Do trees absorb RF?
Brian Kelly
Assuming the use of insulated wire such as common #14 THHN do such
antennas suffer losses vs. the same wire installed over the same ground
at the same height but out in the open? I've gotten very good
performance out of a number of "treed" antennas but that's highly
subjective and doesn't mean I haven't been heating the trees along with
working the dx. Ideally I'd run A-B tests and find out for myself but
I'm not in a position to do that. Has anybody done any actual studies or
published any articles or papers on the matter?
>
Tnx,
>
Brian Kelly w3rv
Reg Edwards
losses induced in them as does the ground.
Also a dense undergrowth reduces the effective height of an
antenna. But not to the full depth of the undergrowth.
Because trees are mostly air-spaced they behave as a lossy
capacitance between antenna and ground. Tree sap and wet
bark has a relatively low resistance but the loss can be
considered as a high value resistance in parallel with the
antenna-to-ground capacitance.
A crude estimate of the loss is calculable for a given
density and height of vegetation relative to antenna height.
In general, loss in vegetation above ground is less than
loss in the ground. And loss in the ground never keeps
people off the air.
--
------------------------------------------
Regards from Reg, G4FGQ
http://www.btinternet.com/~g4fgq.regp
------------------------------------------
Brian Kelly wrote -
Richard Harrison
"Has anybody done any actual studies or published any articles or papers
on the matter?"
Yes, for the U.S. Army in WW-2.
"Recent measurements using antennas somewhat below treetop level in
densely wooded areas indicate that with both the transmitting and
receiving antennas so situated, the loss is about 3 or 4 db for
frequencies in the 20- to 40-mc band, and 10 db in the 70- to 100-mc
band. With horizontal polarization, the losses are negligible at 20 to
40 mc and quite small at 70 to 100 mc."
There is more about dense rain forests and scattered trees, but the
trends are clear from the above.
Best regards, Richard Harrison, KB5WZI
CAM
> I've gotten very good
> performance out of a number of "treed" antennas but that's highly
> subjective and doesn't mean I haven't been heating the trees along with
> working the dx.
You better watch out, Brian. I hear that if Gore gets elected, he
will make the RF exposure rules apply to trees as well as humans.
--
http://www.mindspring.com/~w6rca
Brian Kelly
Reg Edwards wrote:
> A crude estimate of the loss is calculable for a given
> density and height of vegetation relative to antenna height.
> In general, loss in vegetation above ground is less than
> loss in the ground. And loss in the ground never keeps
> people off the air.
Good, you're right about getting it on the air losses be damned.
>
I ran a less-than-optimum 80m inverted L in ENDFEED, the ground losses
are 7.3%. Another 7.3% is not gonna drop me out of the other guy's
radio.
>
> --
> ------------------------------------------
> Regards from Reg, G4FGQ
> http://www.btinternet.com/~g4fgq.regp
> ------------------------------------------
>
Tnx, Reg,
>
w3rv
Brian Kelly
Richard Harrison wrote:
> Brian Kelly, W3RV wrote:
> "Has anybody done any actual studies or published any articles or papers
> on the matter?"
>
> Yes, for the U.S. Army in WW-2.
>
I made a bet with myself that if anybody did any testing in this area it
would some signal corps or another.
>
> "Recent measurements using antennas somewhat below treetop level in
> densely wooded areas indicate that with both the transmitting and
> receiving antennas so situated, the loss is about 3 or 4 db for
> frequencies in the 20- to 40-mc band, and 10 db in the 70- to 100-mc
> band. With horizontal polarization, the losses are negligible at 20 to
> 40 mc and quite small at 70 to 100 mc."
>
Am I correct in translating this as vertical HF antennas strung thru trees
have higher losses than horizontal antennas? Vertically-polarized loops in
the woods are a bad idea? "Inductive coupling" to the trunks? Heh.
>
> There is more about dense rain forests and scattered trees, but the
> trends are clear from the above.
>
Sure does. Fortunately I'm in "semi-scattered trees" situation and seldom
wander above 14Mhz.
>
>
> Best regards, Richard Harrison, KB5WZI
>
Thankew, Sir.
>
w3rv
Brian Kelly
CAM wrote:
>
DON'T EVEN GET ME STARTED ON THAT HOGWASH!
>--
> http://www.mindspring.com/~w6rca
>
Flaming #$@!xx&# nitwits . . .
>
b.
Richard Harrison
"Am I correct in translating this as vertical HF antennas strung through
trees have higher losses than horizontal antennas?"
Exactly what measurements showed. The loss figures posted were for
vertical antennas. Horizontal polarization produces less tree loss.
Vertical antennas on both ends of the circuit below treetop level and
operating below 20 MHz have a combined loss from the trees of less than
4 db. The loss from the trees is less when the antenna polarization is
horizontal.
Paul
much less than half my wintertime range when the leaves are down. 6M is not
a whole lot different.(vertical polarization).
Paul N8ULS
Brian Kelly wrote:
> Trees make great freebie, inconspicuous supports for HF wire antennas.
> Assuming the use of insulated wire such as common #14 THHN do such
> antennas suffer losses vs. the same wire installed over the same ground
> at the same height but out in the open? I've gotten very good
> performance out of a number of "treed" antennas but that's highly
> subjective and doesn't mean I haven't been heating the trees along with
> working the dx. Ideally I'd run A-B tests and find out for myself but
> I'm not in a position to do that. Has anybody done any actual studies or
> published any articles or papers on the matter?
> >
> Tnx,
> >
> Brian Kelly w3rv
Brian Kelly
Richard Harrison wrote:
>
...........................
>
> Vertical antennas on both ends of the circuit below treetop level and
> operating below 20 MHz have a combined loss from the trees of less than
> 4 db. The loss from the trees is less when the antenna polarization is
> horizontal.
>
Roger that. Two dB per. Not exactly a signal killer. I'd love to know why.
Given my upcoming heavily forested qth I'd best rethink using verticals on
20 and above. Probably not an issue below 20m.
>
>
> Best regards, Richard Harrison, KB5WZI
>
Tnx agn.
>
w3rv
Brian Kelly
Paul wrote:
> Where I live with 2M antennas below tree top level my summertime range is
> much less than half my wintertime range when the leaves are down. 6M is not
> a whole lot different.(vertical polarization).
> Paul N8ULS
>
I'd never have guessed it could be that much but it's very much in line with
the other posts. The attenuation seems to go up pretty quickly with frequency.
>
Tnx,
>
w3rv
Dean Arthur
to hunt up documentation for "hidden tree antennas" I have laying around
here - somewhere?
Reg Edwards
Brian Kelly
Reg Edwards wrote:
> Yes. They also cause acid rain. Chop 'em all down. ;o)
>
I'll take the acid rain vs. chopping down my towers!
>
w3rv
W1RFI
>"Recent measurements using antennas somewhat below treetop level in
>densely wooded areas indicate that with both the transmitting and
>receiving antennas so situated, the loss is about 3 or 4 db for
>frequencies in the 20- to 40-mc band, and 10 db in the 70- to 100-mc
>band. With horizontal polarization, the losses are negligible at 20 to
>40 mc and quite small at 70 to 100 mc."
>
>There is more about dense rain forests and scattered trees, but the
>trends are clear from the above.
There may be a word missing in that paragraph -- is that 3-4 dB/mile? On HF,
it certainly can't be 3-4 dB/tree. :-)
Too, this would be for propagation between two stations located on the ground
-- direct or ground wave. The typical HF antenna located in the trees would be
launching energy skyward for most paths, passing through about 0.002 mile of
tree, typically.
73,
Ed Hare, W1RFI
Richard Harrison
"---is that 3-4 dB/mile?"
Good question.
The quotation is from the War Department`s engineering manual. I think
the authors were measuring loss between two points on the ground within
line of sight distance. The military used frequencies between 20 MHz and
100 MHz for vehicular communications in WW-2. Old timers will remember
the "tank sets" with chromium push-buttons, etc.. We used the same
radios aboard our navy ship to communicate with other ships in convoy,
and tanks on shore, or on our tank deck. We didn`t consider FM
communications at VHF a violation of radio silence except when the times
were considered perilous, like dawn, dusk and night.
What is missing from the War Department data is the distance between
measurement sites. The authors probably wanted to give a feel for the
problem without being held responsible for exact numbers. There is wide
variation in foliage and density. Terrain variations could obviate the
foliage problem. Radio users could be on opposite sides of a valley or
on opposite sides of a mountain.
The data show that lower frequencies are less affected by foliage than
higher frequencies. Also, horizontally polarized waves are less affected
than vertically polarized waves as would be expected, unless tree growth
were primarily horizontal instead of vertical.
From the trend of the measurements, I think trees are of little concern
if you are using horizontally polarized sky-waves for short and medium
distances, and not too serious a problem at low elevation angles for
great distances.
Richard Harrison
"There is wide variation in foliage and density."
Of course, there is seasonal variation. An afterthought is that generals
refight the last war. The military schemers probably emphasized Europe
in their tree thoughts. Forests in Europe tend to be swept clean of
underbrush. The practice is to gather everything for fuel and other
uses. Even the lower branches are plucked.
A relatively clean wooded area was probably used for the War
Department`s tests of foliage attenuation. Attenuation from trees
outside Europe is probably worse. Just speculation.
Set'em up Joe
Might be kinda fun to hammer two
copper pipes, one each into adjacent trees,
then run some ladder line up to your rig and
see what kinda SWR you get. The ultimate
stealth verticle. Hi.Hi.
Better yet, if you could get a copper
paint that actually had copper in it you could
spray paint selected branches to form a sort
of a fixed monster size Yagi.
"Excuse me, Mr. Ham. But doesn't
spray-painting your trees constitute a violation
of the Homeowner's Association Covenents ? "
" Oh that ? That just insecticide treatment
for bag-worms !"
73,
Andrew
KG4JBM
>
Ian Jackson
<cath...@erols.com> writes
radiators (such as tapping into a trunk about 1/3 of the way up and
gamma-matching it like you would a vertical mast). Probably best when
'the sap is rising', so efficiency would be seasonal!
Ian.
mike....@ziplog.com
>In article <39fbc619....@news.erols.com>, Set'em up Joe
><cath...@erols.com> writes
>I believe that experiments HAVE been done to try and use trees as
>radiators (such as tapping into a trunk about 1/3 of the way up and
>gamma-matching it like you would a vertical mast). Probably best when
>'the sap is rising', so efficiency would be seasonal!
>Ian.
My 40 meter 4 square is, more or less, an embedded in the tree problem.
It's an elevated creature, 9 feet up off ground.
There is a difference in the SWR overall ring-around-the-rosy seasonally
adjusted and F/B ratio I do see when these Post Oak trees go from dormant
to operational! Despite the fact, however, that ' d posies bloom in 'd
spring, au contrare to the Royal Court .. All don't stand up and All fall
down at all!
For practical purposes, nightly performance of this array at a
seven megahertz rate ..
seems unaffected ...
by the rise in sap
Huge grin...
--> Sleep well; OS2's still awake! ;)
John S. MacKay
set installation in a tank... it would receive but not send. It had been
assumed that it wasn't sending because nobody could receive it. My
examination showed "no fault found". The tank was under a tree and
camouflaged. Moving it a few feet away from the tree resolved the peoblem.
VE7AFN/M. news:39fbc619....@news.erols.com...
Richard Harrison
"Moving it away from the tree resolved the problem."
The Type 19 tank set was a combo of two radios, one 2 to 8 MHz, or
thereabouts depending on late or early production, and another, a UHF
radio, operating 230 to 255 MHz.
Vertical trees next to vertical whips wouldn`t work well for either
component radio set.
Joe Buch
>>
>I believe that experiments HAVE been done to try and use trees as
>radiators (such as tapping into a trunk about 1/3 of the way up and
>gamma-matching it like you would a vertical mast). Probably best when
>'the sap is rising', so efficiency would be seasonal!
>
>Ian.
>
During the Vietnam War Bell Laboratories ran some tests of a device
which transformed trees into vertical antennas. The problem was that
in the dense jungles of Vietnam the absorption was so great even at HF
frequencies that small vertical whips were worse radiators than
normal.
The Bell Labs engineer called his device a "HEMAC". That was an
acronym for something but I can't remember what. The HEMAC was a
loosly wound air-core toroid. The tree trunk went through the center
of the doughnut formed by the torroid. The torroid coil was resonated
with a variable tuning capacitor.
Comparative test results in a densely wooded area showed the
HEMAC/tree had significant gain over an electrically short whip. I do
not remember the numbers. A photo of this device being used on a tree
on the Bell Labs campus appeared in a trade magazine but I never saw
or heard of the device again. The accompanying short article said the
same technique could be used to excite metal light poles. I would be
interested in anything anyone in this group could add on this subject,
especially references that might be accessible via the Internet. An
Altavista search on HEMAC got 26 hits but none related to the device I
remember seeing in that magazine.
Joe Buch
josep...@dol.net
Richard Harrison
"The accompanying short article said the same technique could be used to
excite metal light poles."
A math treatment of toroidal coupling to grounded towers is given by B.
Whitfield Griffith, Jr. in "Radio Electronic Transmission Fundamentals",
Library of Congress Catalog Card Number: 61-13756, pages 379-383.
Griffith says: "The toroidal coupling system which we have just
described has thus far not found application in standard broadcast
stations. The large toroidal inductor (mean diameter 6m and mean
circumference, 18.86m, 1,06 turns per meter, or 20 turns in the
example), would be expensive and difficult to construct and insulate."
Griffith says that shunt feed is a simple and effective alternative to
toroidal excitation.
rrac...@my-deja.com
> A math treatment of toroidal coupling to grounded towers is given by
B.
> Whitfield Griffith, Jr. in "Radio Electronic Transmission
Fundamentals",
> Library of Congress Catalog Card Number: 61-13756, pages 379-383.
>
You have cited what is, in my opinion, the best book ever written to
teach the underlying fundamentals of antennas, transmission lines, RF
networks, and propagation without getting into the level of math with
which they teach those subjects in BSEE programs. In fact, most EEs
would understand those subjects much better if they studied the book.
The book went out of print in the 1960s and has become increasingly
hard to find over the past 25 years. I have good news for anyone who
is interested in getting a copy, though. A reprint edition has just
come out. It is available from:
Pardon me for straying off topic. I wanted to share the news. I
arranged for the reprint edition and traded the publisher my clean copy
for that purpose, but have no financial interest in the project.
Oh yes, I have always been kinda fascinated with Griffith's treatment
of toroid feeding of an antenna.
Ronald Rackley, PE
Sarasota, Florida
AD4WG
Sent via Deja.com http://www.deja.com/
Before you buy.
Richard Harrison
"I have good news for anyone who is interested in getting a copy
has just come out. It is available from:
That is good news! I had no idea. Griffith is probably one of the best
explainers that ever was. It is a pity that this book ever went out of
print.
Thank you Ronald for helping a reprint get published. I, too, have no
financial interest or even acquaintance with the principals in this
venture.
CAM
> That is good news! I had no idea. Griffith is probably one of the best
> explainers that ever was. It is a pity that this book ever went out of
> print.
Is it worth 9% of my social security check?
--
http://www.mindspring.com/~w6rca
Richard Harrison
"Is it worth 9% of my social security check?"
Griffith is good but value is in the eye of the beholder, they say.
40 years passed while I looked for a cheap copy of Laport`s "Antenna
Engineering" to replace the copy I left behind on Tierra del Fuego.
Before paying a high price for a book, I`d check "Half-Price Books",
etc.