Topband: Signal attenuation from foliage near 160m antenna

[Guy Olinger K2AV]

On Thu, Aug 1, 2013 at 10:21 PM, Mike Waters <mikewate at gmail.com> wrote:

> This subject has been discussed here plenty of times in the past, and the
> consensus has always been that trees and other foliage near a 160m antenna
> has a negligible effect on the transmitted signal. That's what I have
> believed for some time.
>
>
I can't agree that trees have no effect. Quite the opposite observed here.

I put up my 160 3/8L in the overhead space in our micro-forest that was
vacated when a huge 140' poplar was turned into toothpicks and other debris
by a lightning strike.  The L vertical portion that was possible went up
almost in a stand of sweet gum trees next to a 115 foot loblolly that had
the support rope for the bend. With a couple years growth, the fast growing
sweet gums were starting to cause snaggle problems in storms.

When I had the tree guy in to remove lightning struck trees, including one
that was going to crush the fence around my neighbor's pool when it fell, I
had the sweet gum trees removed from under the L. The sweet gums were in
juicy full foliage mode at the time.

Before and after was observed at W4KAZ RBN particularly to see what the
changes due to removing trees would be.  Keith is local, only 7.2 miles
(11.6 km) away, bearing 78d in Cary, NC.

The resonance of the antenna system, seen at the shack, went up 18 to 20
khz from the cutting down, and the signal at W4KAZ, conservatively taking
the least change figure implied by a sequence of readings, went up 2 or 2.5
dB.  Dry day morning to early afternoon signals from here to W4KAZ don't
vary, and the change was consistently present in following days.

With the high current on the vertical up near the bend (105 feet from the
far end of the horizontal), the sweet gum trees were closest to high RF
current.  Both ends of the wire were in the clear, well away from the sweet
gums, with the feed, matching box and FCP on the other side of the driveway
from the support tree and the sweet gums.  It does not appear that loss to
foliage is restricted to dielectric loss due to proximity to the voltage
ends.

As an aside, from dead-of-no-sap-winter to juicy sappy summer foliage is a
drop of 6-10 dB noontimes on the path to W4KAZ RBN, every year.  I have no
way of knowing, given when the trees were removed, whether the 2 - 2.5 dB
change would have been seen in the dead of winter.  Looking at the straight
line path on Google Earth, it is clear there is a lot of foliage between
here and there.

I have a considerable amount of anecdota confirming trees cause loss from
other sources, but I'm trying to stick to something that has some amount of
specific measurement.

73, Guy.