Topband: Screening Effect of Trees

[n4is at comcast.net]

Hi Roger

That is a very important question and the answer may surprise a lot of folks
on Topband. Kay KE4PT worked several years on Motorola simulation RF in
hollow bodies  for pagers and handheld close to the human body
Kai Siwiak KE4PT, QEX Editor. Kay prepared this presentation for our local
DX club.

https://qsl.net/k4fk/presentations/2018-May-02-KE4PT-LiveTrees-SFDXA.pdf

A Dipole 0.2 Wavelengths from a Tree looks like a 2-element Yagi on 30 MHz.
on 160m the F/B only 1 db.

Regards
JC
N4IS



-----Original Message-----
From: Topband <topband-bounces+n4is=comcast.net at contesting.com> On Behalf Of
Roger Kennedy
Sent: Saturday, April 3, 2021 2:08 PM
To: topband at contesting.com
Subject: Topband: Screening Effect of Trees


There always seem to be a lot of discussions on here about Antennas . . . so
thought I'd start a new one.

I've never seen any definitive figures for it . . . but I've always felt
that any antenna with lots of trees around - especially Verticals, where the
current maximum is near the ground - is BOUND to suffer a certain amount of
radiation loss, simply due to the screening effect of those damp trees.

I've certainly always noticed that any antenna wire that goes near any tree
affects the resonance quite substantially.

A lot of people refer to proper Field Strength tests carried out by Medium
Wave broadcast engineers when talking about 160m Vertical Antennas . . . but
I doubt if any have of them had their antennas anywhere near any trees, so
can't offer any figures for their effect.

I've also always been pretty sceptical about the accuracy of the various
Computer Modelling programmes when it comes to 160m antennas . . .
particularly as it can't properly predict what is happening regarding the
ground with horizontal antennas that are very low in wavelength terms (and I
mean Ground, not Radials) . . . but equally, I don't see how it can properly
predict the effect of trees (or various buildings, for that matter)

Roger G3YRO


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