For the past 10-15 years I have had a four square array on 80 M. I use the
Comtek phasing system. My array consist of four, full sized, dipoles strung
around my 160 meter vertical. The insulated feed point on the vertical is 27
feet above ground, which makes the tower 150 feet tall, perfect for the 80
meter stuff.
The centers of the dipoles are pulled out to 46.5 feet from the center, making
the side of the square 66 feet as required for the array. The lower halves of
the dipoles are brought back to the center and fastened about 20 feet up the
tower, making the lower half sort of droopy and loopy. This configuration is
not ideal, but I was able to get a 20 dB f/b out of it and it worked fine for
many years.
It stopped showing directivity a couple of years ago, so I decided to refurbish
it this Summer. The dipoles are all on pulleys, so they can be let down whilst
standing on the ground.
So far I have found two problems: 1) lightning had burned one trace in the
phasing box. I repaired that. Nothing else seemed to be damaged.
2) oxide had formed on the outer shield of the 75 ohm coax feeding all the
dipoles, and also on the antenna wire itself. Rain water must have seeped in
the cracks and the moisture helped the oxide develop. The antenna wire is 168
strand #14 flex weave covered with polyethylene coating. The wire is tough as
nails. You could tow a car with it. But, the oxidation had formed under the
coating at the ends of the wire. Again, I think rainwater is probably the
culprit. Davis RF sells a bare Flexweave that is tinned and they advertise that
the tinning impedes oxidation. They don’t have a product that is both tinned
and coated with polyethylene. I think that would help.
So, out of the four dipoles, two had faulty continuity measurements on the
shield of the feedline. The fault developed about 25 feet from where the coax
connected to the center insulator. That was the low point in the droop in the
span between the center insulator and where the redline entered the center of
the tower for its trip down to the phasing box. I suspect water collected and
pooled at that low point.
I am going to try to cut the coax at that point and splice on an equal length
of new coax. Each feed line is 135 feet long and I don’t want to spend that
kind of money to replace it all.
I’m not quite finished with the job. I have one dipole restrung and am working
on the second of the “good” dipoles. The other two will have to wait until I
get some fresh RG59/U in from somebody.
FYI— the “80M” dipoles resonate at 3300 KHz for proper operation. The feed
lines are cut to an electrical three-quarters wavelength measured at 3600 KHz.
This array and rework was also the source for my previous posting on “Antenna
Wire Maintenance”.
73,
Dave, K4TO
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