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.
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