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 includes a link to download a pdf copy of my article from the June 2017 RadCom.

   In the third part the distances are based on the 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 100 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].

                 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.

   You can download the June 2017 RadCom article (pdf), which includes tables of distances for each band. The article is used here with permission from the RSGB (copyright © RSGB 2017).

   A working group is revising the ICNIRP guidelines and hopes to have a draft ready for public comment by the end of 2017 [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. A link to a downloadable pdf is in the ICNIRP section above.

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 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. Downloadable pdf is accessible here.  Copyright © RSGB 2017, www.rsgb.org. Used here with permission.

[7] Updates will be 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


 

Author Information

    Peter DeNeef, AE7PD, is an Extra class amateur radio operator in the US. This Web site has no ads or conflicts of interest.
Email:  HamRadioAndVision "at" gmail "dot" com.

 

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rev 8/3/2017