[TowerTalk] Inv L inTree

[Dan Zimmerman N3OX]

> So the 40' vertical portion starts at 10' up
> the trunk of the tree and 15' away from the trunk.

<snip>

> Now that the leaves are gone I thought about
> climbing the tree and installing the vertical wire inside a 40' fiberglass
> pole and attaching the pole to the tree trunk about 30' above the ground.
> This would give me almost 64' of vertical wire but it would be within 2 " of
> tree trunk for at least 40'.

<snip>

> Am I about to
> screw up a good working antenna just to gain vertical height? Your thoughts?

My thoughts?

Try it and when you try it, make an objective measurement, like a
field strength measurement, that would do two things:

1) You will KNOW what happened.  You won't have to guess, you won't
have to fret that you screwed up your antenna and your one shot at a
nice DX station next time you fail to get through a pileup.  You will
have an immediate result:  that you made it worse, you made it better,
or you couldn't tell the difference.  In any case, if the measurement
is done with reasonable care, you'll KNOW.  There are a few
complications that can make for confusing data but models and taking a
few measurements in different directions and at different distances
can be helpful (make sure to mark your spots for after you make the
change!)

2) You would provide an experimental data point that would be helpful
for this whole "trees" discussion.  I haven't seen very many (any?)
comments to the effect of "I had a cornfield and a forest and I put up
two 40m verticals with identical radial systems and there was XX dB
difference between the two" or "I had my insulated vertical wire
stapled to a tree and moved it 3 feet away and saw YY dB improvement."
   You would help the people who want to model this develop a
realistic model by providing some validation.

A simple field strength setup requires only a small signal source,
your rig, and a voltmeter.  Details here, including some of what I did
to deal with the messiness of having to do nearfield measurements on a
small lot:  http://n3ox.net/projects/n3oxflex/

What you're doing is considerably trickier than my measurement in the
sense that you have a lot of hard work to do to swap the antenna and
that gives more time for random variables to change (soil moisture,
tree moisture, etc), but if you mount the pole, etc, before you do the
"before" measurement, that will speed things.   When I switched from a
40 foot to a 60 foot base loaded vertical on 160m I used a very
similar field strength test setup to check the improvement, and I was
happy to measure a real, substantial improvement (about 6dB).  That
swap took a couple hours and I only did the comparison once so there
was inherently the potential for more error in the measurement than
there was in a measurement where I could swap back and forth twelve
times.

But I was still glad I did a measurement.   I suspect it is more
likely to be definite and satisfying than trying to figure out the
truth from an internet discussion without many reports of dB
differences between "trees" and "no trees."    Some solid comparisons
of models and real world results are very useful and will lead to good
insight and ability to predict stuff.  But if no one actually provides
real world results of the kind of detail needed to check models then
we don't learn anything.  And if we all wait for other people with
more resources or fancier equipment to make the measurements,
real-world measurements become very, very rare.

No one is obligated to spend their time setting up FS measurements.
Sometimes when I suggest them people get upset, and I can already feel
some people reading this and saying "OMG THIS IS A HOBBY, LIGHTEN UP."
 :)   But it's really not that hard and I find it valuable for those
things I'm genuinely on the fence or confused about or don't trust the
model predictions to be accurate for whatever reason. Some things just
have to be checked in real life with an objective, (probably)
repeatable measurement provided I care about the answer.  Sometimes I
decide I don't actually care about the answer.  Maybe the
measurement's too hard or boring or I couldn't actually fix a problem
if I found one, but I try to make the "I don't care unless it causes
an obvious on-air problem" decision as explicitly as I can.   I know
from the tests and experiments I HAVE done that there is not really a
way for me to catch a few dB with no real-time A/B comparison and no
objective FS measurement, but sometimes I decide I don't care.

73,
Dan