When I was a teenager chasing Gus Browning on 40 meters I caught a tree on fire
at night because the end of my dipole was in some dead tree branches!
My parents were not happy...
73
Frank
W3LPL
----- Original Message -----
From: "Tim Shoppa" <tshoppa@wmata.com>
To: "Tom W8JI" <w8ji@w8ji.com>, "Rick Stealey" <rstealey@hotmail.com>,
topband@contesting.com
Sent: Thursday, August 8, 2013 6:04:24 PM
Subject: Re: Topband: Effect of trees- tree appreciation
Different folks here have promulgated a "high voltage point loss" model for
antennas in trees. I think this could be a valid model for a doublet hung with
wires directly touching wet tree branches, I'm sure at high enough power level
this could burn away some of the tree probably at a high voltage point on the
wire.
But my feeling (having done some VHF/UHF foliage loss modeling in my day job)
is that the "heat distributed over several thousand feet of tree trunk" or
"heat distributed over several million leaves" is the correct model if the wire
isn't actually brushing against a tree.
Interestingly enough one of my favorite VHF/UHF foliage loss models notes that
vertical polarization losses at VHF are higher than horizontal polarization,
and they say this is due to "tree morphology" i.e. the tree is taller than it
is wide :-). They also note that most of the models that work best at VHF/UHF,
do underestimate loss at HF, so maybe there is no good verified and published
model for HF loss much less MF loss in trees.
Tim N3QE
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
Sent: Thursday, August 08, 2013 12:40 PM
To: Rick Stealey; topband@contesting.com
Subject: Re: Topband: Effect of trees- tree appreciation
>
> Just thinking here - I can hold my hand on a 75 watt lightbulb for a
> few seconds. 10 of those lightbulbs-worth of heat isn't very much
> heat, dissipated over the surface area of the bark of a tree, 60 feet
> tall, in winter, with low temps, and maybe even some wind can't really
> have much in the way of visible impact, could it? Gone up in flaming glory?
>
Good logical thinking. It's difficult to know a 6 dB change without
measurement, let alone three dB.
Take a tank coil as an example. Even 50 watts of loss could make it smoke or
melt if the loss is concentrated in one small area, say just a few turns.
If loss is spread over enough area, even 1000 watts power loss might not be
noticeable.
This not only applies to trees, it applies to everything from ferrite cores
to transmission lines. 0.1 dB loss in a coax connector can melt it down
at amateur power levels, 10 dB loss in a 100-foot cable might be unnoticeable.
73 Tom
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