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

John Smith I wrote:
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.

Not to be too hostile here, but that suggests ignorance rather than
knowledge.
a) one of the early studies that triggered the whole "currents of death"
fiasco happened to be one that showed that radio amateurs (among some
other groups) happened to have higher incidence of some forms of cancer.
Later shown to be statistically insigificant and confounded by other
factors, but there it is.

b) There is ample evidence of adverse effects of RF exposure in this
frequency and power range. One of the more interesting is the ankle and
wrist pain experienced by workers on the flight line in a high RF field
environment. The eventual analysis was that the pain was likely due to
RF currents flowing through the body from hand (on or near airplane) to
feet(then to ground). There's also some interesting cases of things
like people on transmitting towers when the transmitter was turned back
on (or the power turned on), but that's not necessarily a credible
situation for a ham.

Common sense is a good start (don't look into the waveguide with your
remaining good eye)(don't turn on the transmitter when someone is
working on the antenna), but it actually takes a bit more thought to
figure out the RF exposure hazards in a off-nominal situation. A dipole
30 feet up in the trees is easy, so is a 3 element yagi on a 100ft tower.

But something like a flagpole vertical in your yard, or an attic dipole,
or a compact loop on a picnic table is a much trickier situation.



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

And so can MF, HF, VHF, and UHF...

Jim

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?


It's probably not the health consequences you have to worry about
(although you should...). It's losing your license for not following the
basic RF exposure safety rules. Or, it's the legal exposure for
violating those rules.

Lots of hams rely on the "safe harbor" limits to avoid the need for a
routine evaluation of RF safety hazards. How many realize that if
they're holding a cell phone, or use an HT at the same time as their HF
rig, the safe harbor doesn't apply (the multiple transmitter rule)?

The last thing you want is your HOA or other busybodies being able to
shut you down for operating in an "unsafe" manner.

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,

separation being large - many loop diameters
i.e. in the radiating far field.

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