On 4/9/14 7:59 AM, Roger (K8RI) on TT wrote:
On 4/9/2014 10:04 AM, Brian Alsop wrote:
I'm not sure one would ever notice the tree effects on HF. There are
lots of other things (e.g., no balun at the feedpoint, feedline
routing and objects in the near field, ground terrain variations )
that would probably be larger effects.
I have Satellite, and off the air TV. Several trees grew into the
"line-of-sight" between the antenna and UHF station. It's not true that
with digital you either receive, or don't receive the signal. The range
between signal and no signal is much narrower than it was for VHF
analog, but there is a range. We noticed that on occasion the signal
would "pixelate" and pause.
http://en.wikipedia.org/wiki/Eb/N0
Coding makes the slope steeper
typically, with coding, there's about a 1-2dB range where it goes from
"no significant errors" (1E-7) to "a few errors" (1E-4) to "too many
errors" (1E-2)
Sort of like on FM where you go from unreadable to noisy to full
quieting, but steeper.
and nothing like AM/SSB where there's a nice gradual transition in
intelligibility over 5-10 dB.
Any vegetation seems to block the satellite signal. Which is much
higher in frequency. The same goes for wireless Internet.
Satellite is Ku-band, at say 12 GHz. the dB/meter for trees is pretty
high, so any trees is enough to drop you 10 dB, and that's probably your
entire margin.
Wireless internet is (depending on the provider) probably single digit
GHz. Not as bad as 12 GHz, but still several dB/meter with link margins
in the 6-10 dB range..
It's surprising how far away an antenna for 75 and 40 can be from others
and still show effects even in SWR and resonant point although they were
apparently not sensitive to trees.
I did a bunch of modeling of trees against vertical antennas..
Trees are going to change the Z (dielectric loading, etc.), but they're
not very lossy in the big sense. If you think about a 1 foot diameter
tree trunk 10 feet from a vertical, the tree is only intercepting a few
percent of the field (63 feet circumference for a 10 ft radius circle),
so no matter how lossy it is, the far field just isn't going to see a
big change, and likewise, there's just not that much coupling. It's not
like a resonant wire 10 ft away.
A 40 meter vertical antenna (AV640)displayed shifts in the resonant
frequency when slopers were erected several wavelengths distant. (50 to
100 KHz)
The base of the AV640 is about 25 feet above ground.and 50-75 feet from
the trees.
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