RadioBanter - View Single Post - RF Exposure from Small Transmitting Loops

Jim Lux · July 30th 07, 09:33 PM posted to rec.radio.amateur.antenna

wrote:
I am curious as to whether RF exposure concerns are greater for a
small transmitting loop [like the MFJ tuned loop] compared to a dipole
radiating the same power.

yes.

It would seem that close to the loop, the
RF power density may be greater [than it would be at the same distance
from the dipole apex] since the radiating volume is smaller. Can I
just assume that the power is evenly distributed on the surface of a
sphere having a radius equal to my distance from the loop antenna,
calculate the power density on the sphere surface, and use that number
for evaluation
No
- or are there some near-field considerations not
captured using this approach?
yes

The big problem is this: a small loop stores a lot of energy in the
fields around the loop (if the loop has a Q of, say, 100), and you're
radiating 100 Watts, that implies that there is 10kW circulating in the
loop between the loop itself and the tuning capacitor. The energy
moves between the magnetic field of the loop and the E field of the
capacitor every 1/4 cycle.

A particularly egregious example is the tabletop small loop shown in QST
a few months ago. The Operator is sitting about 1 meter from the loop,
and unless he's running very, very low power, he's exceeding the RF
exposure limit by quite a bit. The worst thing is that the article
makes the assertion that there's a field null along the axis of the
loop, which is true in the far field, but certainly not true in the near
field.

As a practical matter, the field is pretty uniform (within a factor of
2) within a couple loop diameters.

Some useful practical numbers:

For a 1 meter loop, with a current of 10 Amps, the H field at 2 meters
away (along the axis, normal to the plane of the loop) is about .16 A/m
(or right at the Maximum Permissible Exposure (MPE) for controlled
environments at 30MHz, 100% duty factor)

In the plane of the loop, you get down to that level at a distance of
about 1.6 meters.

Here's the letter I sent to QST about it:
http://home.earthlink.net/%7Ew6rmk/qstrfsafety.htm

Jim, W6RMK