## Safety Compliance Distances

for VHF Antennas

RF exposure safety is an isssue that affects every amateur radio operator. Absorption of RF energy can cause injury without warning signs of pain or even a sensation of warmth.

**Safety Guidelines**

The guidelines for maximum permitted RF exposure depend on the country where you operate. In the US, standards are set by the Federal Communications Commission (FCC) [1]. Standards developed by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) are used in the UK and many other counties [2].

In some countries other guidelines are used. Check with an amateur radio association where you operate if you have questions. For example in 2015, Health Canada adopted somewhat more restrictive exposure limits than the FCC and ICNIRP guidelines [3].

**I used 1997 FCC guidelines [1] and the 1998 ICNIRP guidelines [2] for the examples in this article.** The FCC and ICNIRP guidelines were reviewed in 2005 and 2009, respectively, with no change. An ICNIRP working group is reviewing the guidelines and hopes to have a proposal for public comment by the end of 2017 [4].

In general, the FCC and ICNIRP guidelines for dosimetry (maximum permitted exposures) differ for the amateur HF bands, but they are in agreement for the VHF frequencies, 30 - 300 MHz.

**General Purpose Formula**

For some commonly used antennas you can use a general purpose formula for calculating compliance distances [1]. NEC antenna simulations at the ARRL Lab verified that the formula gives conservative estimates for a number of dipole, ground-plane, and Yagi antennas [5].

I used the general purpose formula for the examples in this article. There are three resources for using this formula: (1) VP9KF's online calculator [6]; (2) the ARRL General Purpose Tables [5]; or (3) You can use the formula directly, as demonstrated in the Formulas section below. If guidelines are changed, you can enter the new limits in Eq 2 below.

Both the FCC and the ICNIRP distinguish between two types of exposed poplulations or areas around an antenna. An occupationally eposed population is trained to be aware of the potential risk and to take appropriate precautions. This group includes amateur radio operators. Members of the general public can not be expected to take precautions to avoid exposure, so the safety standards are more conservative. Also, the public includes individuals of varying health status and may include particularly susceptible individuals. The FCC defines these two tiers of exposure limits in terms of controlled and uncontrolled areas.

**Half-Wave Dipoles and J-Poles**

The solid lines in **Figure 1** show FCC/ICNIRP compliance distances *vs *average power for horizontal or vertical VHF half-wave dipoles. Distances are measured from the part of the antenna closest to the point of exposure. The numerical gain in free space used in the calculations is G = 1.64 (2.15 dBi) [5].

Figure 1. Compliance distances vs average power for half-wave dipoles (solid lines) and vertical quarter-wave monopoles (broken lines) from 30 MHz to 300 MHz.

For a classic J-Pole and a non-standard version, W4RNL calculated gains in free space near 2.5 dBi [7], so compliance distances are a little longer than those in Fig. 1 for half-wave dipoles.

There are many variations on the J-pole design for which you might not know the gain. The options in that case are discussed in Other Antennas below.

**Vertical Quarter-Wave Antennas**

The broken lines in **Figure 1** show compliance distances for vertical VHF quarter-wave monopoles—ground-planes and mobile whips. The free space gain is 1.26 (1.0 dBi) [5].

**Discones**

The estimated gain of a VHF/UHF discone antenna example in

Ref. 1 is 2.0 dBi. This conservative (high) estimate gives safety distances similar to those for the half-wave dipole in Fig. 1.

**Yagi Arrays**

The general purpose formula has also been validated for use with Yagi arrays [1,5]. **Figure 2** shows compliance distances for a 5-element horizontal VHF Yagi array with a free-space gain of 8.71 (9.4 dBi) [5].

Figure 2. FCC/ICNIRP compliance distances *vs* average power for a 5-element horizontal VHF Yagi array (30 - 300 MHz).

**Longer Verticals**

Higher gain vertical antennas for base stations and mobile use have a longer, coil-loaded vertical element. The manufacturer specifications usually quote a far-field gain that includes ground reflections, which is higher than the free space gain used in Eqs 3 & 4. The formulas include a field multiplication factor of 1.6 for the effect of ground currents near the antenna, so using a "real-world gain with ground reflections" includes ground effects twice. However, the resulting extra-conservative estimate might be useful when the free-space gain is unknown.

**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] The formula is not accurate for all types of antennas. For example, it underestimates safety distances for small loops [5] and end-fed inverted-L antennas [8].

Measuring near field strengths accurately is not trivial. Another option for calculating compliance distances, antenna simulation with an NEC-based computer program, is discussed below.

If the antenna is not one of the types for which the general purpose formula has been validated, you can consult an expert in an amateur radio association or in the organization that develops the RF safety guidelines.

**Handheld Transceivers**

The general purpose formula is not accurate when your body is very close to the antenna. Manufacturers are required to limit the specific absorption rate (SAR) for RF devices that are used close to the human body. SAR can be calculated using a specialized simulation program or from measurements of the 3D electric field distribution (or, less commonly, the temperature distribution) inside a phantom containing a liquid electrolyte.

**Mobile Antennas**

Researchers at Motorola used an electrolyte-filled, human-equivalent phantom to perform SAR measurements in the back seat of a vehicle with a vertical quarter-wave VHF monopole mounted on the trunk lid [9]. RF power "leaks" into the vehicle interior by diffraction through the rear window aperture, which has dimensions smaller than a wavelength. For transmitters with 100 - 120 W power, the exposure they measured is below the FCC limit for an uncontrolled area.

In Ref. 5, W1RFI recommends a conservative approach of mounting a mobile antenna in the middle of an all-metal roof to shield the occupants from RF.

**Reflections**

The general purpose formula does not include the effect of nearby metal objects that can reflect a signicant amount of power and create dangerous hot spots.

**Formulas**

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

S = 2.56 * P * G / (4 * pi * R^{2 }) [W/m^{2}] (1)

where P = average power input [W],

G = numeric free space gain (not dB),

R = distance between the point of exposure and the nearest part

of the antenna [m].

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, S_{MAX}. Solving Eq 1 for D,

D = 0.45 * SQRT (P * G / S_{MAX} ) [m] (2)

The free space gain comes from far field antenna patterns generated with no ground in the antenna model. Table 5.7 in Ref. 5 shows typical free space gains from some antennas. Table 3 in the OET Bulletin converts gain in dBi to numeric gain G [1].

The FCC and ICNIRP maximum permitted power density exposure is independent of frequency in the VHF range, 30 - 300 MHz: 10 W/m^{2 }in occupational/ controlled areas and 2 W/m^{2} in public/ uncontrolled areas. Using these limits in Eq 2 gives the formulas for the occupational/ controlled compliance distance (D_{O}) and public/ uncontrolled distance (D_{P}):

D_{O} = 0.143 * SQRT (P * G) [m] (3)

D_{P} = 0.319 * SQRT (P * G) [m] (4)

**Antenna Simulations**

The ARRL NEC tables in Ref. 5 give FCC compliance distances that are more accurate than estimates from the general purpose formula. For example, the table for a 2-meter band quarter-wave ground-plane [100 watts, 146 MHz, 6 ft (1.8 m) above ground] shows the controlled and uncontrolled compliance distances are 1.2 m and 2.9 m, respectively, compared to 1.6 m and 3.6 m in Fig. 1.

Eqs 2-4 are estimates of the worst case compliance distance—the farthest point from an antenna where the RF field strength is in compliance. The NEC tables give horizontal compliance distances at various heights for an antenna at a specified height. For the 5-element Yagi example, the NEC tables show that even at 400 watts, exposure is in compliance with occupational limits at every point more than 2.4 m (8 ft) below the antenna.

A modeling error can lead to NEC compliance distances that are too close to the antenna. The EZNEC program displays a disclaimer with all near field calculations [10]:

*"ATTENTION: This software CANNOT BE USED TO tell whether (1) the amount of electromagnetic energy being emitted from an antenna I unsafe to anyone; (2) an antenna subjects anyone to potentially hazardous electromagnetic exposure. LICENSOR DISCLAIMS ANY AND ALL WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. *

* Other disclaimers apply. Refer to the EZNEC on-line help for the complete text. DO NOT USE this software to determine whether an antenna is emitting an unsafe or hazardous level of energy." *

**Conclusions**

The compliance distance formula gives conservative estimates for a number of VHF dipole, ground plane, and Yagi antennas. Current ICNIRP and FCC standards for the VHF bands are the same, so you can use VP9KF’s online calculator, FCC and ARRL tables, or the general purpose formula to find FCC and ICNIRP compliance distances for some VHF 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 should 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. Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields, OET Bulletin 65b (1997). Accessible at this FCC site (pdf).

2. 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).

3. 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. Some examples are in Safety Compliance Distances for Amateur Radio Antennas in Canada.

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

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

6. P. Evans, VP9KF, Amateur Radio RF Safety Calculator.

7. Some J-Poles That I Have Known, L.B. Cebik, W4RNL. Accessible at this archive.

8. Safety Compliance Distances for Three Multi-Band, End Fed Inverted-L Antennas, P. DeNeef, AE7PD. Accepted for publication in QEX.

9. Field Strengths and Specific Absorption Rates in Automotive Environments, D.O. McCoy, D.M. Zakharia, and Q. Balzano, IEEE Transactions on Vehicular Technology 48(4) 1287-1303 1999

10. R. Lewallen, W7EL, EZNEC Software and User Manual. www.eznec.com

**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.

RF Exposure Safety Articles Index

7/17/2017

rev. 7/24/2017