accessible areas.6 However, at higher power levels or with different facility configurations, higher exposure levels cannot be completely ruled out. Even though this study was designed to evaluate typical stations, it represents only a small sampling of many possible amateur radio facilities. There is a wide variety of possible amateur station characteristics and operating parameters.
It is important to emphasize that continuous "key down" amateur transmissions, such as those measured here, represent a worst-case and would not be typical of most amateur communications. Rather, a duty factor would be associated with routine amateur transmissions.
The ANSI/IEEE guidelines specify a six-minute period for time-averaging of field strength and power density levels at most frequencies and in "controlled" environments. Assuming that amateur stations fall under the "controlled" category, this means that during a given six-minute period if a station transmitted for only one minute of the six-minute period, the time-averaged exposure would actually be one-sixth of the exposure level resulting from the one-minute of signal transmission. During the one- minute of "on" time, the allowed exposure could be as high as six times the exposure limit (Maximum Permissible Exposure or "MPE") specified by the guidelines. For example, if the applicable limit were 1 milliwatt/cm2 (1 mW/cm2), or 1000 microwatts/cm2 (1000 µW/cm2), the allowed exposure during the one-minute period of transmission would be six times the limit or 6 mW/cm2 (6000 µW/cm2), so that the average over the six minutes would be 1 mW/cm2.
An excellent discussion of controlling RF exposures at amateur radio stations can be found in the ARRL Handbook for Radio Amateurs (Reference 5). In general, precautionary measures should be sufficient to prevent exposure of the amateur operator or other persons to RF levels in excess of protection guidelines. Examples of such measures are:
using only the minimum power necessary for a transmission; minimizing transmission time so that time-averaged exposures are acceptable; identifying high-field areas and restricting access to them while transmitting; mounting antennas as high above ground as practical.
We hope that this study will provide amateur radio operators with information on environmental RF fields that will help ensure the prudent and safe operation of amateur facilities. We encourage further study and research into the measurement
6 This study focused on field strength and power density measurements and how they compare with the RF protection guidelines. However, it should be noted that the ANSI/IEEE 1992 guidelines for Maximum Permissible Exposure (MPE) also include limits for induced and contact RF currents (see Appendix B). Measurements to determine such currents were not made as part of this survey, but this could be the subject of a future study.