RF Exposure from Small Transmitting Loops
 

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

RF Exposure from Small Transmitting Loops
 

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

RF Exposure from Small Transmitting Loops
 

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.

RF Exposure from Small Transmitting Loops
 

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.

RF Exposure from Small Transmitting Loops
 

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

RF Exposure from Small Transmitting Loops
 

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.

RF Exposure from Small Transmitting Loops
 

Jim Lux wrote:

[...]

I know of only one fellow who worked on radar for the military and used
to warm his hands by a slit on the waveguides, who seriously thought he
suffered any disease from rf; his complaint was that it increased the
severity of his arthritis in his hands.

Now, I can see that "cooking" your hands with microwaves is not good ...
how he determined that this caused more pain from arthritis was probably
less than scientific--however, it would be quite easy for me to believe.
I also note that he did have to have cataract surgery, however, so did
many in my family who were not exposed to any rf that they were aware of.

My statement stands, other than mentioned here I am aware of no hams
with legitimate injuries from rf from hf antennas (and, I have no way to
determine the validity of such anyway.)

Now, I have heard many, many stories of friends of friends, or someone
who knows someone--somewhere who has suffered noticeable damage from
rf--however, I have also met individuals who have claimed to have been
abducted by aliens ...

Not long ago I had dental surgery and a "rf scalpel" was used. I
suppose I could go find some loony attorney to help me sue the dentist
.... I think I'll pass ... other than the "normal" longer-than-usual time
to heal which is common to rf burns, I noted no abnormal complications.

But, wives tales can make for entertaining reading, feel free to
entertain me ...

Regards,
JS

RF Exposure from Small Transmitting Loops
 

Michael Coslo wrote:
Jim Lux wrote:

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.



Jim, are you really saying that there is 10KW in the loop? Who needs
zero point energy if that is so? Or did you mean 10KV?

technically 10kVA.. it's reactive power circulating between the L of the
loop and the C that tunes it.

The very definition of Q is the ratio of stored energy to that lost per
cycle. In the case of the antenna, assuming it's lossless, the lost
energy is that radiated away, and presumably replaced by the transmitter
(assuming a steady state sort of system). If you have X Joules
radiating away each cycle, there has to be Q*X Joules stored in the
system, and X Joules added to the system.

In a lossy antenna (which these loops will inevitably be, barring
superconductors, etc.), some of the energy is lost to heat, but, again,
if you measure the Q, that's rolled in. (The Q of a lossless resonant
loop 1 meter in diameter at 7MHz would be spectacularly high, since the
radiation resistance is tiny compared to the reactance of the loop)

This is why small loops need HV capacitors and low resistance loops.

For what it's worth, the same sort of problems with near fields crop up
in superdirective arrays, because there's a lot of reactive power stored
in the near field that circulates among the elements. Fortunately from
the RF exposure standpoint, most amateur superdirective arrays (i.e.
Yagis) are mounted several array sizes away from people, and in these
arrays, the high energy density is almost entirely within the volume of
the array. Take a look at the cover of one of the Antenna Compendiums
(#3?) for a picture of this.

jim, W6RMK