Dan,
five wavelengths will be far enough. An indirect measurement can be as good
as a field strength measurement. If you can accurately measure the base
impedance at resonance you can calculate the change in field strength. This is
done
by simulating the antenna over perfect GND. You then know the feedpoint
resistance. In actual use the feedpoint resistance will be higher. The
difference is
due to losses. The radiation efficiency is then calculated. As the GND system
is improved the base resistance will decrease. I use this method and it is
convenient. It has confirmed simulations and articles that say that many short
radials is not much better than a few short radials.
Dave WX7G
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