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Years ago, I did broadcast consulting engineering.
An FCC ground system efficiency proof involves measuring field strength at
points along imaginary lines extending out from an
antenna. The line extends radially in a straight line, and is referred to as a “radial“.
Although this process might be called “measuring radials”, “doing radials”, or
“taking radials”, it actually is the imaginary line being referred to as a
radial. It is not a physical wire radial!
To do a proof, field strength is measured outward along a straight line at
planned intervals. The distance is outside the Fresnel region, and outside the
nearfield of the antenna, where signals decay at normal predictable rates. This
is generally over 1 wavelength, and extends outwards to many miles. The
resulting readings, at multiple points, are recorded.
The FCC publishes reference material in the form of graphs. This is covered
in
section 73.184 of FCC rules. A sample is shown here:
These graphs are available from the FCC upon specific request for various
frequency ranges. The little numbers labeling each graph line are soil
conductivity.
The SLOPE of the measurements along a radial is matched to the closest line
slope in the graph. This then “allegedly” tells us average ground conductivity
along each radial.
From conductivity, we predict field strength normalized to some reference
distance, such as one kilometer or one mile. For example, perfect ground would
be the flat downward slope labeled “100mV/m AT 1 km” Note that line is above the
graph line 5000, which represents 5000 mS/m
This graph covers soil conductivities from 0.1 mS/m to infinite (perfect).
For example, it contains 0.1, 0.5, 1, 1.5, etc. All values are
millisiemens per meter.
Once we find and match the slope, we can predict the expected field strength
at any distance. If we match the predicted field strength for a certain applied
power, the ground system is considered good.
Naturally, field strength readings wobble all around with distance. As such,
engineers often get “creative” in selecting a mean value to match a curve. I’d
guess there is a few dB latitude in matching a curve, depending on scattering in
readings. They should be choosing mean values, or taking more “points” to
establish a mean value, but I’ve seen people not do that and just pick readings
that make the system look good.
If ground conductivity is underestimated, antenna efficiency is
overestimated.
Note the greater divergence in field strength with increasing loss when
distances are large. Because Hams do not normally work extended groundwave, soil
isn’t drastically important for Ham applications.
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