Topband: Trees (not the N6TR kind)

[ZR]

Subject: Topband: Trees (not the N6TR kind)


>> Pine trees taller than 100 feet could be an issue, since they could be 
>> near resonance and lossy - a sad combination when within a wavelength or 
>> so of vertically polarized antennas.  If your trees are 50 footers, they 
>> would probably not be of concern on 160m, but could be on 80m.  For 
>> horizontally polarized antennas, the trees aren't a problem.
>
> Years ago, when some fellow proposed that trees would radiate because they 
> were fractals, I measured the RF resistivity of freshly-cut pine trees. I 
> firmly attached copper or aluminum plates to a thick one foot long trunk 
> section, and measured resistance. These were wet, freshly-cut, sappy, 
> pines. Pines are acidic and very wet inside, so they should reasonably be 
> at the top of tree conductivity.
>
> I can't recall the exact RF resistivity, but I'm pretty sure it was either 
> high hundreds or low thousands of ohms per foot for a one foot diameter 
> log. I do know the number was significant. Resonance, significant 
> absorption, or radiation would be impossible with that much resistivity 
> per foot.
>
> Given a choice, I probably wouldn't have a high voltage area in close 
> proximity or contact with a tree. I doubt a few trees would produce 
> noticeable loss, and they certainly could not be by any stretch of the 
> imagination "resonant" with such high resistivity per linear foot.
>
> 73 Tom


My own experience was with a live 70+ foot pine tree on 80M some 30 years 
ago which I have recounted here before.

I started off with a 6 wire 12" diameter cage vertical suspend from a branch 
about 12" from the trunk and 60 1/4 wave radials on the ground. Resonance 
was well below the band, not very pronounced, and signals on RX were 
severely attenuated. Back up the tree and I moved it to 3' out and reasonace 
came up in frequency but TX results were very poor. Climb again and I 
managed to get it maybe 8-10' out and at that point I could work DX much 
easier than with an inverted vee about 60' away. Resonance around 3650 was 
at 59' and the VSWR pattern was as published.

The next step was a duplicate hanging from another tree, Mother nature was 
good to me since these were the only trees in the back yard that were in the 
lawn area and I didnt want to go way back and do some series chain saw work.

The ARRL CW DX contest that season resulted in the #2 single band 80 US 
score and right behind John, W1FV, the perennial winner. I actually beat him 
in multipliers and he later told me he thought I had won.

That experience proved to me without a doubt that pine trees at least made 
good RF absorbers. With other pine trees being over 100' away it wasnt like 
those phased verticals were in the middle of the deep woods. They also 
worked reasonably well on 40 as half waves but not competitive for pileups 
or contests. I dont know if it was trees or radiation angle but I soon went 
with a yagi for 40.  K1VR later tried a standard 40M 4 square and wasnt 
impressed and also went to a yagi.

Carl
KM1H