Roy Lewallen wrote in news:13an54e9keccc59
@corp.supernews.com:

wrote:
Thanks all.
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.

Some years ago, I implemented an online calculator based on the method
proposed by our communications regulator (then, the ACA). The calculator
includes several overseas SAR levels, including that later struck by our
radiation regulator (ARPANSA).

The key difference between the model used and todays regulatory
environment in Australia is that the modelled results are not acceptable
below 10MHz.

If readers want to play with the model, it is at
http://www.vk1od.net/tl/emrcc.php . (The model assumes the antenna is
100% efficient, it it isn't, then adjust the input power to the expected
radiated power.)

Is assessing the radiation hazard of the loop, the mode is very important
to the outcome, and for reasons I don't understand, the FCC, then
apparently the rest of the world, recommended a very high average/peak
ratio for SSB telephony.

If one was really concerned about the loop, a simple measurement
instrument could be made from a small loop terminated in a resistive load
and detector with a small battery powered LCD panel meter. The loop
Antenna Factor can be determined from an NEC model, the detector can be
calibrated on a signal generator, and the whole lot then calibrated in mV
DC to Field Strength in dBuV/m. I have done this for a 0.6m square loop
and the measurement results at locations in the induction and radiation
near field areas around a 20m dipole reconciled reasonably with
expectations based on the calculator above understanding that the
calculator's method is conservative.

Owen