As an exercise, thought I'd try to write how this seems to me. From what
I've read and experimented about the subject, which is not nearly as much as
many here, there seem to be (at least) several variables which are
interdependent.
1. Radiator portion electrical length and configuration. Obviously, 127 feet
or so perfectly straight up would be better ; ) Less vertical length means
more horizontal component, less low angle radiation. Longer than 1/4L
verticals introduce different current distributions, which may help or hurt.
2. Height of feedpoint above average terrain. The higher the better, it
would seem; although one could be too high, it's not likely given the sheer
vertical challenge. The closer to the ground, the more tightly coupled the
radials will be. If the antenna is fed at ground level (or shunt fed, for
that matter), all I can do is lay down more copper, doesn't seem to be any
use in elevating radials - the coupling has already happened and there's not
much left to decouple.
3. Quality of ground in near field. Any vertical over salt water will work
better than up here in the mountains. This interplays with (2), since
poorer ground will be lossier than better ground, or, the ideal, salt water.
Improving the ground under elevated counterpoise systems should help reduce
ground losses and promote maximum current in the radiating section (vertical).
4. Spacing and length of radials/counterpoise. Like a dipole close to
earth, the coupling makes the resonant point lower. Improving the
conductive ground screen under a vertical usually results in narrowing the
bandwidth. Less of the RF is going into lossy ground and more is
reinforcing the radiating element. Lengthening elevated radials beyond
resonance doesn't make much sense, because the current distribution in the
radial system should be balanced, leaving the vertical portion as prime
radiator. Probably the ideal would be pairs of resonant 1/4 wave elevated
radials, up to eight or so. More than that, and it seems hard to avoid some
imbalance between pairs. This is the crux of the elevated radial issue, I
believe.
Here are crude portrayals:
|
____| Vert. w/ one resonant radial 1/8L over salt water.
_______________
Radiation pattern is both vertical and horizontal.
|
____|____ Vert. w /two res. radials 1/8L over salt water.
_______________
Horizontal radiation currents cancel out, and nearly all radiation is
vertical. It's hard to imagine adding more radial pairs would improve
things much, since the ocean is an ideal near and fresnel zone.
Now, extend this to more pairs of resonant elevated radials, and complicate
the picture with less than perfect, and uneven ground, and the counterpoise
becomes difficult to balance. Seems to me the main advantage in using a
counterpoise is to provide an artificial current return system that is at
least somewhat isolated from the actual lossy earth, so if the radial system
is not balanced, what's the use? As anecdote, I remember working someone in
New Mexico, possibly N5IA?, who was using a 1/4L vertical, with elevated
feedpoint and resonant counterpoise a couple of years ago. His was the
strongest Barefoot! signal I've ever heard from that distance.
I wonder if anyone has gone so far as to carefully trim resonant radials,
monitor current flow in them, and actually measure the extent to which
radial pairs work together to balance and cancel horizontal radiation at the
resonant frequency? If so, please let me know. For my money, with poor
ground conductivity, I will continue to isolate my current return path as
much as I am able for vertical radiators. This means pairs of elevated,
resonant, tuned radials, with as much ground enhancement beneath as can be
practically managed. The next problem is the fresnel zone, where 3db or so
can be won or lost. Not much to be done about that in these rocks, which is
why I'm still using horizontally polarized antennas wherever practicable.
John K4IQ
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