Billy,
If you view Rudy's excellent power point and look at the pictures, you
find that Rudy uses symmetric radials as is usually done with verticals
when there is adequate room. His site has a uniform layout in all
directions, so local earth differences and coupling are minimized. His
measurement tools are often home made or basic, ie the radial current
transformer is simple to replicate with circuit board soldered together,
hobby copper tubing (or brass), and a core suited for the band being
tested. A relative voltage is developed which represents current in the
radial being measured.
HP RF millivoltmeters are commonly available to hams from surplus
sources at reasonable prices. These are good for reading the voltages
developed from transforming radial current.
I see nothing exotic needed in instruments if symmetry and good
practices in vertical antenna construction are followed. 20 MHz
oscilloscopes are common and reasonably priced, and would also also
suffice for voltage measurements for the lower bands.
Back before WW2, hams were measuring radial and feed line currents with
thermocouple RF current meters and they are adequate for work at 40m and
below today as well, and likely most are marked as good up to 20 MHz,
thus good for at least 20m and as relative indicators somewhat beyond.
They can be used in opposite pairs of radials to preserve symmetry and
detect any relative radial current variation other than that of the
measurement device. Most would require use of a transmitter as your
signal generator, but some exist that have a 0 to 1 Amp RF scale.
The modeling studies of L.B. Cebik, W4RNL, show that differences in
which place you feed an antenna can alter the pattern slightly. (Such
as on a symmetric Horizontal Loop).
If your local soil varies so much that your radial currents are not
balanced, the effect will likewise alter your pattern, but that does not
prevent the vertical from working as expected, it just may favor one
direction over another, slightly.
The variations in the propagation has a much greater effect on the
signal and for that we have no control, once the signal leaves the
antenna. Most of the signal influence once radiated depends on the
earth out beyond the Fresnel Zone, some 5 wavelengths beyond the antenna
location and usually out of the ham's control as to what is on the earth
at that distance---buildings, ponds, different terrain or elevation.
Extensive radial systems are needed at AM band because their main
interest is a nice controlled ground wave pattern to cover their
intended broadcast area, and nulls in the directions of any co channel
station to which they might cause interference.
Hams on the other hand have different concerns in their main use of sky
wave signals.
-Stuart
K5KVH
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