I often see the quote that making a vertical (or inverted L) longer than
a quarter wave reduces ground loss because the radiation resistance goes
up. I would like to believe that, because that is what I am using for
an antenna, but I don't want to kid myself. I'll admit that the
radiation resistance goes up, but whether ground loss is lower or not is
questionable. Sure the current going into the radials at the base of
the antenna is lower, but as you move away from the antenna the current
density in the ground increases, not decreases as it does with a quarter
wave antenna. So when the current density become greater at a distance
further from the vertical you need more radials in that area unless you
want more loss, so you need a much better and longer radial system for a
longer antenna. How much better does the radial system have to be, is
the real question and I can't answer that. Doing NEC analysis for a
simple vertical includes a lot of guess work to start with (associated
with the ground), and a longer than quarter wave is even more difficult
because there is no measured data for these antennas which can be used
for comparison.
ON4UN in his book referenced a calculation for ground current density
for a half wave antenna. The point of maximum current density occurred
at 0.35 wavelengths from the base of the antenna. So if you used the
traditional quarter wavelength radials, you wouldn't have any radials at
all in the place where they are needed the most.
I have had a lot of success at calculating the feedpoint impedance and
resonant frequency of these antennas, but I have little confidence in
the gain numbers when comparing to other shorter vertical antennas.
I was going to let the "useless horizontal waves" comment pass, but
while I'm at it I may as well take a swipe at that too. Most people
that use dipoles, inverted vees, and Yagi's don't think they are useless.
Jerry, K4SAV
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