HF Small Transmitting Loop
Compliance Distances
(Article revised 7/4/2023)
FCC Compliance Distances
RF exposure safety limits depend on the country where you operate. The compliance distances in this article are based on the US Federal Communications Commission (FCC) guidelines. These guidelines are different from the International Commission on
Non-Ionizing Radiation Radiation Protection (ICNIRP) guidelines used in many other countries.
Effective May 3, 2021, the FCC changed the rules for evaluating RF exposure from amateur radio stations. The changes are summarized on the “RF Exposure” page of the ARRL web site [1]. That page includes a link to the ARRL’s RF Exposure Calculator, which is based on a simple, general purpose formula for compliance distances for many amateur radio antennas. Unfortunately, the formula underestimates distances for small loops [2]. The compliance distances quoted below are from published results of analytical calculations by QEX editor Kai Siwiak, KE4PT.
A False Sense of Security
There is a null in the far field antenna pattern along the axis perpendicular to the loop. That might give you a false sense of security if you expect a similar null in the near fields. The RF magnetic field near a small loop is strongest along the axis perpendicular to the loop.
Small Loop Compliance Distances
In the Technical Correspondence section of QST, May, 2017, QEX editor Kai Siwiak, KE4PT, summarized his calculations of compliance distances for typical loops used by amateur radio operators. He lists compliance distances for a 1 m diameter loop for 5 watts and 10 watts average power [3], as well as references to previously published distances for 150 watts [4] and 1500 watts [5].
KE4PT used FCC guidelines for maximum permitted exposures. He assumes that transmissions are continuous, and he includes the effects of ground reflections.
Table 1 below summarizes his results for controlled and uncontrolled areas of exposure. In controlled areas people are trained to be aware of the potential risk and to take appropriate precautions. This includes amateur radio operators. In uncontrolled areas—where the general public could be exposed—the limits are more conservative.
Distances are measured in feet from the center of the loop.
Table 1. HF small transmitting loop FCC compliance distances (ft) calculated by
KE4PT [3-5].
Average Controlled Uncontrolled
Power Compliance Distance Compliance Distance
5 W 4.0 ft* 5.6 ft*
10 W 4.9 ft* 6.9 ft*
150 W 6.6 - 7.9 ft** 9.2 - 13.8 ft**
1500 W 9.9 - 19.4 ft*** 17.4 - 42.4 ft***
*From Ref. 3. At low power, compliance distances are nearly constant for the bands 40-10 meters.
**From Table 17 in Ref. 4: Distances in controlled areas increase from 6.6 ft for 40 meters to 7.9 ft for the 10 meter band. In uncontrolled areas they increase from 9.2 ft for 40 meters to 13.8 ft for 10 meters.
***From Ref. 5: Distances in controlled areas increase from 9.9 ft for 80 meters to 19.4 ft for the 10 meter band. In uncontrolled areas distances increase from 17.4 ft for 80 meters to 42.4 ft for 10 meters.
KE4PT warns about overexposure to RF from a pedestrian mobile or backpack loop, even at remarkably low operating power if you are very close to the loop [3].
References
[1] http://arrl.org/rf-exposure
[2] Ed Hare, FCC-RF Exposure Regulations: See his comment in the section “Tables Developed from Formulas”
http://www.arrl.org/fcc-rf-exposure-regulations-the-station-evaluation
[3] Kai Siwiak, KE4PT, QST Technical Correspondence, May, 2017, pp. 64-65
[4] Kai Siwiak, KE4PT, Table 17 in Supplement B to the FCC OET Bulletin 65, 1997. Accessible at https://www.fcc.gov/general/oet-bulletins-line (Currently under revision.)
[5] Kai Siwiak, KE4PT, QST Technical Correspondence, March, 2012, p. 54
Author Information
Peter DeNeef, AE7PD, is an Extra class amateur radio operator in the USA. This website has no ads or conflicts of interest.
Email: HamRadioAndVision "at" gmail "dot" com.
Index to RF Exposure Compliance Articles
Rev 7/4/2023