Indirect RF Hazards

Part 3 on Safety

Safety is an important topic in ham radio.  There are 11 questions on electrical hazards in the USA Technician class license exam pool, 13 questions on tower safety and associated grounding, and 13 questions on radio frequency (RF) hazards.

Part 1 on general electrical hazards and Part 2 on contact RF hazards were posted previously.  This post will address indirect RF hazards.  In case you are not familiar with the specifics of RF energy, refer to our post on the subject.

Here we are concerned about non-contact RF energy.  A long and involved topic (sorry about that) but full of useful detail.

While it involves radiation, RF energy radiates at lower wavelengths where it is least hazardous.

radiation spectrum.JPG

From the electromagnetic spectrum diagram above we see that radio waves are on the low end of energy levels.  As the frequency increases (wavelengths decrease) the energy in electron volts increases exponentially.  Energy above 250eV (or so) is ionizing, which in addition to radiation burns can cause cell damage and mutations, leading to cancer and other maladies, as would radioactive material.


Fortunately for hams, all radio frequencies are well below the ionizing radiation energy levels.


Ham radio operators are radio  active, not radioactive. 🙂Amateur radio activeNow just because RF radiation is non-ionizing doesn’t mean it is completely safe.  Besides the direct contact hazard, exposure to radio frequency energy may cause localized tissue heating, particularly in the eyes and male reproductive area (here’s where a lady ham has an advantage, hihi).  Non-thermal effects of RF radiation are being studied constantly because, while compelling, they are somewhat ambiguous and unproven.

Because RF energy has this radiated exposure risk, rules and regulations have arisen to protect people from such hazards.  In the USA this is done at the federal level by both the FCC (radio communications) and OSHA (occupational).  There are also guidelines for RF radiation published by the ARRL and the IEEE.  Internationally, most countries apart from the US have similar guidelines, as does the World Health Organization (WHO).  References to some of these are given at the end of this presentation.

Specific to US radio amateurs, the FCC instituted RF field exposure limits called Maximum Permissible Exposure (MPE).

Frequency is a factor in MPE.  Due to human physiology, tissue heats more at certain frequencies than others based on the specific absorption rate (SAR).

T0C05-2018.pngThe frequency range with the lowest MPE is 30-300MHz so the VHF band is of greatest concern for radiation:MPE Table

Note the difference in a controlled environment vs an uncontrolled one.

In a Controlled Environment you know where people are standing in relation to your antenna and you can do something about it.  Higher power density is allowed because you can make adjustments if needed.  6 minute average exposure.

In an Uncontrolled Environment you have no control of people near your antenna.  Lower power density is allowed because you cannot control or adjust the exposure of people.  30 minute average exposure.

MPE ChartThe most sensitive level includes the popular 6m (50MHz) and 2m (146MHz) bands.  Of the exam question choices, only 50MHz is in this range, although.


So we see that frequency is a factor in RF exposure.  Intuitively we also know that power level is also a factor; more power will heat tissue faster; less power not so much.  RF power also affects the body by the inverse square of the distance (geometric dilution) so proximity to (or rather, distance from) the radiator is yet another factor, as is the radiation pattern.


Proximity to an amateur radio antenna of any radiation pattern is the single most influential factor.  If your antenna is dozens or hundreds of feet away from the control operator position, it’s likely that you’ll be perfectly safe.


Conversely,  sitting right next to a directional antenna and using high transmit power on VHF bands is the most hazardous scenario.

Duty cycle is also a significant factor in MPE because it determines exposure over time (critical long-term exposure), unlike immediate or instantaneous levels which are not as critical to biological RF absorption.  It make sense that a transmission with a low duty cycle presents less exposure than a high duty cycle one.


Duty cycle is how long a transmitter is active in a given period.  Basically how much time you are transmitting vs receiving, measured in percent (%).


To help relate mode to duty cycle, refer to the chart below.  While generalized, it gives a basic idea for comparison of commonly-used mode duty cycle.

mode duty cycle

A higher duty cycle means higher average power density and exposure but it’s the average that counts; the MPE limit are based on average exposure.  So with a 50% duty cycle you can double the power density and remain in the safe limits of MPE.


In summary, to reduce RF exposure there are five things you can do:

  • Relocate or reorient antennas
  • Raise the antenna
  • Reduce antenna gain
  • Reduce RF power output
  • Change to a lower duty cycle mode

All US amateur radio stations must comply with the MPE limits, regardless of power, operating mode or station configuration. This requires an exposure evaluation.

There are three acceptable methods, all listed in the Technician license exam question:


The first is done with an online calculator referenced by OET Bulletin 65 and is fast and simple (recommended).  Computer modeling is also possible if you have the software and the knowledge and skills to use it (somewhat difficult).  Actual measurement of your RF fields requires calibrated equipment which is unavailable to most hams (expensive, difficult).  Obviously we recommend the online calculator.

Fortunately the FCC presumes that certain stations are safe without an evaluation. Those are:

  • Amateur stations using a transmitter power of less than 50 W PEP at the transmitter output terminal.
  • Mobile or portable stations using a transmitter with push-to-talk control.

This means that if you have a transmitter with less than 50W output, or if it has only a microphone for voice mode, an RF exposure evaluation is not required.  This includes most VHF/UHF mobile FM transceivers.


Once you have evaluated your station and are in compliance you should re-evaluate whenever anything related to RF emissions is added or changed (antenna, radio, new frequencies/mode, etc.)


RF exposure references

Ham radio school topic, RF Exposure

Ham radio school topic, Evaluating Your Station

US Federal Communications Commission (FCC) RF Safety FAQ

FCC Overview of RF Exposure Concepts and Requirements (includes detail on controlled vs uncontrolled environments)

ARRL topic, RF Radiation and Electromagnetic Field Safety

US Department of Labor OSHA topic,  Radiofrequency and Microwave Radiation

World Health Organization topic, Electromagnetic fields

IEEE Standards relating to RF radiation

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