Quoting Tom Rauch <w8ji@contesting.com>:
>> That has been the belief in the broadcast industry for as long as I can
>> remember. Every presentation at NAB and IEEE I have ever listened to or
>> read seem to support the thesis that a 50x50 or 100X100 piece of
>> expanded copper mesh under a 90 degree tower AM broadcast tower
>> establishes the near field FSM normallty made at one mile from the
>> tower.
>
> That's interesting because Brown Lewis and Epstein concluded
> a screen does nothing with an adequate field of radials.
>
> One could also claim as the inverse "that with an adequate ground screen and
over good ground, burying wires in every direction does little to change the
near field FSM.
I was a international sales type for Gates Radio (Gates-Harris-Intertype Corp
at the time) out of Quincy. (My office was just across the hall from Gates
Radio founder Parker Gates.
Being interested in 160 I spent way to much time quizing the engineers in the
ATU and Antenna department to death on this. I was told of a company antenna
test plot the company had used with a 100X100 foot square ground screen and
120 1/4 radials laying on excellent farm country ground spaced a few degrees
apart. A remote FSM one mile away recorded the findings as each radial was
coiled up a laid at the edge of the ground screen. I remember design Engineer
Bob Brown telling me personally that after all radials radials were coild up
the reduction in the measurments were "insignificant". BTW I still use as
many long radials as I can here. However every radio station I have ever
worked at I found large screens of cooper mesh as standard equipment.
One of them, in the Virgin Islands, when the Consulting Engineer and I were
doing a proof we began looking for radials by holding a FSM upside down with
the probe close to the ground as sort of an RF metal detector. What we
learned was that almost all the radials were gone. We learned later that
local "Frenchtown" fisherman in St. Thomas had been pulling them up for years
to make fish traps! Still the FCC checkpoint at one mile was a few mv within
specs. This checkpoint was on near a pier at Crown Bay and the path to the
tower was mostly over sea water. So I guess my experience proves little
excpet that if you near an almost perfect ground the radials past the screen
may not imporve the near field that much. Similar findings may approach this
with excellent soil characteristics. On the flip side of that I would believe,
according to the NAB Engineering Handbook, that with very poor soil the 360
radials 1 degree apart, would be the best approach.
It is obvious I am not an engineer as my degree is in Geography and Music.
Yet my interest in what works best on 160 meters has always led me to inquire
on this topic.
73
Herb Schoenbohm
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