Hi John,
In a message dated 96-09-30 14:27:48 EDT, you write:
>
>These comments are essentially correct if one assumes the antenna is a
>standard quarter-wave vertical whose radiation resistance is theoretically
>around 36 ohms. However, one can make a case for more extensive radial
>systems when dealing with verticals with lower radiation resistance.
It is important to remember that while a more extensive ground system helps,
there is virtually no difference between 50- 60 radials and 100-120 radials
IF the radials are only 1/4 wl long.
At 3 MHz, the Brown Lewis and Epstein study measured no difference at all
between 60 and 113 radials, or with or without a ground screen, at antenna
heights down to 25 degrees. In order to increase efficiency with more
radials, the radials had to be made longer. So the case I outlined includes
base loaded 1/14th wl verticals.
Because the FCC wanted to insure long life and conservative operation in AM
BCB ground systems, they included a base screen and an extra "safe" number of
radials. "If 50 or 60 is perfect for 1/4 wl radial, let's make it twice than
number" is the motto.
We seem to have copied that approach, even though very extensive field
strength measurements have shown it to be much more conservative than
necessary.
73 Tom
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