Jim and the list. This is a topic near and dear to me. I live in a house
surrounded by forest - mostly oak, poplar and maple with a couple pines
scattered about. I have a 90 foot tower, the top of which was above tree
top twelve years ago when I installed it. The clearing that it is in is
just big enough to get the tower in. I have often wondered (and never
gotten an answer ) to the question "How does the forest affect the radiated
signal?" I have 40M through 10M on the tower - horizontal beams (dipole for
40M). The direct signals from the antennas are radiated without problem
above the trees. The reflected path goes down to earth through the trees,
is reflected, and goes back out through the trees, to be combined with the
direct signal in the far field. To what degree is the reflected signal
attenuated? This will obviously affect the over-all antenna pattern,
probably with reduced lobes and nulls. How?
I am working toward putting the tower on the air on 160M. I'll be dealing
with the vegetation attenuation (we need a better word for this!). No
choice with the location I have.
Really appreciate all of the good, positive chatter on this reflector.
Wayne N1WR
--------------------------------------------------
From: "Jim Brown" <jim@audiosystemsgroup.com>
Sent: Tuesday, November 03, 2009 11:08 AM
To: "TopBand List" <topband@contesting.com>
Subject: Re: Topband: Improve efficiency?
> On Tue, 3 Nov 2009 10:45:48 -0500, Guy Olinger K2AV wrote:
>
>>Some interesting anecdotal-level experiments that suggest that there is no
>>great loss.
>
> Since I live in a dense redwood forest, I've done my best to learn as much
> as I can about this. While I agree with Guy as to the difficulty of
> learning anything solid, I don't think it's quite as rosy as W8JI's
> research suggests ON ALL BANDS.
>
> My take, which is the combination of my experience and that of my
> neighbors
> who think in solid engineering terms, is that losses due to trees:
>
> 1) Are greatest for vertically polarized antennas than for horizontal ones
>
> 2) Increase with frequency (although probably not linearly)
>
> 3) are no big deal for horizontal antennas at HF and 6M, start getting
> significant at 2M, are a big deal at 440MHz, and are a wipeout at 900 MHz.
> A colleague with considerable experience at UHF and above says, "at cell
> phone frequencies, think -3dB per tree." I can tell you that no one's cell
> phone works up here, and propagation to 440 repeaters is quite poor.
>
> 4) are probably not a big deal on 160M for vertical antennas on 160M, but
> may be significant at 40M and above. I have, for example, tried a 40M
> vertical over the same set of radials that I installed for 160M, and it
> was
> a complete waste of time. It's in the middle of big clearing (roughly 75
> ft
> radius).
>
> I have a vertical dipole for 40M hanging from a 170 ft redwood, with the
> feedpoint at about 80 ft. It is a block of wood compared to my horizontal
> dipoles at 120 ft. Part of that difference is predicted by the pattern of
> the antenna over ground, but my experience with it suggests that there is
> also some loss from the tree. I could be wrong, and one of the things on
> my
> "to do list" is to set up a second version of this antenna that is not
> close to a tree.
>
> 73,
>
> Jim Brown K9YC
>
>
> _______________________________________________
> 160 meters is a serious band, it should be treated with respect. - TF4M
_______________________________________________
160 meters is a serious band, it should be treated with respect. - TF4M
|