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

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

The method you describe is valid only in the far field. There are higher
order terms to the field strength (field relative to distance) in the
near field, and they're strongly a function of the distance and the
antenna geometry. Using the method you propose can produce very
erroneous results close to the antenna.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
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

The method you describe is valid only in the far field. There are higher
order terms to the field strength (field relative to distance) in the near
field, and they're strongly a function of the distance and the antenna
geometry. Using the method you propose can produce very erroneous results
close to the antenna.

Roy Lewallen, W7EL

A good question and an interesting, but not very helpful response.

It seems to me that there are elements of truth in both the original
proposition and in the response comment, but that each is only a partial
truth. The essential aspect is surely the separation distance relative to
the size of the loop antenna.

However, the obvious comment is that the small physical size of a loop is
likely to lead to use in a situation (for example, indoors and close to the
operating point) that would/could lead to excessive levels of RF exposure.
For an electrically small loop (the typical loaded loop less than 0.1
wavelength), then it is probably fair to assume that all of the input power
is radiated through the sphere surrounding the loop provided that the
separation is reasonably large, however, for a large loop (eg half-wave or
larger) its probably best to approach the RF exposure issue as you would
with any other antenna such as a dipole or vertical.

Keith G Malcolm
VK1ZKM
28 July 2007

In article , who knows
wrote:

However, the obvious comment is that the small physical size of a loop is
likely to lead to use in a situation (for example, indoors and close to the
operating point) that would/could lead to excessive levels of RF exposure.

Does it really matter? What are the odds of serious health consequences
from RF exposure? If I quit smoking and avoid RF over-exposure will I
live forever, or will I be dead as a doornail 100 years from now just
like everyone else currently participating in this newsgroup?

--
-30-

Anonymous wrote:
In article , who knows
wrote:

However, the obvious comment is that the small physical size of a loop is
likely to lead to use in a situation (for example, indoors and close to the
operating point) that would/could lead to excessive levels of RF exposure.

Does it really matter? What are the odds of serious health consequences
from RF exposure? If I quit smoking and avoid RF over-exposure will I
live forever, or will I be dead as a doornail 100 years from now just
like everyone else currently participating in this newsgroup?

Well, like exposure to anything, it depends on how much exposure you
get.

It is possible to die from drinking too much water for example.

So, unless your plan is to live fast, die young, and leave a good
looking corpse, it is probably a good idea to know how much RF is
too much and avoid it at dangerous levels.

--
Jim Pennino

Remove .spam.sux to reply.

Anonymous wrote:

...
Does it really matter? What are the odds of serious health consequences
from RF exposure? If I quit smoking and avoid RF over-exposure will I
live forever, or will I be dead as a doornail 100 years from now just
like everyone else currently participating in this newsgroup?


I worry little about exposure to rf 30 Mhz and 1kw or less ...
I am NOT aware of any malady/disease which strikes hams any more often
than any other group ... which suggests the safeguards in place (simple
plain common sense) is/are more than adequate.

However, as frequency of the rf increases so does my concern ...
everyone is aware microwaves can cook, maim and kill biological entities.

Regards,
JS

Calculating, or even defining, power density in the near field is a bit
dicey to say the least. But the field strength (E or H, which aren't
necessarily in phase or oriented at right angles in this vicinity) can
readily be determined. Here are some values for the E field as a
function of distance from the center of an octagonal loop about 3 feet
in diameter at 7 MHz, with 100 watts of applied power, compared to the E
field calculated using the proposed simple spherical method (and further
assuming, incorrectly, that the wave impedance is 377 ohms resistive):

Dist m E V/m E sph apx V/m
1 683 54.8
2 133 27.4
4 34.2 13.7
8 11.1 6.85
16 4.58 3.42
32 2.15 1.71
64 1.06 0.856
1000 0.0548 0.0673

As you can see, the approximation might be adequate at some distances
and for some purposes but not for others.

Roy Lewallen, W7EL

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