Re: NEC-LIST: short towers near tall trees

Grant:

The late Al Towne told me that a row of eucalyptus trees had to be
removed along a roadway adjacent to the transmitter site of the 1520
directional antenna in Port Hueneme, CA before the night pattern could
be adjusted to meet the minima requirements. As I'm sure you know, Al
was a very knowledgeable and distinguished engineer, so I would take him
at his word. Also, in tuning up the 1130 array in Detroit, Ron Rackley
and I discovered a lone deciduous tree in an otherwise totally grassy
area in a nearby park - where the H field was the equivalent of a few
hundred mV/m - which was a significant re-radiator, whose effect could
be discerned for at least several tens of meters. I've seen indications
of that sort of behavior in the past, but usually masked by other
possible effects due to all the other kinds of clutter that affect H
field measurements at MF. So I suspect that if the "reradiating" tree
were close enough - say less than 1/8 wavelength - to be a realistic
parasitic radiator it might generate enough field to produce a far-field
effect.

Some trees do have very substantial moisture content, and with the
dissolved constituents of sap should be pretty good conductors.

So I believe that under some conditions the answer to your question is yes.

Ben

Benj. F. Dawson III, P.E.
Hatfield & Dawson Consulting Engineers, LLC
9500 Greenwood Avenue North
Seattle, WA 98103 USA

GrantBingeman_at_cs.com wrote:
> I was wondering if anybody knows of any studies of the absorption of RF by
> trees within the near-field of electrically short towers operating at the high
> end of the AM broadcast band, for example, a 75 foot tower on 1610 kHz less
> than 50 feet from a 100 foot tall tree-line. Would this impair signal intensity
> at 10 miles, say.
>
> A recent question I could not immediately answer, in a similar vein, is "Can
> a tree close to a short antenna act as a lossy parasitic element, thus causing
> a directional radiation pattern?" Does anybody have any experience or
> insight with this type of behavior? Thanks. g
>
> p.s: I am familiar with the Viet Nam studies of RF propagation in the jungle.
>
> Grant W. Bingeman, P.E.
>
>

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