Dennis:
You have not reached a point of diminishing returns here. Although you see no
virtual change in the antenna VSWR, the radiation efficiency of the antenna
system improves every time you add additional radials under the antenna. There
is published data in the Proceedings of the IRE that show that once you reach
120 radials you see no additional improvements to the efficiency of the antenna
by adding more radials. With regard to length, the magic cut-off number for no
improvement to radiation efficiency is 0.4 wavelengths. That is why when I
worked in the broadcast industry, we always installed 120 radials of at least a
quarter wavelength long under all AM broadcast towers.
You have reached a point where the transmitter is happy with the antenna match
but you can improve the antenna's signal radiating efficiency by adding still
more radials under the antenna. You are trying to reduce the ground losses in
the near field of the antenna with the ground system and when you finally reach
120 1/4 wavelength radials, the antenna will be about as good as it can be.
You should also not see any VSWR change by adding the additional radials.
If you can't get to a 1/4 wavelength in all directions, you can add more
radials in the shortened areas to help improve the radiation efficiency in
those directions where the ground system is shortened. If you can't get 120
radials down, put as many down as you physically can and make them as long as
you can.
73.................de Goose, W8AV
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