Mike Walkington wrote:
Finally, I've been reading Moxon's HF Antennas for All Locations, and he
doesn't seem to recommend quarter wave radials for ground planes. I'm not
sure I comprehend why. Is anyone familiar with his concerns?
He argues that unbalanced currents in radials will result in radiation
at very high angles, which is essentially wasted. He says that resonant
quarter wave radials are likely to be affected by nearby objects, so
unless take special care to measure the current in the radials and
adjust them, the currents are likely to be unbalanced.
For that reason he recommends shortening the radials and feeding them
through a common inductance (it can be a coil or a stub) to resonate the
whole system; that is much more likely to result in equalized currents
in the radials.
I have a 33-foot vertical with four radials made of tubing, each 8 feet
(2.4m) long. All of the radials are connected together at the base, and
fed through a small coil. I adjusted it with an MFJ analyzer (the
adjustment was somewhat critical) and I get an SWR of about 1.2:1 at
resonance, and it's broad enough to cover most of the 40 m band with SWR
under 2:1. It's not far from another 45-foot tower, so there's some
interaction. Modeling it with EZNEC indicated that the gain was about 1
db lower than full-size resonant radials (of course this assumes that
the radial currents are perfectly balanced).
I also use a choke balun at the feedpoint which consists of a bunch of
ferrite toroids that fit tightly on a piece of coax. I bought a kit to
make it from The Wireman <http://thewireman.com/index.shtml>. This is
advisable for two reasons: 1) without it, current flows on the outside
of the coax which causes radiation upwards as discussed before, and 2)
without it, noise generated in the shack from computer equipment, etc.
can flow up the coax and enter the system at the antenna.
I also have an inverted V with the center at 40 feet with a balun at the
center. Comparing these two antennas I find that the dipole is always
quieter, and is usually better for stations closer than about 1500
miles. For stations 2500 miles away it's a tossup, and for DX usually
the vertical is better. A whole lot of the time there's no difference.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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