Safety Compliance Distances
for Amateur Radio Antennas
in Canada

 

   In 2015, Health Canada revised Safety Code 6, "Limits of Human Exposure to Radiofrequency Electromagnetic Energy in the Frequency Range From 3 kHz to 300 GHz." [1]. These guidelines determine compliance distances—the separation to keep between an antenna and people in the area around it.

Half-Wave Dipole Example

   The red data points in Figure 1 show examples of Health Canada compliance distances in the amateur bands for a half-wave dipole fed with 100 watts average power. At low frequencies they are more conservative than distances based on the US Federal Communications Commission (FCC) guidelines as shown in blue [2].

Figure 1.  Controlled area compliance distances vs frequency for a half-wave dipole (average power = 100 W).

   Figure 1 applies to Controlled areas, where people are trained to be aware of the potential risk of RF exposure and to take appropriate precautions. Controlled area limits apply to amateur radio operators. Compliance distances in Uncontrolled areas that include the general public are more conservative, as shown in Figure 2.

Figure 2.  Uncontrolled area compliance distances vs frequency for a half-wave dipole (average power = 100 W).

   The horizontal dotted line in each figure shows distances based on guidelines from the International Commission on Non-Ionizing Protection (ICNIRP) [3]. These standards are used in the UK, and many other countries. They were published in 1998 and reviewed without change in 2009. The 2015 Health Canada exposure limits include dosimetric refinements in recent years. A working group is revising the ICNIRP guidelines and hopes to release a document for public comment before the end of 2017 [4].

Calculating Compliance Distances

   I calculated the distances in Figs. 1 & 2 using Eq 2 in the Formulas section below. The compliance distance in meters is

     D = 0.45 * SQRT (P * G / SMAX)    

where P = average power in watts, G = numeric free space gain, and SMAX = maximum permitted power density in watts per square meter

   For the dipole example at 14 MHz:  P = 100 W, G = 1.64 (2.15 dBi), and SMAX = 10 W/m2 [from Table 6 in Ref.1], so Eq. 2 becomes

     D = 0.45 * SQRT (100 * 1.64 / 10) = 1.8 m.

Other Antennas

   The ARRL Lab verified that the general purpose formula gives conservative estimates for a number of dipole, ground-plane, and Yagi antennas [5]. However, the formula is not accurate for all types of antennas. For example it underestimates safety distances for small loops [5] and sometimes for end-fed inverted-L antennas [6].

Other Methods

   • Antenna simulation with an NEC based computer program is often used for calculating compliance distances.

   • You can ask at an amateur radio association for referral to an antenna safety expert.

   • Measuring near fields accurately is not trivial.

   • VP9KF's online Amateur Radio Safety Calculator [7] and the tables in the ARRL RF Exposure book [5] are based on FCC guidelines.

General Purpose Formula

   The general purpose formula is based on a far-field approximation that gives an estimate of the power density [1,5]:

     S = 2.56 * P * G / (4 * pi * R2)    [W/m2]          (1)

with P and G as defined above, and R = distance in meters between the point of exposure and the closest part of the antenna.

The FCC-recommended factor 2.56 is an estimate of the increase in power density around an antenna due to ground reflections. It corresponds to an increase in field strength by the factor 1.6.

   At the compliance distance D, the power density is the maximum permitted exposure, SMAX. Solving Eq. 1 for D,

     D = 0.45 * SQRT (P * G / SMAX)       [m]       (2)

   The free space gain is often calculated from an NEC antenna model without ground. Table 5.7 in Ref. 5 shows typical free space gains for some antennas. Table 3 in the OET Bulletin converts gain in dBi to numeric gain G [2].

Below 10 MHz

   The maximum permitted power density (to use in Eq. 2) for frequencies below 10 MHz is not specified in the Safety Code 6. For the general purpose formula the FCC uses the effective plane wave power density E2 / 377 [W/m2] in this frequency range, where E is the maximum permitted electric field [V/m]. From Table 3 of the Safety Code this limit is E = 193 / SQRT(f) for controlled areas, where f is the frequency in MHz. So, the maximum permitted power density
SMAX = E2 / 377 = 99 / f  [W/m2]. For example, at 7 MHz,
SMAX = 99 / 7 = 14.1 W/m2, and for the dipole example the controlled area compliance distance from Eq 2 is 1.5 m.

Conclusions

   The 2015 Health Canada exposure guidelines include recent refinements in dosimetry. The general purpose formula gives conservative estimates for a number of dipole, ground-plane, and Yagi antennas.

Disclaimer

   This article is provided for educational purposes only, and it is presented without warranty of any kind. It is not intended to provide advice or specific compliance distances for you to use. You can choose compliance distances by using the references listed below, or consult an expert. I do not accept responsibility for claims that result from the use of this information.

 

References

1. 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.

2.  Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields, OET Bulletin 65b (1997). Accessible at this FCC site (pdf).

3.  ICNIRP Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic and Electromagnetic Fields, Health Physics 74(4) 494-522 1998. Accessible at this ICNIRP site (pdf).

4.  The ICNIRP Work Plan and updates are accessible at this ICNIRP site.

5.  RF Exposure and You, Ed Hare, W1RFI, ARRL, 1998

6.  Safety Compliance Distances for Three Multi-Band, End Fed Inverted-L Antennas, P. DeNeef, AE7PD. Accepted for publication in the Nov-Dec, 2017, issue of QEX.

7.  Amateur Radio Safety Calculator, Paul Evans, VP9KF

 

Author Information

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

 

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rev. 9/29/2017