On Mon, Dec 31, 2012 at 1:12 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
> My take on the issue, which seems to agree with what science I've seen
> published on the topic, is that vegetation, including tall trees, has little
> if any effect at HF, but begins to show attenuation in the VHF range that
> becomes increasingly significant with increasing frequency. No cell phones
> work up here in the mountains, and I see a lot of attenuation of UHF
> broadcast signals.
...[snip]...
> My 160M antennas are all verticals with the best radial systems I can do, and
> I just worked #130 on Topband since moving here seven years ago. All my other
> antennas are horizontal, all surrounded by the trees, and at least 60 ft
> below the top of those trees, and I just worked #301. So I'd say from an MF
> and HF point of view, consider them a blessing -- organic antenna supports --
> and don't waste much effort above 100 MHz.
A lot of this needs to be put onto a scale. To a cellphone guy, losing
a TOTAL of 3 dB to trees would operationally NO loss. To a ham putting
up an L next to a tree, and losing 3 dB, and knowing it, would be a
different matter.
Cellphones are in the order of magnitude frequency range of microwave
ovens, whose frequency is chosen to maximize excitation of water
molecules. Dielectric loss to trees and leaves for high UHF channels
and cellphones could hardly be worse. On the extreme other end trees
can still modify results on 160 meters, so there is a continuum of
loss.
Probably the biggest modifier of tree loss on HF is the switch to
horizontal polarization, which takes the trees out of it, rather than
there being some vast hole in how trees respond to RF at HF. I've
heard enough stories about someone putting up a 40 meter four square
in the woods, poor results, then beaten by an inverted vee hung in the
same woods.
I recently had a tree man on my grounds to take down some lightning
struck trees gone dangerous, and took advantage of the occasion to cut
down some fast-growing sweet gums reaching for the sunlight next to my
100' loblolly antenna tree for supporting the bend in my 160 3/8 L.
Unfortunately the side of the loblolly they were growing on was the
wire stringing side, and they had grown up to the point that the
vertical part of the L was getting snagged in the branches in a wind
storm, sometimes tangling the support ropes as well.
After the three sweet gums were felled, my daytime signal strength
went up ~ 3 dB at W4KAZ, 8 miles away in Cary, NC, a change which has
persisted. The tuned resonance of the L went up from 1838 to 1850.
There were no changes in antenna deployment, height, etc. Only change
was that three sweet gums growing up alongside and into the vertical
wire were removed. Leaves were already down for some weeks. Only
wood was removed.
All of this was just the removal of a dielectric material very close
to the wire, Removal of the dielectric would be expected to raise the
velocity factor on the wire, making the antenna electrically shorter,
and sending resonance higher.
One could dicker about the 3 dB change at W4KAZ, but the 12 kHz change
in resonance is an easily measured fact, in this case only explained
by removal of dielectric loss in the close field. Going up, the
vertical wire varies 20 feet to 10 feet from the supporting pine
trunk, same as it was before the sweet gums were felled.
Due to the 3/8 length, the current max was well up the vertical wire,
basically in the area where the sweet gums were interfering. This
probably maximized the effect.
There also remains the nagging idea that some species of tree could be
a lot worse than others for dielectric loss. I cast one vote for sweet
gums on the list of lossiest trees.
73, Guy
On Mon, Dec 31, 2012 at 1:12 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> On 12/31/2012 8:02 AM, Wayne Willenberg wrote:
>>
>> The problem I may have is my house is located on about 21/2 acres of
>> landscaped land that is surrounded by a forest in the remainder of the 5
>> acres.
>
>
> Hi Wayne,
>
> My QTH in the Santa Cruz Mountains is in a very dense redwood forest, and
> I've tried to learn as much as I can about how they affect my antennas. I've
> operated all bands from 160M to 450 MHz, an FM talkie on 1.2 GHz, and use a
> cell phone in the 800 MHz and 1.8 GHz range, and just bought a couple of 900
> MHz Motorola radios that I'm about to fire up on a ham repeater system.
>
> My take on the issue, which seems to agree with what science I've seen
> published on the topic, is that vegetation, including tall trees, has little
> if any effect at HF, but begins to show attenuation in the VHF range that
> becomes increasingly significant with increasing frequency. No cell phones
> work up here in the mountains, and I see a lot of attenuation of UHF
> broadcast signals. Jim Lux, an EE who works at JPL and is a contributor to
> the ARRL Handbook, has done some serious literature searches, and come up
> with similar conclusions. Google searches will, for example, find extensive
> published work on attenuation at VHF and UHF in jungles, and I've seen
> believable anecdotal comments from guys installing cell phone networks in
> Georgia pine forests indicating strong attenuation at those frequencies.
> When I moved here, WA6NMF, my neighbor and colleague on the AES Standards
> Committee a very serious engineer who has been building VHF and UHF systems
> for most of his adult life
, said to me, "Jim, at cell phone frequencies, think -3dB per tree."
>
> My 160M antennas are all verticals with the best radial systems I can do, and
> I just worked #130 on Topband since moving here seven years ago. All my other
> antennas are horizontal, all surrounded by the trees, and at least 60 ft
> below the top of those trees, and I just worked #301. So I'd say from an MF
> and HF point of view, consider them a blessing -- organic antenna supports --
> and don't waste much effort above 100 MHz.
>
> 73,
>
> _______________________________________________
> Stew Perry Topband Distance Challenge coming on December 29th.
_______________________________________________
Stew Perry Topband Distance Challenge coming on December 29th.
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