Jump to content

How to do an RF exposure evaluation

From HamWiki
How to do an RF exposure evaluation
Applies toEvery US amateur station, under 47 CFR §1.1307 and §97.13(c)
TimeAn hour or two for a typical home station
Skill levelBeginner to intermediate
You will needYour station's power, antenna and band details, a tape measure, and a calculator or the ARRL's online calculator
Key dates
Rules took effect3 May 2021
Deadline for existing stations3 May 2023
New or changed stationsBefore they go on the air

How to do an RF exposure evaluation walks through checking that a US amateur station keeps the radio-frequency fields around its antennas within the FCC's limits, as every licensee must. Under rules that took effect on 3 May 2021, a station no longer escapes the requirement just because its power is low.[1] Each licensee first checks whether the station qualifies for an exemption, and if it does not, carries out a routine evaluation and fixes any problem it finds. For what RF exposure is and why the limits exist, see RF exposure. These are US rules; other countries set their own, as described below.

Before you start

  • Know the two sets of limits. The FCC's maximum permissible exposure (MPE) limits in §1.1310 come in two tiers. The controlled limits may be applied to you and to members of your household who have been told about the exposure and know how to reduce it. Everyone else, including neighbors and passersby, is judged against the stricter uncontrolled, or general population, limits.[2]
  • Know that power is averaged. The limits apply to exposure averaged over time: over 6 minutes for the controlled limits and 30 minutes for the uncontrolled limits.[3] Two things bring the average down: the duty factor of the mode, and the share of the averaging period you spend transmitting rather than listening.
  • Gather the numbers. For each band and antenna you use, write down the transmitter power, the feed line loss, the antenna gain and the modes you use. Then measure the shortest distance from any part of each antenna to places where people can be: rooms in your house, your yard, a neighbor's yard, a sidewalk.

Step by step

  1. Work out the power that reaches the antenna. Start with the transmitter's output and subtract the feed line loss. A long run of thin coax on 2 meters can lose a good share of the power, which counts in your favor here.
  2. Apply the duty factor of the mode. The ARRL calculator uses 20 percent for conversational SSB without speech processing, 50 percent for SSB with heavy processing, and 40 percent for conversational CW.[4] FM, AM, RTTY, AFSK data modes such as FT8, and a carrier for tuning all count as 100 percent. If you don't know the duty factor of a mode, use 100 percent.[4]
  3. Apply time averaging. Estimate the most time you would transmit in any 30-minute period (or 6 minutes for the controlled limits). An FT8 station that transmits in every other 15-second period is on the air half the time. Multiplying power, duty factor and transmit share gives the time-averaged power.
  4. Convert to ERP. The exemption rules use effective radiated power (ERP), which is the time-averaged power multiplied by the antenna's gain over a dipole.[5] A simple dipole has a gain of about 0 dBd, so its ERP is close to the power fed to it. If the gain is given in dBi, subtract 2.15 to get dBd.
  5. Check the exemption formula. §1.1307(b)(3) lets a station skip the routine evaluation when its ERP is no more than a threshold set by frequency and by the distance R, in meters, to the nearest person.[5] The formula may only be used where R is at least λ/2π,[5] a little under a sixth of a wavelength. If you run several transmitters at once, add up each one's fraction of its own threshold; the total must not exceed 1.[5]
  6. Do a routine evaluation if the station isn't exempt. The ARRL's online RF exposure calculator asks for the power at the antenna, the mode's duty factor, your transmit and receive times, the antenna gain in dBi and the frequency, and reports the minimum compliance distance for both the controlled and the uncontrolled limits.[4] Compare those distances with your measurements. The FCC's OET Bulletin 65 and its Supplement B, written for amateur stations, give the same kind of equations and tables of typical distances.[6] Computer modeling or measurement with suitable instruments are also accepted.
  7. Run every combination. Repeat the check for each band, antenna and mode you use, and for each direction a beam can point. The highest-power, highest-duty case on the band with the strictest limits usually decides the result.
  8. Fix anything that fails. If the fields could exceed the limits anywhere people can go, §97.13(c) requires the licensee to act to prevent exposure.[2] Lower the power, move or raise the antenna, transmit for less of each period, keep people out of the area while you are on the air, or point a beam elsewhere.
  9. Keep a record. The FCC does not ask for evaluations to be filed, and the rules do not require you to keep them, but the ARRL advises keeping them on file.[6] Note the date, each band and antenna, the power, duty factor and transmit times you assumed, the distances you measured, the method you used and the result.
Redo the evaluation whenever you add an amplifier, put up or move an antenna, add a band or start using a high-duty mode such as FT8. Under the rules, a new or modified station must be evaluated before it goes on the air.

The exemption formula

In each formula below, R is the distance in meters from the antenna to the nearest person, f is the frequency in MHz, and the result is the largest time-averaged ERP, in watts, that is exempt.

Frequency Threshold ERP (watts)
0.3–1.34 MHz 1,920 R²
1.34–30 MHz 3,450 R² / f²
30–300 MHz 3.83 R²
300–1,500 MHz 0.0128 R² f
1,500–100,000 MHz 19.2 R²[5]

Two examples show how this works. A 100-watt SSB station on 14.2 MHz, without speech processing and transmitting half of the time, has a time-averaged power of about 10 watts, and with a dipole its ERP is about 10 watts. At 5 meters from the nearest person, the threshold is about 428 watts, so the station is exempt. By contrast, a 50-watt FM station on 2 meters transmitting half of the time into an antenna with 3 dBd of gain has an ERP of about 50 watts. At 3 meters the threshold is only about 34 watts, so that station needs a routine evaluation.

The λ/2π condition matters most on the low bands. On 80 meters it works out to roughly 13 meters, and on 160 meters roughly 25 meters, so many stations on those bands can't use the formula for nearby people and go straight to a routine evaluation. Separate exemptions cover sources very close to the body, such as a handheld radio held at the face: one based on specific absorption rate (SAR), which applies only between 300 MHz and 6 GHz and at distances of 0.5 to 40 centimeters, and one for any source of 1 milliwatt or less.[5] A 2-meter handheld is below that frequency range, so it does not qualify for the SAR exemption. The ARRL calculator should not be used for antennas less than 20 centimeters from a person.[4]

Common problems

  • An indoor or attic antenna fails → Indoor antennas are close to people by nature. Keep them as far as possible from bedrooms and living areas, use less power, and see Stealth antennas for ideas.
  • A neighbor's yard is inside the uncontrolled distance → Raise the antenna, move it toward the far side of your property, or reduce the power on that band.
  • A beam fails in one direction → Avoid long, high-power transmissions toward that area, or raise the beam so the strongest field passes over people's heads.
  • FT8 or other digital modes fail where SSB passed → Their duty factor is 100 percent, so the same output power counts several times more heavily. Many digital operators use less power than they would on voice.

Outside the United States

The steps above follow the FCC's rules and apply only to stations under FCC jurisdiction. Other countries set their own requirements, and many base their limits on the guidelines of the International Commission on Non-Ionizing Radiation Protection (ICNIRP). In the United Kingdom, for example, Ofcom's EMF license condition, which applies to stations transmitting above 10 watts EIRP, requires compliance with ICNIRP's limits for the general public.[7] Check your own regulator's rules and guidance.

See also

References

  1. ↑ Updated Radio Frequency Exposure Rules Become Effective on May 3, ARRL, April 2021.
  2. ↑ 2.0 2.1 47 CFR 97.13, eCFR.
  3. ↑ 47 CFR 1.1310, eCFR.
  4. ↑ 4.0 4.1 4.2 4.3 RF Exposure Calculator, ARRL.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 47 CFR 1.1307, eCFR.
  6. ↑ 6.0 6.1 Additional Information for Amateur Radio Operators, ARRL VEC, June 2021.
  7. ↑ EMF licence condition: what you need to know, Ofcom.
Retrieved from "https://hamwiki.org/index.php?title=How_to_do_an_RF_exposure_evaluation&oldid=2925"