In article ,
Nate Bargmann wrote:

I'm curious whether anyone has run an NEC type of analysis on the Hustler
G7-144, and/or the G6-144 antennas. I'd like a gain/pattern comparison
to a 1/2 wave vertically oriented dipole. However, I can't find any
manufacturer pattern plots to compare these antennas.

I put together a couple of NEC models for the G7-144 a year or two
ago. If I recall correctly, this antenna is composed of three
5/8-wave radiators in a collinear arrangement (stacked one above
another) separated by relatively short inductive loading/phasing
coils. It has quarter-wave radials at the bottom and is fed at the
base through a tapped coil.

I did two different models - one using shorted transmission-line stubs
as the phasing elements, and another using lumped inductances. With
the stub lengths, and the inductance values tweaked to give the
correct current-phase relationships in the 5/8-wave radiating
sections, the two models produced almost indistinguishable results in
the pattern analysis, and a gain figure which matched the
manufacturer's claims quite well. I can't claim that the models are
precisely correct, but I'd guess that they're fairly close.

The pattern seemed reasonably clean... most of the energy went
straight out towards the horizon. There were minor lobes upwards and
downwards at an angle of around 45 degrees, as one would expect from a
5/8-wave radiator. I didn't see any systematic "squint" (up-tilt or
down-tilt) in the pattern. The "donut" of the pattern was
significantly shallower than that of a half-wave reference dipole...
not s surprise.

I added a random wire down from the feedpoint to simulate the "RF on
the outside of the coax or on the mast" issue, and found that there
was little current and little effect on the pattern... according to
the models, at least, the quarter-wave radials do a pretty effective
job in decoupling the antenna from the mast and feedline.

I compared these results against those for another antenna design I've
been considering for our repeater - a pair of "plumber's delight"
copper-pipe 5/4-wave extended double Zepp antennas, stacked one above
the other (about a quarter-wave separating the tips of their inner
arms), fed in phase. The results were roughly comparable.

I've got the models on my laptop PC at home, I think... if I can find
'em I'll email them, and perhaps some results plots to you.

The biggest concern I have about the G7-144 isn't its pattern - it's
some aspects of its construction. The one we're using went bongo
after a few years up in the weather - the repeater began suffering
from extremely severe desense - it was so bad as to render the
repeater unusable unless you were within a mile or so of it. We
temporarily swapped the Hustler for a simple J-pole and got back most
of the performance.

When we tore down the G7-144 we found that the bottom end of the
base-matching/feed coil was high-Z from ground. The construction of
the antenna base depends on a heavy-gauge press fitting between two
sections of aluminum to establish the connection between the base of
the coil and the N connector, and about five years of weathering (in a
fairly mild climate) had led to enough oxidation to allow the
connection to go open-circuit at DC! I believe that there were also
some connections which were done with ring terminals and rivets. The
antenna actually worked, in the sense that it transmitted and showed a
low SWR, but there was enough micro-arcing and rectification taking
place in the oxidized connections to create plenty of broadband noise
which was swamping the receiver and leading to many dB of
desensitization.

Our chief metalworking guy tore the whole thing apart, and rebuilt the
base with a better N connector and a pair of soldered connections
between the coil base and the ground shell of the N connector. He
also cleaned up all of the sliding-tubing connections between the
radiators and phasing coils, roughened them up a bit, gooped them with
a conductive anti-oxidant compound, re-fit and re-tuned them, then
drilled through the tubing and riveted them together, and then
used heat-shrink tubing over all of the phasing-coil connections. I
think he believes in wearing suspenders, a belt, *and* duct tape :-)

We put it back up, and found that the desense was down to undetectable
levels. It's been up in the air for 18 months now, with nary a problem.

We're going to have to move our antenna tower in a few years, and at
that point we'll probably retire the G7-144 and replace it with either
a stacked pair of EDZ antennas (homebrew) or a stacked folded dipole
system (commercial). Both of the candidate designs use fully-soldered
or fully-welded radiator construction... they're DC-grounded and would
be free of sliding or press-fit connections which could oxidize and
degrade over time.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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