Topband: Trees and RF

[Gary and Kathleen Pearse]

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