> From: Guy Olinger, K2AV
>
> >
> > The 120 radial wires comes from the FCC's recommended ground
> > of 120 1/2 wl radials for BC stations, where they want a LOT of
> > headroom on the conservative side.
>
> Hmm... If you talk to JI, you get 60, and if you talk to the FCC you get
> 120?
>
> There was one other advantage to 120, that over time some of them may go
> bad, for any number of reasons. Even if you lose half of them, you still
> have 60 and still are dense according to JI, unless they all go bad in
> the same place.
>
Guy,
You missed a key piece of information ...
The FCC standard is 120 *HALF WAVE* radials for AM broadcast.
1) it maintains the .02 wavelength separation at the ends ("dense"
radial field)
2) it provides about 1/2 dB headroom above the "knee" in the loss
curve and thus some protection from serious performance shortfalls
with radial deterioration over time.
3) limits the interference potential due to signal (efficiency) changes
with moisture level in the ground system.
For an amateur, the half dB improvement in signal between 60 x 1/4 wave
radials and 130 x 1/2 wave radials does not justify the cost of four
times as much copper (30,000 ft. sted 7,500 ft.) much less the labor
to install it.
73,
... Joe, K4IK
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