[Elecraft] Vetical Antennas

[Vic K2VCO]

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