Around Halloween, and with the arrival of perpetual below freezing
temps in northern KL7, we tree-hangers often see changes in our
vertical antenna resonance. For example, my 160M Inv-L, vertically up
70' next to a tall Spruce, then over the top and out horizontally a
variable 50', rises in resonant frequency from 1.830 to about 1.845
with the onset of cold weather. Happens every year so far.
Today I'll let out enough of the #12 insulated folded wire from the
bottom end of the vertical section to lower the resonant frequency.
In the past that's been about 15-20" of wire. I use seven elevated
insulated radials up about 15' that are tuned via the dipole method
to 1.830. The beast is fed with a commercial current choke and UNUN,
and is ungrounded at the feedpoint. Testing of the radials before and
after freeze-up has indicated little change in their resonance.
Last January it warmed into almost the 50's for a few days enough to
melt snow, and apparently parts of the supporting tree. The ground
under the antenna remained snow-covered and well frozen down a few
feet below the surface. The resonant frequency fell to the 1815 range
when the tree thawed, then climbed back to about 1.830 when it
refroze for the rest of the season. I left the vertical length the
same to note the daily change in resonance during the brief warm spell.
Folks with a better understanding of the reasons for the frequency
excursion might offer an explanation. I just vary the vertical wire
length to fix and live with the required adjustment.
73, Gary NL7Y
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160 meters is a serious band, it should be treated with respect. - TF4M
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