[TowerTalk] Four Square notes
k4to at arrl.net
Wed Apr 5 13:37:57 EDT 2017
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”.
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