Stuart K5KVH wrote, regarding 120 buried radials...
>That is because that is what is done with AM broadcast
>stations, who do NOT use resonant verticals, (Theirs are
>typically shorter than a quarter wave), and therefore really
> need a good RF ground even more than Hams need it.
While it is definitely true that AM broadcast towers are rarely exactly
resonant on the frequency of interest, it has been my experience that many AM
station verticals (particularly towers that are part of directional arrays)
are often right around a quarter-wavelength high . Many others are taller
still, up to 5/8 wave tall. It is more rare to find shorter towers. All
benefit from radials to return the ground currents.
The two most common broadcast ground system installations I've seen or built:
120 buried radials up to 1/4 wavelength long, plus a 24' x 24' copper screen
around the base of the tower
or
120 buried radials up to 1/4 wavelength long, plus 120 more radials 50' long.
Metal fences, tuning houses, etc are also bonded to the ground system using
copper strap. In directional antenna systems, the radials from adjacent
towers are usually bonded together using strap along the the line where they
intersect.
The goal is to increase the ground conductivity as much as possible,
particularly close in to the tower. The ground system is definitely *not*
resonant.
In spite of all this, papers I've read show better performance per radial
from elevated, above-the-soil ground systems, and some stations employ these
in varied configurations. That's what I would do if I were to set up a
vertical for HF.
73 Steve Johnston WD8DAS
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