Topband: In L in Tree

Fri Oct 19 20:31:39 EDT 2018

I sometimes get around to posting on these things only when I'm not buried
by something else. So late as this may be...

I'm sure that the writer truly meant "worked very well", but that is not a
scalar description. "Worked very well" is a degree of satisfaction, however
else someone may measure that and render it in objective scalar fashion. I
do not doubt Mike in the least that his L was quite satisfactory to him.
Whether that should be tendered as advice is another issue.

There decidedly ARE loss issues with trees, even if not adequately
described, explained and quantified to hamdom in general (they really are
not). Take as example my own day of reckoning with the issue.

A few years back, I unwisely allowed three fast-growing sweet gum trees to
grow up underneath my 160m inverted L, "inside the bend" of the L. As they
grew, the sweet gums started tangling the inverted L in storms, and I began
plotting their demise.  When a massive unhealthy loblolly pine began
drifting off vertical toward my neighbor's swimming pool, I called my tree
pro to remove it. While they were here I had them take down the three sweet
gum trees.

Being curious what difference it might make to TX signal strength, when the
tree guys were ready to take down the sweet gums, I took a 1.825 MHz RBN
reading from W4KAZ, 7 miles away. I had tree guys wait until I got the RBN
reading and *ran* out and told them to start. It took them 11 or 12 minutes
to get all three on the ground. When the last one was down, I *ran* back to
the house and redid the RBN. Running to minimize the time between
readings.

The RBN reading immediately increased 2 or 3 dB, and remained increased on
following days. The elapsed time between readings was 14 minutes, at
roughly solar noon.

The weather on the day the trees were cut down was cloudless, summery hot
and dry and we had been without rain for more than a week. During "summery
months", readings from W4KAZ in continuing summer dry weather will remain
constant (ground wave) a few hours before solar noon through 2 to 4 hours
after solar noon. From summery day to summery day, eg Monday noon and then
Tuesday noon, the before-cutting-down-trees solar noon RBN reading would
vary on average less than a dB from the previous day.  This entire improved
pattern continued after the tree felling, elevated 2 or 3 dB over
with-sweet-gum readings. The difference persisted until the next time the
inevitable branch fell across KAZ's 9AY used for his RBN SDR antenna, and
the 9AY was rebuilt from scratch.

This rather violent change after cutting down the trees (~ half my
transmitted power restored) was a considerable surprise to me and sent me
off on a quest to find out why. The answer is on k2av.com in more detail in
the section "Place an Inverted L", look for "no trees inside the bend."

This is a factor particular to an L, a much lesser extent to a T, where the
field intensity (NEC4 near field tables) is FAR higher inside the bend of
the L than on the outside, 6 to 8 times more. The loss in watts from a
given dielectric mass inside the bend will increase as the SQUARE of the
increase in the field, making those sweet gums I just cut down 36 to 64
times more loss-causing than had they been in front of the bend, more lossy
only because they were inside the bend.

So we'd best not advise someone, "just don't worry about the trees." In
different degrees of urgency, particularly if added up, the total
miscellaneous dielectric material issues at a given QTH can approach the
losses of a very poor counterpoise or radial field. See the "loss list" on
k2av.com.

The scalar objective answer is complex and situational. In certain building
lot physical circumstances, an L could be inadvisable, e.g. lots of trees
inside the bend, which cannot be removed for any one of a pile of reasons.
An L in the woods, run up next to the trunk of a tree and then thrown over
the support tree and adjacent tree canopy, depending on the insulation of
stranded antenna wire to prevent arcs to the tree, is the absolute worst of
all worlds. I know of one such case where the RBN was at least 10 dB below
what should reasonably be expected.

We have figured out a procedure for measuring loss data through the woods,
but mass data gathering has not begun. The experiment is sitting there
waiting to get done. A lot of work.

It is a good thing to avoid/clear out dielectric mass outside and
especially inside the bend of an L, retain the 88' foot top load that forms
the bend of the L. The top load increases the current up high on the
vertical wire, where radiation has to go through a lot fewer trees in the
woods getting to the sky than radiation from down low.

73 and hope to see folks on the Pre-Stew tomorrow,

Guy K2AV

On Tue, Oct 2, 2018 at 8:58 PM Mike Waters <mikewate at gmail.com> wrote:

> Jose (et al), I respectfully and strongly disagree that this QST article
> applies to 160m to the degree described in that PDF (which does not even
> mention 160 meters!). I DO agree with the findings in that PDF *for those
> higher bands*.
>
> However, I do agree that it's probably a bad idea to have the vertical
> portion of an inverted-L stapled to a tree trunk. :-)
>
> The effects of nearby trees or woods and their attenuation at 160m have
> been discussed to a great length here in the past by many Topbanders far
> more experienced than I am. Before anyone takes exception to what I am
> saying, please do a search in the Topband Reflector Archive for "trees",
> "woods", etc.
>
> My inverted-L (http://www.w0btu.com/160_meters.html) was in a dense woods
> of oak trees, and anecdotally I can say that it worked very well indeed,
> thank you very much!
>
> And it'll be back up before winter. :-)
>
> 73, Mike
> www.w0btu.com
>
> On Sun, Sep 23, 2018 at 11:19 AM JC <n4is at n4is.com> wrote:
>
> > Hi Ed
> >
> > One of most impressive study about Tree and antenna performance was
> > presented few month ago by Dr. Kay Siwiak, KE4PT during the South Florida
> > DX
> > association. The study was based on years or resources at Motorola about
> RF
> > on living body.
> >
> > Live tree does affect RF performance. There are more information on QST
> Feb
> > 2018 pg 35 to 39 , Kai Siwiak, E4PT,and Richard Quick, W4RQ
> >
> > Here you can download the presentation
> >
> >
> > https://qsl.net/k4fk/presentations/2018-May-02-KE4PT-LiveTrees-SFDXA.pdf
> >
> > Very interesting stuff. I love the last comments on page 18
> >
> > "In Practice, everything works, but we don't know why"
> >
> > " In Theory, we know everything, but nothing works"
> >
> > We combine Theory and Practice:
> > NOTHING WORKS, AND WE DON'T KNOW WHY!
> >
> >
> > 73's
> > JC
> >
> > -----Original Message-----
> > From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of
> Edward
> > via Topband
> > Sent: Sunday, September 23, 2018 1:40 AM
> > To: topband at contesting.com
> > Subject: Topband: Inv L in Tree
> >
> > Has anybody snaked a wire up a tall tree trunk to make an Inv L?
> >
> > Any interaction?  Success??  Has to be stealthy because the tree os my
> > neighbor's :-)
> >
> > Thanks,
> > Ed NI6S
> > _________________
> > Topband Reflector Archives - http://www.contesting.com/_topband
> >
> > _________________
> > Topband Reflector Archives - http://www.contesting.com/_topband
> >
> _________________
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