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Jim Lux · July 30th 07, 09:49 PM posted to rec.radio.amateur.antenna

Roy Lewallen wrote:
wrote:

Thanks all.

Roy, I was not implying that I believe one can assume that the power
is from a point source and one can consider the power density passing
through a sphere to determine RF safety. I was looking for some
guidance as to how to determine a "safe" distance from a small tuned
loop assuming a particular frequency and power.

It appears that the simple sphere approach works reasonably well
beyond a wavelength or so, and may be an acceptable first-order
approximation at 1/2 wavelength [from a small loop].

There's no distinct boundary between the near and far field, but at a
wavelength, or even a half wavelength, you're pretty much in the far
field of a small antenna. So far field approximations such as the one
involving power density on the surface of a sphere are quite reasonable
at those distances.

Somehow, though, I suspect that many people will operate within a half
wavelength of a 40m or 20m compact loop, and that's where it gets a bit
stickier.

It's the loop or short whip on the balcony railing or picnic table, with
a high duty cycle mode (like psk31, rtty, or SSTV) and turning up the
power knob beyond a few watts that raises the concern. The antenna
doesn't radiate well, and the QSO is a bit marginal, so the OP turns up
the gas a bit.

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