[TowerTalk] 80M antenna wire size

[Jim Brown]

Feedpoint impedance is strongly dependent on height above ground and 
soil characteristics. I have three 80M dipoles and two 40M dipoles at 
125-135 ft. All the 80M dipoles and one of the 40M dipoles are built 
with bare #8 copper stretched to approximate #9 hard drawn, as I've 
posted here. I have NOT done comparative tests of wire size. I do that 
with the wire for physical strength, and so that it won't stretch over 
time, not for bandwidth or power handling. When your wires are rigged 
between tall redwoods with attachment points at least 150 ft up, tree 
sway can put a lot of tension, so any mechanical weaknesses will 
eventually result in a failure. All of these antennas are rigged with 
pulleys in both trees, a hard tie down on one end and about 90# on the 
other.

My ground quality is pretty poor (rocky, mountainous), and my dipoles 
measure in the 80-90 ohm range at resonance. This is an in-shack 
measurement, with the feedline subtracted using SimSmith. The feedlines 
are Davis RF RG11, measured parameters essentially equal to Belden 8213.

What I HAVE done (about ten years ago) was to add a second #10 THHN to 
my Tee vertical for 160M, spaced roughly 8-10 inches from the first, 
wired in parallel top and bottom. The vertical section is 100 ft. The 
measured result was to approximately double the SWR bandwidth.

My experience has been that when I correctly define the model (wire, 
insulation, height, soil parameters) NEC gives me pretty good 
correlation with what I measure when the antenna is built and installed. 
An example: for a CQP site we used for several years, I modeled a 40M 
dipole at the height we were able to rig it in the scrub trees at the 
site, and over poor soil, which is what is there. The model predicted 75 
ohms, so we brought RG11 for it. Our SWR bridge that was referenced to 
75 ohms read close to 1:1 at resonance.

73, Jim K9YC

On 1/5/2020 9:57 AM, Jim Thomson wrote:
> 10  gauge  wire  provided  for a   much wider  BW,  vs  the   16  +18  gauge  I  tried  previously.
> 2  other  folks here  in  town tried the  same experiment...with  same  results.