[TowerTalk] 80 M 4 SQUARE MODELING
k4to at arrl.net
Wed Jul 19 21:32:12 EDT 2017
Good evening Bill,
I have an array of 4, vertically oriented, dipoles controlled by the “Old” Comtek control. The “new” Comtek control evidently does not work with dipoles, only verticals.
Nevertheless — I believe the following principles apply, based on my experience.
My array is strung on ropes and pulleys around my elevated 160 meter vertical. The center tower is 150 feet from ground to tip. The pulleys for the 80 meter array are tied at the 140 foot level. The centers of the dipoles are pulled out to 47 feet from the center and tied off to masts located at the appropriate distance from the tower. The feed lines, all 75 ohm RG6, go back to the center and are insulated from and fed down through the center of the tower to the ground. The lower ends of the dipole are pulled back to the center and tied high enough so the “droop” clears the ground and allows the tractor to get under it.
The feed points, being 47 feet from the center, form the “square” that is 67 feet on a side.
The control box for the phasing is at ground level.
Here is the tricky part (for me). My 150 foot tower has an insulator section at 27 feet above ground where it is fed as the 16O meter vertical. There are 7, full sized, radials connected together at 27 feet and there is where the shield of the 160 meter feed line is connected. The 160 meter vertical is ground plane, not a grounded, vertical. So now if I haven’t lost you, you might imagine a rather messy interwoven set of wires. BTW, the guys are all phillystran. In order to keep interaction at a minimum between the two antennas, there are multi-core common mode chokes on each of the four feed lines from the 80 meter array placed at the point where the feed lines cross the insulators on the 160 antenna. This decouples them from 160 and keeps the impedance bump from affecting the feed point of the 160 antenna.
Both antennas work extremely well. I have worked all continents on 160 using 5 watts. On 80, I get over 20 dB front to back and good front to side ratios. I can’t recall right now what the number is. I regularly get answers from Europe when I call CQ with 5 watts on 80.
Finally, to make my point … I have two vertical antennas operating very well in an environment that says there “should” be close coupling and interaction. They work well.
Your situation is that the antennas you will be close to are all horizontal, which should give you a break in keeping them isolated. Your feedlines and radials will all be at ground level, which is good news, again, for any interaction with the higher, horizontal antennas.
Having said all of that… go ahead and model it all to your satisfaction. But, modeling programs are only as good as the files used to run them and then only if the programs are sufficiently sophisticated to handle a system like this.
Good luck and let us know how it goes.
ps — the system has been in place for close to 20 years now.
More information about the TowerTalk