[Lowfer] NC Booming in on ground wave

[John Andrews]

Here's another thought.

The output of a receiving antenna is the vector sum of the various signals
that arrive at the antenna. For LF purposes, we commonly think simply of a
desired signal coming by surface or sky wave, and all sorts of noise. The
noise may atmospheric and arriving in the same way, or may come from the
"near fields" of noise from local conductors. But things are not always so
simple. Long conductors near the antenna can re-radiate the desired signal
as well. And vectors being what they are, the re-radiated component may
subtract from the desired signal. It MAY also add, but Mr. Murphy seems to
prefer subtraction.

For example, taking AM broadcast field measurements in an urban or suburban
area is a major exercise in walking around trying to find why the signal
isn't as strong as you expected. Re-radiated signals from power lines, metal
fences, buried pipes and so forth can have a major effect on the readings.
Even with those shorter wavelengths, moving 20 feet or so can make a big
difference. (Remember that you are usually dealing with the near-fields of
those re-radiated signals.)

If you drive on a country road with overhead power lines in an area with not
too many other conductors, you will frequently notice peaks in a
medium-to-weak AM signal that occur every half wavelength. Or it may be
easier to notice the "valleys" that occur a quarter wave later. This is
coming from the vector sum of the desired signal plus the re-radiated signal
from the power lines.

My point is that unusual effects may occur if you put your receiving loop
near other conductors. Mounting it on a steel tower would seem to be an
invitation to odd behavior, though some of it may be positive as far as the
desired signal is concerned. And since those conductors are also potential
noise sources, you may be limiting your results. If you are lucky to have
enough land at your disposal, I'd always suggest locating the RX loop as far
from conductors as possible.

John A.