Re: NEC-LIST: short towers near tall trees

Copied to Grant Bingeman and Roy Lewallen in addition to NEC-LIST to maintain
thread.

Hi Brian,

Thanks for the input. Yes, it was a toroidal excitation as you said. I recall
the AP paper and seem to think that it was a banana plant or something similar.

Best regards,
Duncan Baker.
________________________________________
From: Abkaustin_at_aol.com [mailto:Abkaustin_at_aol.com]
Sent: 20 July 2008 10:21 AM
To: apc_at_acenet.co.za
Subject: Re: FW: NEC-LIST: short towers near tall trees

Hi Duncan,
 
Yes, I still subscribe to NEC-list and did see that request for info regarding
trees. Our experience at Wits (for which you were the external examiner, you'll
remember) involved a technique pioneered by Ikrath (I don't have the reference
details in my head but it was in IEEE A/P 1970s) where he showed how trees could
be made to perform as antennas. The excitation, by the way, was done by means of
a toroidal coupler. A loop would have to lie parallel to the tree trunk to
induce current into it. The toroid is really a whole lot of little loops in that
plane. Interestingly enough, those tests done by my students did show coupling
to the tree in terms of an increase in input resistance but how much of that was
due to loss and how much to radiation is unknown, of course. What I was able to
show was that a 10W TX driving the 50 ohm match provided by an ATU connected to
the toroid enabled my to make contacts on the 7MHz amateur band, one if I
remember correctly with a fellow as far away as East London (Cape not UK!). So
trees do radiate and therefore I see no reason why they wouldn't also act as
parasitic elements or, as Roy Lewallen discovered, as pretty effective absorbers
especially when approximately resonant.
 
If you'd like to post these comments to NEC-list go right ahead. I'm having to
rush off so don't have time right now. I trust you and Lynne are well.
 
Regards,
 
Brian

-----Original Message-----
From: Duncan Baker [mailto:apc_at_acenet.co.za]
Sent: 20 July 2008 07:51 AM
To: 'Roy Lewallen'
Subject: RE: NEC-LIST: short towers near tall trees

Hi Roy,

Good to know you are still active.

My comments to Grant are below - I have not copied them to NEC-LIST because I
get nasty responses about not being registered.

"Hi Grant,

I have been retired several years now. However, I do recall the odd experiment
to use banana trees (or something) as an antenna published in the literature.
Brian Austin, then at the University of the Witwatersrand and now in the UK, had
a capstone project where students used a loop around a tree to stimulate it as
an antenna. I hope he responds. In short, if these did show some signs of
working, albeit ineffectively, one could surmise that there could be an
influence on radiation patterns of short masts from nearby trees.

Many years ago I had to advise on a military installation where the nearby VHF
antenna mast acted as a parasitic element for an HF transmitter. The ATU would
not tune in until the mast was moved some distance away.

Be interesting to see if anyone has any real world experience with the problem
you have posed."

I also had somewhere a copy of a WWII report dealing with the influence of
tethering cables for military balloons on HFDF systems - even at some
considerable distance.

Have a great weekend,
Duncan Baker.

-----Original Message-----
From: nec-list-bounces+duncan.baker=ieee.org_at_robomod.net
[mailto:nec-list-bounces+duncan.baker=ieee.org_at_robomod.net] On Behalf Of Roy
Lewallen
Sent: 19 July 2008 11:53 PM
To: nec-list_at_robomod.net
Subject: Re: NEC-LIST: short towers near tall trees

Many years ago I was experimenting with a two element 40 meter phased
array. It was composed of two quarter wavelength vertical radiators with
moderately good ground systems, spaced a half wavelength apart, in my
back yard. The radiators and ground systems were, as close as I could
make them, identical. A friend made some measurements of field strength
as I excited each element in turn. The unused element was open circuited
at the base, and a directional wattmeter was used to make sure the
applied power was constant. I was startled to discover a 10 dB
difference between the two! And this was a capably made measurement, not
some estimation from an S-meter. Standing at each antenna and looking
toward the friend's house, the only apparent difference was that a stand
of a few Douglas fir trees of approximately a quarter wavelength height
were in the path from the weaker antenna. They were in my yard, roughly
a quarter wavelength from the weak antenna.

Receive signal measurements showed at least 6 dB difference between the
two antennas, only in the direction through the trees, providing added
evidence that the trees were responsible for the attenuation.

Not long afterward, I moved the weak element and its ground system a
quarter wavelength toward the strong one and repeated the measurements
with the friend. The path from the weaker element went through the edge
of the stand of trees, but no longer through the center. The measured
difference between the elements dropped to 4 dB.

The only way I can think of to absolutely prove that trees can have this
profound an effect would be to set up a test giving similar results,
then cutting down the trees and remeasuring, but that's a test I've
never had the opportunity to do. In the meantime, I'm convinced that I
have observed 10 dB of attenuation of a vertically polarized HF signal
caused by absorption by trees, although I can't point to exactly what
criteria must be met to effect this level of attenuation.

Roy Lewallen

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