On 28 jul, 00:33, "
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. 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 - or are there some near-field considerations not
captured using this approach?

Thanks,

-JJ

Hello,

When you are close to the loop, let say less then 0.1 lambda, the
exposure for the loop will be significantly higher with respect to the
full size HW dipole.

The reason for that is that at short distance the reactive fields
dominate (that are the fields that obey "DC/lumped AC" calculus).
While the radiation H field has 1/r relation, the reactive field has a
relation between 1/r^2 to 1/r^3. So you cannot calculate the field
strength (both H and E) based on the 1/r relation.

Some years ago I did a calculation on the H field from a loop with
D=3m, radiation efficiency 22%, input power 50W (so radiated power is
just 11 W), 3.6 MHz. The H-field at 2m would be about 1.33A/m,
while the ICNIRP reference level for the general public is 0.22A/m. At
4.5m from the loop, the field drops to 0.2A/m

The reason for the strong local magnetic field is the high Q factor of
the loop (about 1500), while a HW dipole will have a Q of about 12.
The same radiated power for a HW fipole would result in a about 0.5A
feed current. This would result in about 0.04A/m at 2 m distance from
the center of the dipole.

At the higher HF bands, the levels for a loop and HW dipole will come
closer as the reactive fields vanish faster with respect to distance
and (with same size of loop), the Q-factor decreases because of higher
radiation resistance (hence lower circulating current in the loop).

Best regards,

Wim
PA3DJS
www.tetech.nl