> > Elevated radials SHOULD have the feedpoint decoupled
through
> > a common mode choke. if you ground the radial common
point
> > is causes increased loss as well as current on the
feedline
> > shield.
>
> How would you go about this with an antenna like the
AV-640 which has a
> built in tuning network that is grouneded. Isolate the
entire antenna from
> the tower, feed it through a choke, and atach the radials
to the bottom of
> the antenna where it would normally atach to the mast
driven into the
> ground?
To be truly unbalanced a system should have equal and
opposite currents flowing into the shield and center
conductor at both ends, and the shield has to have
essentially zero voltage to the environment around the
antenna including the feedline.
Any vertical antenna with anything less than a large radial
system has significant voltage on the radial common-point
with respect to the surrounding of the antenna. That
includes earth, the feedline, and so on. This is true even
with 1/4 wl groundplanes using four full size radials.
Substantial pattern and SWR changes occurred when feedline
length was altered in a commercial 40 MHz groundplane I
designed. The cure was to isolate the radials from the mount
and decouple the feedline with a common mode choke.
One of the things I didn't initially believe when I read the
first article (based only on models) was an earth path would
reduce antenna efficiency when using elevated radials. When
I actually tested systems, I found it was true.
Small groundplane verticals are a worse case situation. They
are simply nasty for common mode feedline currents. Most of
the small groundplane antennas like the 640 include
decoupling, but it is often not enough. When I worked for
Ameritron the majority of "I bought and amp and everything
in my shack goes nuts when I use it" complaints were with
these small groundplane verticals, even though they had some
factory decoupling.
I'm not sure how a person could cure every antenna like
that. We generally advised a large common mode feedline
choke before the feed cable entered the shack as well as
keeping the feedline away from other conductors between the
antenna and that choke.
I don't know how significant a problem this is, it varies
with every installation.
I'm only pointing out small radial systems are a headache
for a number of reasons. They are frequency sensitive, they
radiate like crazy in the area around the radials, they
couple to everything around them, they produce feedline
common mode problems. In my tests and every BC station I've
seen or heard of where the radials were actually A-B'ed
against a regular system of 50 or more radials, and as my
friends have changed them, the results were always that a
conventional system was better.
Even if it isn't better, it is virtually impossible for a
conventional system with 50 or so radials to be worse. With
50 or so radials efficiency is in the high 90% range.
That's true no matter what type of soil is under the
radials, if the 50 radials are shallow buried, or if they
are elevated. Conventional systems are not critical. They do
not have common mode or near field coupling issues. In
addition to RF advantages they also work well for lightning
protection of equipment.
I'm very skeptical of any system that is reported to clearly
beat a system known and proven to have nearly 100% of the
maximum possible field strength. Of course it is work to
install 50 ground wires, and that alone is good reason to
not use them if ease of installation outweighs the
electrical disadvantages of small counterpoise systems.
73 Tom
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