Patrick
A note on some past modeling in this arena...
A few months ago, a fellow ham wanted to shunt feed a tower
and put up elevated radials in a space too small for 1/4-wave
straight radials. I took a VERY QUICK look at three different
types (all 8 feet abover ground) --
1) A bunch of short radials (about 1/8 wave long)
2) a grid or screen of radials and interconnecting wires,
occupying about 1/4 x 1/4 wave square space with the
vertical in the middle.
3) four 1/4-wave radials bent or "spiraled" to fit available space.
If the NEC-2 models are to be believed, the spiraled 1/4-wave
radials were somewhat better (>1 dB) than either the grid
or the short radials. When you add the fact that additional
inductive loading is needed to obtain a match with the short
radials and grid, the extra loss makes the spiral radials
look even better. The spiral radials system was about 1.5 dB
down from four straight 1/4-wave radials, not too bad for
half the diameter.
The spiral radials need to be longer than a free-space quarter
wave to make them resonant, since they are bent and current
cancellation occurs.
It also appears that the spiral concept applies to top loading,
as well -- getting more effective loading in the same space as
a conventional top hat.
Like I said, this was a really quick analysis, done just for the
curiosity of the moment. It may be worth taking a closer look.
73, Gary, K9AY
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From: Wintheiser, Patrick[SMTP:Patrick.Wintheiser@PSS.Boeing.com]
Sent: Monday, September 21, 1998 12:55 PM
To: antennaware@contesting.com
Subject: RE: [antennaware] Homebrew vertical construction
My first attempt at replying to antennaware.
I've modeled elevated radials in a 'bowtie' shape and tested. Length of
radials is about 172/f . The 'bowtie' is just as you imagine. String 2
sets of elevated wires(each wire 344/f) and close the ends on 2 opposing
pairs. Closing all 4 sides will result in radials of about 160/f but
more wire needed.
--
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