RF Radiation Exposure and conducted RF

Radio World, 8 May 1991

by Harold Hallikainen

We've recently been discussing what a station chief operator should do to insure compliance with the FCC and ANSI specifications for human exposure to RF fields. Last month I suggested a report be prepared based on predicted and measured fields. This report would show what areas are safe for human exposure for unlimited periods of time, those areas safe for limited exposure times, and those areas where a transmitter power reduction is required. The report would contain sufficient measurement and calculation detail to fully justify its conclusions (such as, how much must the transmitter power be reduced when a climber is a certain height on the tower).

I received a call from Richard Tell of Richard Tell Associates, Inc. (phone 702 645 3338). His company specializes in RF radiation consulting. Prior to that, he spend 18 years with the Environmental Protection Agency studying the potentially hazardous effects of electromagnetic fields. In the call, we discussed "RF hot spots" and RF contact currents. He also sent me a copy of a paper on this subject from the NAB 1990 Engineering Conference Proceedings.

The study of human exposure to RF fields appears to be a very interesting and controversial subject. The ANSI guideline the FCC is currently using is being revised to include various new ideas. The FCC may or may not adopt the new specifications (it'd appear that to adopt the new specifications, the FCC would have to go through the notice of proposed rulemaking, comment and report and order process, which can be slow).

Hot Spots & Contact Current

Hot spots are small areas of high RF field in areas of much lower RF fields. They are generally caused by reradiation. For the NAB paper, Richard Tell measured hot spots generated by reradiation from a mail box on a metal post, a metal window frame and a guy wire. In each case, the field close to the metal object was substantially more than the "ambient" field. "Surface fields" were measured about 5 cm from the surface. The "E squared" and "H squared" fields were increased to 5.9 to 15.8 times ambient. If these fields are above the ANSI limit, is exposure to these fields allowed?

Ideal Transducers

Ideally, a transducer or measurement instrument does not affect or "load" the parameter being measured. For example, an ideal voltmeter draws no current, an ideal ammeter has no voltage drop, an ideal pressure gauge introduces no volume displacement, etc. An ideal EM field meter does not change the field at the point of measurement (which it might do, due to "loading"). However, when a person enters this area, the radiator is substantially loaded, possibly decreasing the radiated field to within the ANSI limits. When the person actually touches the radiator, the radiated field drops substantially, but a contact current flows. A limitation on contact current is being considered for "hot spot" exposure, instead of specifying the undisturbed field be measured some arbitrary distance from the reradiator.

Wrist Currents

Since mailboxes and window frames are generally touched by hands, and the hand is connected to the remainder of the body by a relatively small diameter wrist, it would appear that the highest heat dissipation due to RF contact would be in the wrist. Using the specific conductance of the various tissues in the wrist and the density of those tissues, a total wrist current can be determined to give a Specific Absorbtion Rate in watts per kilogram of tissue. The SAR can then be tied back to how much temperature rise would be expected in the tissue due to the RF current. The current ANSI standard limits SAR to 0.4 watts per kilogram for the whole body, but allows 8 watts per kilogram in any one gram of tissue, allowing for varying fields, such as those generated by a hand held radio transmitter. Mr. Tell's paper shows a contact current of 75.1 mA in the AM band and 87.3 mA in the FM band for an SAR of 8 watts per kilogram in the wrist. Further, since wrists seem to be able to survive elevated temperatures (if not, I won't wash dishes any more!), an SAR of 20 watts per kilogram has been suggested. This would limit AM contact current to 119 mA and FM to 138 mA.

Lots of Study

Mr. Tell's paper lists 19 references from a wide variety of organizations and publications. A lot of research is being conducted in this field. The current ANSI limits are based on the heating effects of RF exposure. There is also study being conducted into other effects. It might be interesting to see how computed peak SARs (possibly at the tip of a finger) compare with those expected by contact current as the finger approaches a reradiating object. The tip of the finger is very close to the object, resulting in a potentially high field and high SAR, but the reradiator is also "loaded" by the presence of the finger, decreasing the generated field. It would be interesting to see a graph of peak SAR, SAR in the wrist and whole body SAR as a function of the distance between the tip of the finger and the reradiator. The actual field at various places on the finger and wrist would have to be measured to take into account the loading as well as the attenuation due to distance from the radiator. Does the peak SAR approach a limit as the finger approaches the reradiator? How does this peak SAR compare with that expected by contact current (I 2 R)?

This is a very interesting field deserving additional study. There may be a safety advantage to the proposed DAB and digital HDTV systems, since they generally use lower power.

Unlicensed College Radio

I got another call from a college FM station operating without the benefit of a license. Various intentional and unintentional unlicensed radiators are discussed in part 15 of the Rules. 15.211 and 15.221 are quite interesting. 15.211 allows any frequency or power to be used IN A TUNNEL, provided any emissions escaping the tunnel meet the various specifications of part 15. These systems are allowed when the radiator and all connecting wires are contained within the "tunnel, mine or other structure that provides attenuation to the radiated signal due to the presence of naturally surrounding earth and/or water."

15.221 similarly allows educational institutions to use AM frequencies on a noninterference basis with any power or radiated field as long as the field meets the general limitations of part 15 at the campus boundary.

Note that 15.221 permits such operation on AM only. It appears that operation on FM would only be permitted at an "underground university".

We've always assumed that radio required federal regulation because it does not "stop at property lines". These two sections, however, seem to define the word "stop" and, with some extension, could allow radio use on any frequency and power as long as the field at all points on the property line (and perhaps a certain distance above the property) met certain field limitations. Instead of setting a limitation on the field 30 meters from some box that radiates, the field would be set 30 meters from the property line (the property itself becomes a large "box"). I've heard of this reasoning being used to transmit an FM signal in a concrete sports complex. Radiation outside the complex was well below the part 15 limits.

But, as I read the rules, unlicensed operation in the FM band is limited to 250 microvolts per meter measured at 3 meters (15.239(b)).

Continuing Education

Barry Mishkind recently wrote in RW about continuing education. I'd like to suggest RW readers be learners. Too often people cram through four years of college, graduate, and never return to an intense learning environment (of course, we hope we learn on the job and through life in general). I'd like to suggest taking one class a semester or quarter in any field of interest at a local college or university (conflict of interest warning: I teach at a community college). The flexible hours of contract engineers often allow a few hours a week to be set aside for continuing education. Over the years, I've taken many business and computer language courses. I am now relearning calculus (after a 20 year break). It's much more interesting when you don't have to deal with lots of other classes, or, perhaps, when you're a bit older. Many colleges also offer tutoring to students. Working as a tutor (either volunteer or minimal wage) is a very good way of reviewing subjects. I'm currently tutoring algebra and calculus, Basic, Cobol, assembly and C languages, and accounting. The students and the instructors provide a very stimulating environment. Give it a try!
Harold Hallikainen is president of Hallikainen and Friends, a manufacturer of transmitter control and telemetry systems. He also teaches electronics at Cuesta College, San Luis Obispo. He can be reached at 805 541 0200. He can also be reached on internet at harold@hallikainen.com.