The FCC defines a portable device as one that is normally used with its antenna within 20 cm of the body of the user. A handheld radio transmitter, such as a cellular telephone, is an example of such a device, and local SAR determinations are required, since its energy is absorbed in a small area of the body, usually the head. It is not meaningful to determine the power density at the head, as would be applicable for transmissions from an antenna that is farther away from the body. The FCC requires that many handheld radios meet the safety requirement that the SAR must be less than 1.6 W/kg in any one gram of tissue. It is assumed that not all cellular telephone users will be informed about RF safety, so the General Population limit is applied. It is acceptable to either measure absorption in a phantom head, or to perform a computer model of absorption, using one-gram regions of tissue in both cases.
The FCC has exempted amateur handheld transmitters from testing for compliance with SAR limits. There are several reasons for this. One is that hams are assumed to be in the controlled population, and the acceptable peak localized SAR of 8 W/kg is higher than a handheld transmitter would normally be expected to cause. Also, unlike cellular telephones, which transmit continuously when in use, an HT is a “Push-to-Talk” device, that only transmits when the ham is speaking.
The Cell Phone Debate
As the question of cell phone safety continues to be argued in the general society, we, as hams, cannot sit back and be spectators. Many of the same issues that affect cell phone users also affect us. With regard to exposure limits, the FCC considers us to be part of the informed population. We need to understand as much as possible about the issues affecting safe exposure. When using our H-Ts, we need to know why they were exempted from testing and operate accordingly. Since the exemption was based in part on the assumption of a relatively low duty cycle, we should avoid holding the PTT button and chatting nonstop for six minutes at a time. Above all, let’s strive to operate safely and to understand how and why we are doing so.
Editor’s note: Greg Lapin, N9GL, started working in the RF safety world after spending many years first studying cardiac function imaging and then brain tumor kinetics. He serves as chairman of the ARRL RF safety Committee and as a member of the IEEE Committee on Man and Radiation. A former professor of Biomedical Engineering and Neurology at Northwestern University, Lapin now works as a consulting professional engineer in the electronics industry. He was first licensed while a teenager in 1969 and continues to be fascinated by virtually all aspects of Amateur Radio. One of his many interests is electronic design, and he is the author of Chapter 8, “Analog Signal Theory and Components” in The ARRL Handbook for Radio Amateurs. His non-ham interests include making things grow in his garden and serving as commissioner of the local children’s softball league. At other times–when he is not working or helping his kids with their homework–you might find him
with the local emergency services agency, climbing his tower, building a new QRP rig, playing with his APRS setup, responding to QSL cards, going off on a DXpedition, or trying to get that “new one.” You can reach him by email at firstname.lastname@example.org.