas?
Well, I've seen plans for J-poles which mount the socket on one pole
(combination of a screw, and solder), and then run a solid wire across
the other pole to make the feedpoint attachment. With UHF connectors
this is still potentially leaky. However, if you were to use an N
connector at this point, you'd end up with a connection which is far
easier to keep dry, thanks to the internal gaskets.
I use N connectors for all of my outdoor feedlines and antennas...
they're a bit more expensive than UHF, but I like 'em a lot better.
--
Dave Platt AE6EO
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