I have done similar tests with my Hi-Z 8 circle 160m array and agree with
Lee's assessment. My test were based on over the air received signal levels
on individual elements. (There are more uncontrollable variations in this
method as opposed to using C measurements, which is better). My array uses
26' elements with one element being very close to various tree branches
(mainly oak and elm) that had grown close over the past few years. The
branches were close enough that, under certain wind conditions, they
actually lightly brushed against the element (which did have very noticeable
affect on signal levels). Most of the time they were within about two feet
along side the top of the element. I had a tree crew come in (I couldn't
get to them safely myself as the trees were in an adjacent ravine) and
remove the branches and noticed only a small change (maybe a db or so) in
received signal level. I'm guessing the type of tree or foliage, the
moisture content in the foliage at the time, etc., all have some bearing on
how much the branches affect signal levels. In any case, I think if your
foliage is more than 3' or 4' from the element you should be in good shape.
73. . . Dave, W0FLS
-----Original Message-----
From: Lee STRAHAN
Sent: Sunday, November 13, 2016 11:54 PM
To: topband@contesting.com
Subject: Topband: FW: Hi-Z Around Trees
Greetings Top-Banders, I have been away for a couple days, sorry for the
late comments.
The fact is, for me there is no definitive answer to the effect of trees
on the Hi-Z arrays. I have made tests measuring phase shifts and amplitude
changes with a variety of objects around these elements. My original
thoughts after the tests were that any tree or large foliage would have to a
minimum of 5 feet distance from the elements. There are not many trees where
I live so this sort of test is very difficult. One of the things I did was
to measure the capacitance of the elements as a source impedance to the
amps. Changes in this capacitance would make amplitude changes at the
amplifiers. The only real changes I measured after numerous tests were when
I would approach the element with the capacitance meter in operation. When a
6 foot person approached the elements I could see changes in the element at
about 3 feet separation. Nothing at 5 feet. This was with a 24 foot aluminum
element and a low capacity mounting insulator. This capacitance test was
more sensitive than my voltage and phase measurements.
Mike NI7T installed a 4 square system in the forest behind his cabin.
Many pine and redwood trees were within 5 feet of the elements. We were
never able to discern any negative affects to his array. Front to back
rejection was well within nominal and front patterns seemed proper. The
array remained sensitive to weak signals. Larry K1UO had his array in a
brush patch. As I recall I suggested a 5 foot spacing which he did. If you
have not seen the DX Larry posts from Maine you are missing something. In
other words it was apparently adequate spacing.
So with the 24 foot elements ¾ inch diameter that have around a 75pf
source capacitance I would say you are pretty safe at 5 feet. Shorter
elements with significantly less source capacitance would surely need more
spacing. A 12 foot similar element drops clear down to 42pf source
capacitance. I would hazard a guess that arrays using short whip elements at
around 25pf would be quite sensitive to close trees or foliage.
Barring all this I am confident you would be absolutely safe in my
estimation at 10 feet distance of brush, tall foliage, and trees from the
elements.
Lee K7TJR
Hello Jim
My HI-Z 4 square has trees about 15 feet away from two of the elements (I
can make exact distance measurements if that would help you).
I do not "think" the trees interact in my setup - but I am not 100% sure.
Even my HI-Z 8 circle has trees pretty close to 3 elements (10 feet).
Without measuring changes (impedance) to these elements due to location, I
am not sure how else to know. However, given Frank's comments - I will be
trimming some trees today!
I am certain K7TJR is the expert on this has he has heard from users about
element location and performance for years.
I wish you the best.
73
Tim K3LR
________________________________
From: donovanf@starpower.net<mailto:donovanf@starpower.net>
[mailto:donovanf@starpower.net]
Sent: Saturday, November 12, 2016 6:57 PM
To: jim@audiosystemsgroup.com<mailto:jim@audiosystemsgroup.com>
Cc: Lee K7TJR; k3lr@k3lr.com<mailto:k3lr@k3lr.com>; Joel Harrison
Subject: Re: Hi-Z Around Trees
Hi Jim,
My comments about short verticals not performing well near trees
applies only to high impedance verticals. Low impedance are
little affected.
73
Frank
W3LPL
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