> Date: Fri, 12 Jun 1998 20:22:38 -0700
> From: Bob Wanderer <email@example.com>
> I asked Roger Cox of Hygain whether putting radials under a DX-77
> vertical (also a vertical dipole) would be of any benefit. He said NO.
> 73 Barry
Not speaking of Roger, who I personally like....
I've observed over the years most vertical manufacturers promote two
radials cut for each band, or a few ground rods, are enough.
I've also read extensive measurements of antennas that include
extensive field strength data by Brown, Lewis and Epstein, made my
own measurements, read field theory books, talked to others who have
made measurements, and virtually all (with the exception of some
theories that DON'T involve field strength measurements) report quite
I think part of the problem is it always difficult to sell people on
doing work. I doubt many verticals, or other popular antennas, would
be sold by telling the absolute truth to the consumer.
Even a dipole benefits from a good counterpoise system, unless it is a
large height (in wavelengths) above earth.
> It, like the GAP, are 1/2-wavelength antennas. Radials are primarily
> designed for 1/4-wavelength antennas and supply the missing quarter
That's a popular picture used, but unfortunately it is not a very
accurate picture of radials or antennas at all.
1.) A half-wave antenna is, by definition, a half wave long. 68
feet of wire stretched out in a straight line is a half wave for 40
meters. A dipole 30 feet long is a 1/4 wl dipole. The same applies to
2.) If you feed a 1/4 wl long vertical with a sinusoidal current
distribution as a ground independent antenna, it has a loop radiation
resistance only about 1/3 the loop radiation resistance (about 13
ohms) of a conventional 1/4 marconi vertical (about 37 ohms).
For equal loss resistances (normalized to the point where loop
radiation resistance is determined), the "ground independent"
vertical will have three times the power loss of the same radiator
when grounded and fed as a marconi. That's another reason they play
so poorly on lower bands.
3.) The same ground independent vertical, when loaded to resonance as
a psuedo-hertz antenna, require TWICE the loading reactance with
everything equal. That means you have TWICE the loss resistance for
an equal Q loading system. Another reason less than half-wave ground
independent antennas perform poorly on lower bands.
Radials also do not supply anything that is "missing", nor do they
form an "image antenna". If they did, the base impedance of a 1/4
marconi would be about 75 ohms instead of 37 ohms.
Radials provide a "reference" for the feedpoint or antenna to push
against so common mode currents can be forced up into the radiator,
and spread the current required to do this out minimizing losses
caused by current concentration.
Radials also shield the antenna fields from the lossy earth below the
>As pointed out, the higher you get off the ground, the fewer
> radials are needed.
> Remmember Gooch's Law! "The RF gotta go somewhere." (I wonder if
> Kurt N Sterba is familiar with that one.)
Very true. It's only at a distance where radiation all looks like it
comes from one point in space that each radial's radiated field
fully cancels. The higher the antenna, the better each individual
radial's radiation is cancelled by an opposing radial.
73, Tom W8JI
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