Antenna theory has never been a strong suit of this history major, so need some advice from the forum experts.
-I put an Inverted L using single strand 14 ga insulated wire. Goal was to get on 160M, so dimensions are 62ft vertical and 68ft horizontal. Vertical runs up pine tree, horizontal runs thru one tree and is secured to another tree. Have two 65 ft above ground radials (3 ft above ground), one runs under the horizontal leg and the other runs off in opposite direction. Right now both the SWR and the Impedance is too high.
- So here’s my question --- do I shorten or lengthen the wire to lower the SWR and Impedance? Depending on the answer, do I also modify length of radials?
Tnx & 73
AA4PB gives good advice. Your radials are not even close to what you assume they are. They are far too short.
An elevated counterpoise is a very special case. The antenna connection point of the elevated counterpoise, even if it had 1/4 wave radials, has considerable voltage to earth. In your case, because the radials are so short, it has huge voltages to earth.
This means the shield or anything you connect to the radials has considerable RF voltage to earth and other things around the antenna. If you run a ground rod to the counterpoise, your signal level will decrease. If you don't decouple the coaxial feedline, you will bring RF into the shack and noise out to the antenna.
Also, since the voltage at the feedpoint on the counterpoise is out-of-phase with current, the ground looks reactive. This capacitive reactance will make the antenna "look shorter".
You need to decide how to fix the counterpoise, and you absolutely need to isolate the coax shield from the common of the counterpoise with some type of feedline choke. There are several ways to fix this, but the first step is always in recognizing what is really wrong.
1.) ANY sparse counterpoise with less than a few dozen adequate length radials has considerable voltage to earth, and needs decoupled from earth. The smaller the counterpoise the more critical.
2.) Any non-resonant small counterpoise will detune the antenna, and will often change the feed impedance drastically.
Those are the two problems you need to fix *****BEFORE***** adjusting antenna length if you want a system that works properly and reliably.
I have started some stuff here:http://www.w8ji.com/fcp_folded_counterpoise_system.htm
Pay particular attention to counterpoise voltage to earth at 1500 watts.