> Date: Thu, 11 Jun 1998 21:07:12 -0400
> From: Dick Green <firstname.lastname@example.org>
> I don't think radials will help the Gap because it's a vertical dipole.
> 73, Dick, WC1M
Radials help any ground mounted or near-ground vertical, the more the
Radials do two jobs:
1.) They give some antenna designs something to "push"
against, so current can be forced up in the element. The sum of
currents flowing up in the radiator MUST equal the sum of currents
flowing out (or in) at the ground connection point.
2.) Radials divorce or "shield" the fields near the antenna (both
induction fields and the radiation field) from lossy ground around
Some neat things to remember:
A vertical dipole will eliminate problem one, but not problem two
above. The only way to substantially eliminate problem two is with
height (more than 1/2 wl at the antenna base typically), or a darned
good counterpoise (typically one with wire spacing less than .025 to
.05 wl maximum at the wire ends, and wires 1/4 to 1/2 wl long).
Problem one will show up as base impedance changes. Problem two
more often than not won't show up as any abnormal resistance at the
feedpoint! You can "do up" a ground system that makes the antenna
have a low base resistance and still have poor overall efficiency.
For example, I recently installed a ground system that presented a
base impedance of near 30 ohms with a 1/4 wl vertical, and had
poor efficiency (ten or so percent)!
I installed another system that had much higher base resistance
(near 40 or 50 ohms), and obtained fairly good efficiency.
The only thing we can depend on is a field strength meter.
Almost everything else proves or means nothing.
Here's some things I've found comparing systems in an open
pasture (reference system was a 1/4 wl vertical with 60 1/4 wl
Four coil loaded 1/8 wl radials were about 10 dB down, base
resistance was about 31 ohms.
A two-wire elevated resonant 1/4 wl counterpoise was about 8 dB
down, base resistance was 42 ohms.
Four elevated resonant 1/4 wl counterpoise wire was about 5 dB down,
and had 41 ohms of base resistance.
Connecting a ground rod system to the common point of the resonant
systems lowered FS by about one dB!! You can't ground the resonant
counterpoise common point for RF without adding ground loss! That
amazed the heck out of me.
Four non-resonant on ground radials were about 8 dB down, and base
resistance was about 85 ohms.
Sixteen ground mounted radials were 4 dB down, and base resistance
was about 50 ohms.
Sixty ground mounted radials were equal to the reference antenna, and
base resistance was about 38 ohms.
An earth connection via ground rods did not change loss one bit for
ground mounted radials.
FS was measured at 12 monitor points. Base impedance measured with a
Delta OIB. Impedance excursions were removed with a vacuum cap /
roller inductor L network so the applied power could accurately be
maintained for all tests.
On the air A-B tests confirmed the groundwave results. The results
were similar to FS tests I made in Conyers a few years ago.
73, Tom W8JI
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