W6FB:
"In this design the “ground wire” is actually a 1/4 wave ring that wraps
around the base of the antenna."
One issue with a compact counterpoise is that feedline decoupling
becomes more difficult. Imagine a 1" counterpoise with a series
inductance to resonate it. If you were the inner shield current coming
up out of coax, would you rather go there or down the outside of the
shield? You can model this issue to get some idea of its severity.
Another issue is that a compact counterpoise gives up the directivity
provided by a single straight radial. A single radial is not something
I'd ever imagine might work well. However, some of the SOTA people use
one as part of a portable hilltop antenna. The Elecraft KH1 includes
one. When I first modeled a vertical with a single radial, I was
surprised how directive it could be. In one modeling session, I had to
increase the number of radials from two to four to regain the gain at
low angles provided by just one. Symmetric deployment of two or more
radials cancels the electric field directly below, minimizing current
induced in lossy ground. So I had always assumed two radials was the
minimum you'd ever want to consider. A little modeling dispelled this
notion.
For these antenna systems, frequency and height above ground can make a
big difference as can the details of the counterpoise or radial. It's
easy to wrongly generalize by examining too few models. Results also
depend on the ground constant values you use. The generic values your
antenna analysis software offers are inappropriate. You'll find more
realistic generic values here:
https://k6sti.neocities.org/hfgc
Dale, I don't know whether using a single radial for each band instead
of two would be worthwhile for your situation, but it's something to
consider.
Brian
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