Loop Antenna Inter-action With Tree Branches
Niittymaa Family
city lot and fed with 450~ ladder-line through a tuner. I have a couple of
options:
1) Hang a maximum 40M loop in the clear - sloping but at least 2-3 feet from
any branches or metal siding, etc. ranging from 35' at a TV tower feed point
to 20' at the lowest point
or
2) Snake a much larger loop through a couple of maple trees and branches fed
at the same 35' point and averaging about 20-25 feet up, perhaps close to a
80M loop size. Some articles I've read say that snaking across tree limbs
can be used to max out the size without operational impacts.
I want to use the loop on 160M (hoping) but mainly 80M through 10M and would
like to know if snaking through the trees will impact the loop operations on
any of the bands. Will the impact be greater the higher the frequency and
if so, at what band do the branches start having an appreciable impact?
Another option might be to build the 2 loops one inside of the other and fed
with separate feeders but this might be stretching the optics with the XYL?
Any thoughts? ...... Al VA3KAI
Mark Keith
> I want to use the loop on 160M (hoping) but mainly 80M through 10M and would
> like to know if snaking through the trees will impact the loop operations on
> any of the bands.
No. Many of my wire antennas are snaked through trees. Trees are very
handy for antenna supports. All hams should plant a tree on arbor day.
Preferably one that grows to be a tall one. :/ MK
Dick Carroll;
Niittymaa Family wrote:
> Some articles I've read say that snaking across tree limbs
> can be used to max out the size without operational impacts.
Maybe, maybe not. Depends on whether or not you wind up with a high impedance
(thus high voltage) point of the loop touching a tree branch, leaf or twig. If
so, expect some deletorious effect. And when it rains,
or if you live in a damp, foggy area or anywhere condensation is common, all
bets are off. In short, some of the time, and on some bands, you probably won't
notice effects from the tree. Other times, on other bands, you probably will.
If I planned to install an antenna that might touch any part of a tree I'd make
sure that part of the antenna wire was insulated with tough insulation of such
quality that the abrasive action of limbs or twigs wouldn't soon wear through
the insulation.That should lessen the effects of the tree touching it but
probably not eliminate them, at least all the time.
I've used screw-in insulators to a trunk or limb and ran the wire through the
"eye" of the insulator and let it slide back and forth as the tree moves. Expect
to do maintanence since the wire will wear as time passes. Use your ingenuity to
minimize the wear on the wire.
There is capacitance involved, too. An antenna wire lying on a limb or leaf
that contains moisture can not only change the tuning of the antenna but also
induce some RF loss, even if there is no direct connection. If the leaf or limb
is wet the ante goes up.
But the only way for you to know for sure how this is going to work is to try
it.You can always work to imrove things if you don't like the way it works out.
Dick.
Bill Turner
$IR1.7...@news20.bellglobal.com:
> Some articles I've read say that snaking across tree limbs
> can be used to max out the size without operational impacts.
>
________________________________________________________________
Trees have been set on fire by arcing from antennas run through them. A
few watts would probably not be a problem, but be careful.
--
Bill, W7TI
Brian Kelly
> I want to build a horizontal delta loop (~hex-shape) contained within my
> city lot and fed with 450~ ladder-line through a tuner. I have a couple of
> options:
>
> 1) Hang a maximum 40M loop in the clear - sloping but at least 2-3 feet from
> any branches or metal siding, etc. ranging from 35' at a TV tower feed point
> to 20' at the lowest point
>
> or
>
> 2) Snake a much larger loop through a couple of maple trees and branches fed
> at the same 35' point and averaging about 20-25 feet up, perhaps close to a
> 80M loop size. Some articles I've read say that snaking across tree limbs
> can be used to max out the size without operational impacts.
>
> I want to use the loop on 160M (hoping) but mainly 80M through 10M and would
> like to know if snaking through the trees will impact the loop operations on
> any of the bands. Will the impact be greater the higher the frequency and
> if so, at what band do the branches start having an appreciable impact?
The U.S, Army conducted some investigations into this subject a number
of years ago which were discussed in this NG earlier. The answer is
that trees do not absorb RF, leak RF or otherwise affect antenna
performance at frequencies below 30 mHz. There are absorbtion effects
which increase as the freqs go above 30 mHz.
The much bigger problems are mechanical in nature and come in two
flavors. The bigger is the wind-induced relative motions between the
trees. Say you have a 100 foot run of wire between trees A and B. A
blow comes along and tree A deflects your support point five feet West
and tree B only deflects two feet at it's wire support. Not all trees
were created equal. Now the wire has to suddenly be two feet longer
than it was before the wind came along or 'bye-bye loop. The effect
can be much worse in the typical swirling winds found at ground
levels, e.g., tree A bends five feet west and tree B bends two feet
East. Times maybe four trees in some number of permutations. Been
there, done it, ugly. The cure for this problem is to support two
opposing corners of the loop via the pulleys and long-stroke weights
method.
The other mechanical problem is abrasion between the wire and the
branches, etc. This one can be minimized by using wire which has some
particularly abrasion-resistant insulation or another. I've had good
luck with using common hardware store Type THHN stranded "house wire"
run thru trees. The Wireman offers some virtually bulletproof coated
antenna wires in his "Toughcoat" line.
>
> Another option might be to build the 2 loops one inside of the other and fed
> with separate feeders but this might be stretching the optics with the XYL?
There are no (legal) cures for XYL/antenna "interactions".
>
> Any thoughts? ...... Al VA3KAI
w3rv
'Doc
As has been already stated, there will be 'problems'
with running the wire through trees. I don't have the
slightest idea how much inductance, capacitance, and/or
resistance a tree represents, and I doubt if anyone else
does either. That makes guessing what will happen when
you run the antenna through a tree impossible, with any
accuracy.
There have been a bunch of loop antennas erected that
way, though, and in most cases they work just fine. I've
had two loop antennas run through trees that worked on
multiple bands. Sometimes not very well, but certainly
better than not having an antenna for that particular
band, and suprizingly well on other bands!
The 'short and sweet' of it is that you just won't know
until you try it, and I certainly would try it. I'd also
'aim' for the largest loop I could get. (Higher is better
than lower, too.) I'd let the 'straight' sections sort of
sag a bit so that there's some slack for when the trees
move. 'THHN' insulated wire seems to last a reasonable
length of time, but I'm sure there are 'better' types of
insulation that can be had.
Because of all the variables involved, you won't know
how well it will work until you try it. So, try it...
'Doc
Roy Lewallen
>
> The U.S, Army conducted some investigations into this subject a number
> of years ago which were discussed in this NG earlier. The answer is
> that trees do not absorb RF, leak RF or otherwise affect antenna
> performance at frequencies below 30 mHz. There are absorbtion effects
> which increase as the freqs go above 30 mHz.
I'd like very much to see any published work which shows this. On one
occasion some years ago, a friend living one mile away measured 10 dB
difference between two identical 40 meter verticals spaced a half
wavelength apart (with the inactive one open circuited at the base, of
course), in my back yard. The only apparent difference was that the path
from one vertical included a stand of a half dozen or so Douglas fir
trees which were roughly a quarter wavelength high. The same magnitude
difference was observed in listening to other signals from that same
direction -- and only from that direction. Moving the "dead" vertical so
that the path shot through the edge of the trees dropped the difference
to 4 dB. I was and remain convinced that the attenuation was due to the
trees. However, I don't have any more information -- the importance of
tree type, height, time of year, etc. If studies showed no absorption,
I'd like to see how the setups used in the study differed from my
experience.
Roy Lewallen, W7EL
Ian White, G3SEK
So what effect do the trees have on the four-square's performance as a
beam?
(Roy pointed to the trees from his deck, and said there was a
four-square in there somewhere...)
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
K9SQG
book, forget which one but I have the info). The attenuation is primarily
limited to vertical antennas, in the HF range, mounted "close" to the trees.
The same is NOT true for horizontal antennas with nearby trees.
73s,
Evan
Roy Lewallen
>
> So what effect do the trees have on the four-square's performance as a
> beam?
>
> (Roy pointed to the trees from his deck, and said there was a
> four-square in there somewhere...)
The answer is that I don't know -- I don't have another array to compare
it with, or any way to get quantitative data on the directional pattern.
The two verticals involved in the attenuation measurement were
constructed when I first began experimenting with phased arrays many
years ago. I wanted to get a feel for the directionality of the array,
so asked my friend to make some field strength measurements. (He's
capable of making these with reasonable accuracy.) As a baseline, I
excited each antenna in turn with a QRP transmitter. Imagine my surprise
when the field strengths were different by 10 dB! Incidentally, the
feedpoint impedances of the two verticals were very nearly the same, and
I monitored the transmitter's output power to insure that the same power
was delivered to each vertical. The measurements were repeatable, and I
could easily hear and measure a comparable difference between the
antennas on signals coming from the direction of my friend's house (in
the general direction of California).
Anyway, the 4 square (for 40 meters) was put in some time later. One
reason you couldn't see the array very well is that there's a sequoia
tree in the middle of it. I was obviously concerned about that when I
built the array, but couldn't find any obvious effect on the mutual
impedances between elements which had the tree between. The array
directionality was quite good, and I made a lot of QSOs with it,
including quite a few VK and other south Pacific stations while running
1.5 watts. I also worked and confirmed all 50 states and 33 other
countries running 1.5 watts with either that array, a single vertical,
or dipole. So although the sequoia might have had an effect, I'm pretty
sure it wasn't on the order of 10 dB.
As for the stand of firs which produced the attenuation in the
two-vertical experiment, they're just about SW of the 4 square. About
all that's down that way from here is ZL, and I don't frankly recall
whether I did work ZL with that array and my QRP rig. The trees had
grown substantially between the time I made the attenuation measurements
and when I built the 4 square.
Although the 4 square is still standing, the phasing box fell apart, and
I haven't replaced it or used the array for quite a few years now. The
small sequoia is now about 60 feet high, and the firs, which were about
30 feet high during the vertical experiments, are now 70 - 80 feet.
There are a lot of potential variables involved in this absorption
issue, and I have only one data point. It has convinced me that trees
*can* produce a very substantial attenuation at HF. But only under
certain circumstances, and I just don't know what those circumstances are.
Roy Lewallen, W7EL
Dick Carroll;
Brian Kelly wrote:
>
> The U.S, Army conducted some investigations into this subject a number
> of years ago which were discussed in this NG earlier. The answer is
> that trees do not absorb RF, leak RF or otherwise affect antenna
> performance at frequencies below 30 mHz.
I gotta wonder what time of year they did the tests, and it seems like it must have been in the dead of winter when
the tree was dormant and essentially sap-less. I know that you can sure cause sparks and spitzen if you happen to
transmit with amplifier power into an antenna -at least some portions thereof-
touching a well-sapped twig or leaf. And when it rains, or if you live in a
drippy climate, there's no way I'd believe there wouldn't be adverse effects
on the radiated signal when in contact with any part of a tree.
I took down my 160 meter dipole because the XYL's mimosa tree grew so tall it overtook the dipole. And here you are
trying to tell me I did that unnecessarily! :-)
Dick
Dick Carroll;
Roy Lewallen wrote:
Seems I recall something about the Army trying to use trees for antenna
transmitting elements. IIRC that didn't work out too well, but it must have
been thought to have been a possibility or they wouldn't have tested it.
Dick
Dick Carroll;
'Doc wrote:
> Al,
> As has been already stated, there will be 'problems'
> with running the wire through trees.
Just to clarify, running a dipole through trees *without the elements
touching* any part of the tree is an entirely different matter than
running it through leaves, branches, twigs where the antenna element
*does* touch.
I almost always wind up running my dipoles through trees, but am
careful to trim the tree so that it can't touch the antenna.
Dick
Brian Kelly
We've been here before Roy. As you might recall there was an thread in
this NG in late Oct '00 "Do Trees Absorb RF?". There were a bunch of
opinions tossed out but the bottom line as far as actual studies on
the topic are concerned it got down to some work done by the Army in
the WW2 era. To wit:
===========
"I recall seeing a study done by the US military on HF. As I recall,
when I made
some assumptions about the spacing between trees and their figures for
the
attenuation in dB/mile of deducious forest, I estimated about 0.01
dB/tree.
73,
Ed Hare, W1RFI"
===========
>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 (2000-10-22 07:45:47 PST)"
=========
From these I've concluded that it's safe to assume and state that the
performance of a *horizontal* loop will not be measurably degraded by
trees used as supports. I stand spanked for being too general and not
including a caveat about the problem of verticals being affected by
trees.
Somewhere in the dusty U.S. Army R&D archives lies The Documentation .
.
>
> Roy Lewallen, W7EL
w3rv
'Doc
Dick,
I don't worry about it a whole lot. I figure sooner
or later the antenna will either wear the leaf/twig/etc.
off, or catch it on fire. Either way, it won't be a
problem any more. Until that happens it's a simple matter
to tweek the tuner to take care of it.
'Doc
PS - Never saw an antenna catch a tree on fire yet. I'm
still hoping though...
Roy Lewallen
I also would be very surprised to find any significant attenuation
caused at HF by trees to horizontally polarized signals. And I suspect
that even vertically polarized signals are attenuated only under
specific and probably unusual conditions. But I am convinced that trees
can cause very substantial attenuation of vertically polarized signals
under some conditions. I just wish I had a better idea of just what
those conditions are.
Roy Lewallen, W7EL
Brian Kelly wrote:
> . . .
Mark Keith
I do the same. But I have had many that actually touched branches,
etc. I've never had a problem with this as long as the tree wasn't
wet. And if you use insulated wire, it's usually not a problem. After
time, the insulation will be worn off by tree movement in places, so
it's good to take a look at the antenna every once in a while. I've
never noticed any kind of problems running 100w over many years. Only
when I cranked the power up, did I ever see arcing , and it was always
at the ends of the dipoles. I can't even count the number of times
I've simply chunked wire over tree branches for a portable dipole.
Myself, I never worry about tree effects. Even soaking wet is rarely a
problem being I use insulated house wire. But I'm only talking about
obvious "operating" issues as far as wires through trees. I've never
dealt much with any attenuation issues. I had always assumed this was
more a problem at higher freq's, "VHF/UHF" and the more leaves and
foliage, the worse the problem. I've never really worried about it on
HF. Not much I could do about it anyway...:/ MK
KC1DI
Hi Al,
Go for the biggest loop you can it will work great on 80 through 10 ,
I would not expect an 80m loop to do much on 160m though if 160 is
your goal. you'd be better off with an inverted L for that band.
I have a 160m loop up here that's 520 feet around and at an average
height of about 30 feet or so it slopes a little as on support is a 50
feet and all the rest at about 30 feet , but seems to work great on 80
through 10 m.. on the upper bands say 20-- 10m i also have a vertical
to fill in some of the pattern nulls on those bands. my yard is
surrounded by trees and I find no adverse effect of running the
antenna through them.. I would us insulated wire however were ever the
trees might touch the wire.
Good luck
73 Cave kc1di
Bill Turner
> PS - Never saw an antenna catch a tree on fire yet. I'm
> still hoping though...
>
________________________________________________________________
The following was posted on rec.radio.amateur.antenna on June 25, 2001:
-----
Amen brother! I burnt down my house one time because one end of my
inverted Vee had broken loose and dropped down into one of the roof
valleys where there was some metal flashing. I was tuning up a 4-1000
amp and noticed something wasn't right. About ten minutes later the
whole roof went up. I mean big time! Use 'good' insulators and check
the thing every once in awhile. It made a believer out of me!
de Jerry -n6jp-
-----
So keep hoping, Doc. Your time will come. :-)
--
Bill, W7TI
Dick Carroll;
Bill Turner wrote:
> 'Doc <w5...@cwis.net> wrote in news:3EE2C989...@cwis.net:
>
> > PS - Never saw an antenna catch a tree on fire yet. I'm
> > still hoping though...
> >
> ________________________________________________________________
>
> The following was posted on rec.radio.amateur.antenna on June 25, 2001:
>
> -----
>
> Amen brother! I burnt down my house one time because one end of my
> inverted Vee had broken loose and dropped down into one of the roof
> valleys where there was some metal flashing. I was tuning up a 4-1000
> amp and noticed something wasn't right. About ten minutes later the
> whole roof went up. I mean big time! Use 'good' insulators and check
> the thing every once in awhile. It made a believer out of me!
I recall reading years ago where a ham in Kansas City set his house afire
while on the air using an attic antenna fed by his Johnson Valiant on AM
running around 150 watts. No details on the antenna installation were given
but you can be sure it was a poorly installed antenna and not properly
insulated from the house structure. Seems most important to keep the
antenna well away from anything that could possibly cause an arc.
It's something to keep in mind. Such stuff can be avoided with the proper
exercise of care. And it's not something one would automatically think of.
Dick
Brian Kelly
> . . . But I am convinced that trees
> can cause very substantial attenuation of vertically polarized signals
> under some conditions. I just wish I had a better idea of just what
> those conditions are.
Probably boils down to too many unmeasurable variables to ever be able
to make any sense of it by the numbers. Calibrated trees . . . ?
>
> Roy Lewallen, W7EL
w3rv
David or Jo Anne Ryeburn
<el...@gwi.net> wrote:
> Go for the biggest loop you can it will work great on 80 through 10 ,
> I would not expect an 80m loop to do much on 160m though if 160 is
> your goal. you'd be better off with an inverted L for that band.
I don't see why a loop one half wavelength in circumference on 160 metres
would radiate all that badly. It seems to me it wouldn't be that much
worse than a half-wave dipole at the same (low) height. Of course it would
have a very high feedpoint impedance and using coax to feed it would not
be a good idea.
The inverted L, if only 1/4 wavelength in overall length, would require a
good ground system. A balanced high impedance load such as a one half
wavelength loop could well have lower overall losses. Perhaps someone with
EZNEC could compare the two choices. I suspect for NVI, the loop would be
a much better antenna.
David, ex-W8EZE
--
David or Jo Anne Ryeburn
rye...@sfu.caz
To send e-mail, remove the letter "z" from this address.
W5DXP
> I don't see why a loop one half wavelength in circumference on 160 metres
> would radiate all that badly.
It's the matching that is the problem. It has a feedpoint impedance
of thousands of ohms, like a one-wavelength dipole. A 1/2WL closed
loop should be fed with open-wire transmission line. EZNEC says a
1/2WL closed loop fed with 50 ohm coax will have an SWR greater
than 100:1. Not good, even on 160m.
--
73, Cecil http://www.qsl.net/w5dxp
-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 80,000 Newsgroups - 16 Different Servers! =-----
David or Jo Anne Ryeburn
> David or Jo Anne Ryeburn wrote:
> > I don't see why a loop one half wavelength in circumference on 160 metres
> > would radiate all that badly.
>
> It's the matching that is the problem. It has a feedpoint impedance
> of thousands of ohms, like a one-wavelength dipole. A 1/2WL closed
> loop should be fed with open-wire transmission line. EZNEC says a
> 1/2WL closed loop fed with 50 ohm coax will have an SWR greater
> than 100:1. Not good, even on 160m.
I realize that. I also wrote:
> Of course it would have a very high feedpoint impedance and using coax
> to feed it would not be a good idea.
I assumed that anyone trying this would use open wire to feed it. The
question remains: once power is gotten into the thing efficiently, and I
believe that could be done using open wire feedline, what would its
radiation pattern look like and how would it compare with that of a
half-wave dipole at the same height? My guess is that it would be more
nearly omnidirectional than the dipole, and thus would have slightly less
gain in its best direction than the dipole would have in the dipole's best
direction (and slightly more gain in its worst direction than the dipole
would have in the dipole's worst direction), all measured at high
elevation angles of course. Anyone using either antenna for low elevation
angle work should be doing so only because the alternative (a vertical
with a good ground system) is impractical.
David, ex-W8EZE
--
David or Jo Anne Ryeburn
'Doc
Bill,
Thanks! In 20 years as a professional firefighter
I've never seen a tree burn because of RF. Sooner or
later I'll get lucky...
'Doc
J. McLaughlin
touches to a book, I investigated just this issue. I used an octagonal
loop fed in the middle of one of the eight legs and having a perimeter
of 12 meters as a test antenna of resonance and "anti-resonance." Here
is what I sent to him:
Conditions: A loop in free-space with a perimeter of 12 meters
consisting of #10 loss-less wire.
Below 12.2454 MHz the reactive part of the impedance is inductive.
Resonance at 12.2454 MHz (0.5 WL) with the real part of Z = 72310
ohms
Between 12.2454 MHz and 26.162 MHz the reactive part of the
impedance is capacitive.
Resonance at 26.162 MHz (1.0 WL) with the real part of Z = 135.6
ohms
Between 26.162 MHz and 37.699 MHz the reactive part of the impedance
is inductive.
Resonance at 37.699 MHz (1.5 WL) with the real part of Z = 3782
ohms.
Note that I made no effort to investigate sensitivity to segments or the
like. I also commented to him that it would be difficult to couple
power into the loop at its 0.5 WL resonance (or anti-resonance).
Just a data point. 73 Mac N8TT
--
J. Mc Laughlin - Michigan USA
Home: J...@Power-Net.Net
Richard Harrison
"The attenuation is primarily limited to vertical antennas in the HF
range, mounted "close" to trees. The same is NOT true for horizontal
antennas with nearby trees."
"Electrical Communication Systems Engineering". War Department TM
11-486, 25 April 1945 agrees. It says on page 241:
"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 when using vertical
polarization is 3 or 4 db for frequencies in the 20 to 40 MHz band and
10 db in the 70 to 100 MHz band. With horizontal polarization, losses
are negligible at 20 to 40 MHz and quite small at 70 to 100 MHz."
Dense jungle is much worse, says The War Department.
Yuri Blanarovich
some physical properties making it look like some kind of conductor (even poor)
can act as a "semi-wire" and participate in the antenna's performance. Hence
trees are good candidate especially for the vertically polarized antennas. To
what extent, it is fuzzy (and tough to model) but measurable as some of the
examples stated. Tree can also be used as a (poor) antenna by properly coupling
RF to it. The "quality" of the composition (tree material) and electrical
length (yes Virginia) would determine its "performance" and participation in
the vicinity of the array. If it got water and something that can make it
(semi)conducting, then it will show up in the antenna's performance. It might
be a good idea to plant the vertical arrays on the front of the forest for
better F/B performance. The tress would "eat" the back lobe. Trees can be used
wisely to enhance or hinder antenna performance.
Heck, I can make RF conductor out of my hand - I arch the hand so I touch my
nose, inductively (small loop) couple RF to my "hand loop" and by lightly
touching my nose I can create the burning sensation, or by pressing harder, I
can feel the heat in the junction. Just don't report me to RF police for
overdosing myself with RF exposure! (For F.Art, Nathan, Phil et al - I did it
before "regulations" were in effect.)
Yuri, K3BUrn
J. Harvey
> Heck, I can make RF conductor out of my hand -
> I arch the hand so I touch my nose, inductively
> (small loop) couple RF to my "hand loop" and
> by lightly touching my nose I can create the
> burning sensation, or by pressing harder, I
> can feel the heat in the junction.
So you really are loopy ? ;-) !!
Jeff
VE1BLL (/VE3 for just a bit longer).
Mark Keith
> In article <ijv5evcloforncih9...@4ax.com>, KC1DI
> <el...@gwi.net> wrote:
>
> > Go for the biggest loop you can it will work great on 80 through 10 ,
> > I would not expect an 80m loop to do much on 160m though if 160 is
> > your goal. you'd be better off with an inverted L for that band.
>
> I don't see why a loop one half wavelength in circumference on 160 metres
> would radiate all that badly. It seems to me it wouldn't be that much
> worse than a half-wave dipole at the same (low) height. Of course it would
> have a very high feedpoint impedance and using coax to feed it would not
> be a good idea.
Most of the people I know that do that, split the loop at the far end
for 160m use, and run exactly as you mention. Like a folded back
dipole. Works fairly well for NVIS. On 80m, they connect it back up.
MK
Dick Carroll;
'Doc wrote:
Anyone who thinks that green stuff won't burn hasn't spent much time out
west.....
I haven't seen a tree set on fire by RF eitner, and I don't plan to, at
least via my own antenna installations.
Dick Carroll;
Brian Kelly wrote:
To me the logic goes that trees contain water and if water has effects on open wire feedline
there's no reason for it to act differently on an antenna that comes in close contact with
part of a tree containing water, particularily when moisture content is high.
Dick
Richard Harrison
"Most of the people I know that do that (operate an 80-mtr full-wave
loop on the160-mtr band where its perimeter is only 1/2-wave), split the
loop at the far end for 160m use, and run exactly as you mention."
That brings the drivepoint resistance down.
Arnold B. Bailey gives formulas in "TV and Other Receiving Antennas" on
pages 407 and 408. He gives graphs on 408 and 409.
A 1/2-wave in circumference continuous loop is kin to a 1/4-wave short
circuit stub. It has a high drivepoint impedance.
A 1/2-wave in circumference open-circuit loop is kin to a 1/4-wave
open-circuit stub and has a low value of resistance at its terminals.
Brian Kelly
There is no way to get me even close to a discussion involving XYLs
and antennas, never mind a discussion which also includes her pet
tree.
??!
>
> Dick
w3rv
Roy Lewallen
actually seeing that addresses HF, even though it mentions only the
upper HF range and doesn't give any real information about the makeup of
the forest. Although apparently some more work was done during the
Vietnam war, references and specific information are elusive.
Roy Lewallen, W7EL
Richard Clark
wrote:
>Thanks very much for the reference. It's the only one I can recall
>actually seeing that addresses HF, even though it mentions only the
>upper HF range and doesn't give any real information about the makeup of
>the forest. Although apparently some more work was done during the
>Vietnam war, references and specific information are elusive.
>
>Roy Lewallen, W7EL
>
Hi Roy,
Another reference is found in with one I share with Richard (we appear
to be the only two to have copies that I suggested to him in private
correspondence in a e-mail round robin with Walt many summers ago):
Arnold B. Bailey's "TV and Other Receiving Antennas"
Chapter "The Radio Path"
Section "Effect of Wooded Areas on Signal Strength"
Subsection "Horizontal versus Vertical Polarization"
which itself runs to three pages of discussion compared to the 83
pages of the chapter alone and two pages enumerating 47 references.
Discussion within these pages are generally confined to VHF and above,
but a good amount of data encompasses HF as a consequence of his
inclusive and approachable style. I would say that nearly half of his
44 figures in this chapter alone are charts of practical application.
One that is especially bookmarked by myself is Figure 5-13 - "Plot of
the electrical impedance of earth to electromagnetic was as a function
of the frequency" This figure spans from 100KHz to 10GHz for
intrinsic Z over a range of 1 to 1000 Ohms for Air/Vacumm (the
reference), Dry Soil, Wet Soil, Fresh Water, Ocean Water.
His Chapter 6 "The Theory of Signal Interception" runs 63 pages before
even introducing the simple dipole (which is Chapter 7 at 87 pages
long). Bailey's style is very readable and is not given over to the
numbing drone of dry math.
As a reference, it should be the first book read on the subject of
antennas and propagation.
73's
Richard Clark, KB7QHC