Niel,
Fine on your constraints.
Remember that a vertical with raised radials is just a dipole 'on its
side' with one leg (a single radial) bent at 90 degrees to the radiating
element.
So how does only the vertical part radiate you may ask. That happens
because you add another radial physically opposite the first one, and
the radiation of one radial is 180 degrees out of phase with the
radiation from the other, so they cancel leaving only the vertical
portion to radiate a signal.
If the pair of radials are not symmetrical, then you may be just as well
off with only a single radial per band.
The radials must each be tuned (just like one end of a dipole) with the
vertical radiator. The radials must be an *electrical* quarter wave
long at the band of operation - they can be loaded with inductors near
the junction with the coax shield, or they can be end loaded with a "T"
wire, or some combination of both, but they must be resonant.
If they are placed close to the ground (4 feet is close for the HF
bands), there will be some effect from ground coupling that will de-tune
them, so the easiest is to cut them long and do the trimming with the
aid of an antenna analyzer (if there are several radials for each band,
they must be tuned one at a time - connect only one and tune it, then
connect only the next one and tune, etc. until done.
Remember that the radials will not be 'radials' unless they are
symmetrically placed around the vertical, it is of no use to run two
radials for any one band out in the same direction for they will produce
the same radiation pattern as a single radial.
Another 'solution' is to trade that antenna for one that is an
electrical half wave - which does not need radials being an antenna
complete within itself.
73,
Don W3FPR
Niel Skousen wrote:
Hi Don (and all who have offered advice & help)
Thanks to all for the input that has been offered. Based on some of the
comments, I thought I might share the 'rest of the story', both to seek
additional input, and to show the technical trade off's associated with
a compromise antenna environment.
I'm installing a used HF-6v in a CCR environment, so I have some
environment limitations. The CCR 'enforcement' committee is not
horribly aggressive, but if I don't aggravate the situation, I'm much
better off.
So, the HF6 will be mounted about 4' off the ground, a full elevated
radial system is not possible. Choices seem to be the CPK solution from
Butternut or a modified radial system. Two options are being considered
on the modified radials.
First, 3 tuned radials (either individual wires for 40/15, 30, 20, 15,10
or the Butternut tuned twinlead variety) could be connected at the base
of the antenna, then run down (hidden in the bushes) at a 60 degree
angle to ground level, then run under/around the rocks. Performance is
expected to be better than the CPK, but still a compromise. Routing is
not linear. Tuning of this radial environment is expected to be a bear !
The second, the CPK, is expected to be enhanced by the low elevation,
but NOT as effective as a good ground radial system. I might be able to
enhance this configuration by laying out a psuedo radial set of 6-10 8'
wires at the base of the 4' mast, which would more accurately be a
surface capacitive element to enhance the CPK's effect.
In neither case will the near field ground losses be impacted. There
are some spiral wound counterpoise idea's out on the web, as well as
some thoughts on constant angle spirals, which are tempting, but at this
juncture, I expect to lean to the CPK to get it up and running, with
spirals and experiments to follow...
So any additional input, idea's, are solicited and welcome
Thanks again all,
Niel
Don Wilhelm wrote:
Remember (or understand if you did not before) that elevated radials
should be tuned (pruned for length) just like a part of the antenna
(they *are* part of the antenna).
So get out your antenna analyzer and cut one radial a bit on the long
side - attach only that one radial to the vertical and shorten it until
you have the correct readings on the antenna analyzer. Cut the second
radial to the same length - move to the next band and do the same thing
until you are done. Note that the radials can interact on a multiband
setup (just like parallel dipoles), so tune the lowest bands first and
move upward in frequency one band at a time.
The twinlead solution does work, but suffers badly from interaction.
73,
Don W3FPR
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