We just went through the exercise of tuning up the elevated radials on the
KC1XX 160m vertical array in advance of the upcoming CQWW DX Contests.
The first thing is to make each of the radials look as electrically
identical as possible. We assume the length of the vertical element is
fixed and not adjustable. We start by connecting one radial at a time to
the feedpoint of the vertical and measuring the impedance at the vertical
feedpoint. In general the impedances will not be exactly the same. With
unequal impedances, the currents flowing in each of the radials will
generally be unequal, which is undesirable because then the radials generate
unwanted radiation. Suppressing that radiation depends on making the
radial currents as close to equal as possible so that in the far field, the
radiation from each radial is cancelled by the out-of-phase radiation from
an opposing radial. The lengths of each of the radials then need to be
adjusted to equalize the feedpoint impedances of the vertical with one
radial at a time.
We trimmed the radial lengths to make the feedpoint reactance X=0 at the
desired resonant frequency of the vertical. (Note that if there are other
vertical elements present in an array, they have to be detuned to insure
they do not corrupt the measurement.) However when we did this, we found
that the resistance (R) at the feedpoint was different with each individual
radial and it was not possible to equalize the resistances and reactances
simultaneously. Apparently this is because of the non-homogeneous
environment (ground and surrounding trees) around each radial. So, we do
the best we can in equalizing the impedances and accept some degree of
imperfection. The more radials present around the vertical, the more
forgiving the system is of imperfections in radial symmetry.
As a final check we then connect all of the radials to the vertical element
and check the impedance again. The resonance (where the reactance X=0)
should be close to the resonance that was obtained in the radial trimming
exercise for the vertical with one radial at a time.
73, John W1FV
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