HF Small Transmitting Loop
Safety Compliance Distances


FCC, ICNIRP, and Canadian Guidelines


   Safety compliance distances depend on the country where you operate. The compliance distances in the first part of this article are based on the US Federal Communications Commission (FCC) guidelines.

   In the second part the distances are based on the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines. ICNIRP standards are used in the UK and many other countries. This section also includes information on how to receive a copy via email of my 2017 RadCom article on small loop safety distances.

   In the third part the distances are based on Health Canada Safety Code 6.

   There is an important note at the end about the RF magnetic field distribution near a small transmitting loop.

FCC Compliance Distances

   QEX editor Kai Siwiak, KE4PT, recently summarized his calculations of compliance distances for typical loops used by amateur radio operators. In the Technical Correspondence section of QST, May, 2017, he lists compliance distances for a 1 m diameter loop for 5 watts and 10 watts average power [1], as well as references to previously published distances for 150 watts [2] and 1500 watts [3].

   KE4PT used FCC guidelines for maximum permitted exposures [4]. He assumes that transmissions are continuous, and he includes the effects of ground reflections.

   Tables 1a and 1b below summarize 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 (Table 1a) or in meters (Table 1b) from the center of the loop.

Table 1a.  HF small transmitting loop FCC compliance distances [ft] calculated by KE4PT [1-3].

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. 1. At low power, compliance distances are nearly constant for the bands 40-10 meters.

**From Table 17 in Ref. 2: 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. 3: 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.

 

Table 1b.  Table 1a expressed in meters [1-3].

Average               Controlled                     Uncontrolled
 Power          Compliance Distance*      Compliance Distance*

      5 W                    1.2 m*                                 1.7 m*

    10 W                    1.5 m*                                 2.1 m*

  150 W               2.0 - 2.4 m**                      2.8 - 4.2 m**

1500 W               3.0 - 5.9 m***                    5.3 - 12.9 m***

 *See footnotes with Table 1a for explanations of how these depend on the frequency.

   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]. The results of his analysis are also quoted in the June 2017 RadCom article [6], which you can download via a link in the ICNIRP section below.


International (ICNIRP) Compliance Distances

   At most HF frequencies the ICNIRP guidelines are more conservative than those from the FCC. As a result, ICNIRP compliance distances are generally longer.

   Table 2. ICNIRP compliance distances for a 1 m diameter loop from RadCom, June, 2017 [6]. By permission of the Radio Society of Great Britain a free PDF copy of my RadCom article is available via email request to HamRadioAndVision "at" gmail "dot" com.

Average       Longest Controlled          Longest Uncontrolled
 Power         Compliance Distance*     Compliance Distance*

  10 W                    2.1 m                                    2.9 m

100 W                    3.4 m                                    4.7 m

400 W                    4.5 m                                    7.3 m

  *For the 10 - 40 meter bands.

   An ICNIRP working group released a draft of revised guidelines for public comment in July, 2018 [7].
 

Health Canada Guidelines

   In 2015, Health Canada revised Safety Code 6, the guidelines for RF exposure safety in Canada [8]. They are similar to the 1997 ICNIRP guidelines, but with differences due to recent refinements in dosimetry.

   Table 3 shows loop compliance distances based on the revised Health Canada guidelines. Compared with the ICNIRP distances from the same NEC model (Table 2), the only change in the largest distances for 10 - 40 meters is the uncontrolled distance for 400 watts (8.2 m instead of 7.3 m).

Table 3.  Health Canada compliance distances for a 1 m diameter loop.

  Average         Longest Controlled       Longest Uncontrolled
   Power         Compliance Distance*     Compliance Distance*      

    10 W                        2.1 m                               2.9 m

  100 W                        3.4 m                               4.7 m

  400 W                        4.5 m                               8.2 m

*For the 10 - 40 meter bands.

   Tables 1-3 in the June 2017 RadCom article show ICNIRP compliance distances at 7, 14, 18, 21, and 28 MHz [6]. Health Canada distances are the same as ICNIRP distances at each frequency except 28 MHz. An emailed copy of the RadCom article is available on request from HamRadioAndVision “at” gmail “dot” com.

Table 4 shows Health Canada compliance distances at 28 MHz, which are different from the ICNIRP distances in [6].

 Average              Controlled                     Uncontrolled
  Power          Compliance Distance     Compliance Distance

   10 W                      1.5 m                              2.2 m

 100 W                      2.6 m                              4.3 m*

 400 W                      4.0 m                               8.2 m*

*E field limit from high voltage across the tuning gap.

 

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.
 

Calculating Loop Compliance Distances

   There is a simple, general purpose formula you can use to calculate compliance distances for many amateur radio antennas, but that calculation underestimates the distances for small loops [4,9]. Instead, KE4PT uses analytical methods to calculate the near fields. I used his results as a gold standard to check the accuracy of the NEC antenna model in the RadCom article.

References

[1] Kai Siwiak, KE4PT, QST Technical Correspondence, May, 2017, pp. 64-65

[2] Kai Siwiak, KE4PT, Table 17 in FCC OET Bulletin No. 65b, 1997. Accessible at https://www.fcc.gov/general/oet-bulletins-line

[3] Kai Siwiak, KE4PT, QST Technical Correspondence, March, 2012, p. 54

[4] Table 1 in Appendix A of FCC OET Bulletin No. 65b, 1997. Accessible at https://www.fcc.gov/general/oet-bulletins-line

[5] ICNIRP Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields, Health Physics 74(4) 494-522 1998. Accessible at http://www.icnirp.org/cms/upload/publications/ICNIRPemfgdl.pdf

[6] P. DeNeef, AE7PD, Safety Distances for HF Small Transmitting Loops, RadCom, June, 2017, pp 46-47. By permission of the Radio Society of Great Britain, a free PDF of the article is available on request via email to HamRadioAndVision "at" gmail "dot" com.

[7] Updates are posted at http://www.icnirp.org/en/activities/work-plan/details/work-plan-hf.html

[8] Limits of Human Exposure to Radiofrequency Electromagnetic Energy in the Frequency Range 3 kHz to 300 GHz, Health Canada Safety Code 6 (2015). Accessible at this Health Canada site.

[9] Ed Hare, W1RFI, RF Exposure and You, ARRL, 1998. No longer being published. See Section 2 of Supplement B to the FCC OET Bulletin 65, which is reference [4] above.

 

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 Safety Articles

Small Loop Articles

HamRadioAndVision Home Page


rev 11/14/2018