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

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].

-JJ

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.

Roy Lewallen, W7EL

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

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.

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