RF Exposure Calculator

FCC has adopted guidelines and procedures for evaluating environmental effects of RF emissions. The new guidelines incorporate two tiers of exposure limits based on whether exposure occurs in an occupational or “controlled” situation or whether the general population is exposed, or exposure is in an “uncontrolled” situation. In addition to guidelines for evaluating fixed transmitters, the FCC adopted limits for evaluating exposure from mobile and portable devices, such as cellular telephones and personal communications devices.

To use the RF Exposure Calculator, fill-in the form below with your operating power, antenna gain, and the operating frequency. Depending on how far above ground the RF source is located, you might want to consider ground reflections — and then click “Calculate”.

Reference and explanation of terms

Power at antenna

The power at antenna parameter is the radiated power in watts at the antenna. For best results you should account for feedline loss, poor SWR, etc. At VHF/UHF frequencies, the feedline loss with coax can be significant. For example, transmitting 100W into 80 feet of RG-8X coax to an antenna with an SWR of 1.4:1 at 146.52 MHz will result in only 46W of actual power at the antenna. Here is an excellent coax loss calculator. Alternatively, you can be conservative and enter the transmitter’s output power (which is naturally a higher power than at the antenna) if you cannot determine what that actual power is at the antenna. Try to be as accurate as you can.

Mode duty cycle

The mode duty cycle effects the average output power based on usage.

Some mode duty cycles are:

• Conversational SSB with no speech processing, uses a 20% duty cycle which includes voice characteristics and syllabic duty factor.
• Conversational SSB with heavy speech processing, uses a 50% duty cycle which includes voice characteristics and syllabic duty factor.
• Voice FM, uses a 100% duty cycle.
• FSK or RTTY, uses a 100% duty cycle.
• AFSK SSB, uses a 100% duty cycle.
• Conversational CW, uses a 40% duty cycle
• Carrier always on (e.g. commonly used for tune-up purposes), uses a 100% duty cycle.
• For all others, or if unknown, uses a 100% duty cycle as a worst-case catch-all.

To figure out your percentage of transmitting, enter the number of minutes you transmit, followed with the number of minutes you receive. This effects the average output power.

Antenna gain

One of three conditions will likely apply to you.

• You could have a detailed antenna model. If so, along each lobe of the model, or direction of interest, use the dBi gain of the antenna derived from the model. This likely will mean running the calculator several times, but it will result in a the most detailed picture of your situation.
• Or you might have an antenna model from the manufacturer with gain and radiation pattern information. If so, you could use the same process as #1 using the manufacturer’s model. This will give you a good rough picture.
• Finally, if you have no idea what your antenna’s radiation pattern looks like, use the antenna gain reference information below as a first approximation. Refine your picture as you research your situation over time.

Controlled vs. Uncontrolled

This parameter takes into account whether or not individuals are aware of the RF exposure or not. A Controlled Environment is an area where the persons exposed to RF are aware of the exposure and its effects. An Uncontrolled Environment is an area where the persons exposed to RF are unaware of the exposure and its effects.

Effect of ground

The effect of ground parameter, when checked, the effects of signals reflecting off the ground are included in the calculation. Use this setting for low or non-directional antennas. This is a more conservative way of estimating RF exposure.

Antenna gain reference

* Source: Informal discussions within our team, aka. The School of Hard Knocks
** Source: RSGB EM Field Exposure web site at https://rsgb.org/main/technical/emc/emf-exposure/
Calculator: RSGB/Ofcom calculator (3.0 MB MS-Excel Worksheet)

Notes:

(1) Antennas with even moderate directionality may need to be evaluated in multiple directions to get a complete picture of their overall RF exposure.

(2) Small mag loops are particularly difficult to calculate their near field RF exposure. See the ARRL Antenna book, Chapter 5 “Loop Antennas”, section 5.3 “Small Transmitting Loops” and Table 5.5 on page 5.19.

Calculation Sources:
FCC’s OST Bulletin 65, originally issued in 1985 and revised in 1997, provides additional guidance for evaluating compliance with the latest FCC policies and guidelines and is available here. A supplement for the Amateur Radio Service (Supplement B) is available here. Likewise a supplement for Mobile and Portable equipment (Supplement C) is available here.

Credits:
The Javascript contained on this page was derived from a public domain BASIC program written by Wayne Overbeck N6NB and published in the January, 1997 issue of CQ VHF, p. 33. Terms: GNU Licence. To view the Javascript that performs the calculation, right-click on this page and select ‘View Page Source’.