RF Exposure Evaluation for Amateur Radio Operators Marc C. Tarplee, Ph.D. Technical Coordinator, SC Section

Overview This presentation is meant to be a brief introduction to RF exposure (RFE) rules promulgated by the FCC in1998 and RFE evaluation as it applies to amateur stations. For a comprehensive treatment of the FCC rules and evaluation techniques, see “RF Exposure and You”, by Ed Hare, W1RFI. The book is available from ARRL for $15.

A Brief Review of the FCC RF Exposure Rules

The FCC’s RF Exposure Rules On 1 January 1998, new FCC rules regarding RF Exposure (RFE) went into effect. RFE rules were not new – the first FCC rules for RFE date back to The new rules no longer exempted amateurs from MPE limits. The rules set new limits on the maximum permissible exposure (MPE) allowed from radio transmitters.

Major Points of the New Rules Set RF exposure limits that are consistent with the manner in which the human body absorbs RF. Require stations regulated by the FCC to meet the guidelines or file an environmental assessment. Permit amateurs to perform their own evaluation of their station. Provide categorical exemptions from RFE evaluation for stations whose power, frequency, duty cycle, etc. are such that compliance may be assumed.

RF Environments The FCC rules define two classes of RF environments: –Controlled: an environment in which people are aware of their RF exposure and are able to take action to reduce their exposure if necessary. Exposure in this environment is averaged over a 6 minute period. –Uncontrolled: an environment accessible by the general public, who may not be aware of their RF exposure. Exposure in this environment is averaged over a 30 minute period.

RF Environments Typically, the controlled environment is an amateur’s residence and yard. The uncontrolled environment is everything else

RF Exposure Limits Freq Range (MHz) Controlled Environment (6 minute average) Uncontrolled Environment (30 minute average) E Field strength (V/m) H Field strength (A/m) Power Density (mW/cm 2 ) E Field strength (V/m) H Field strength (A/m) Power Density (mW/cm 2 ) /f4.89/f900/f /f2.19/f180/f f/ f/1500 > Note that the limits are field strength limits, not power limits!!

MPE Chart

Field Strength vs. Power Although the limits are given in terms of field strength, it is possible to relate them to transmitted power and the distance from the measurement point to the antenna, quantities which most amateurs are able to measure. The FCC’s OET Bulletin 65 contains numerous tables for this purpose, so it is not necessary to directly measure the field strength.

Time-Weighted Averages The limits are “time-weighted averages”. The exposure is averaged over the specified time period to get the average value. Here is an example: A person is exposed for 4 minutes to an electric field of 700 V/m. What is the time-weighted exposure in a controlled environment? –Solution: Avg RFE = (Field strength*time)/weighting period = (700*4)/6 = 2800/6 = 467 V/m

Time-Weighted Averages The example can be reworked for the uncontrolled environment, for which the weighting period is 30 minutes: –Solution: Avg RFE = (Field strength*time)/weighting period = (700*4)/30 = 2800/30 = 93 V/m

Exemptions from Evaluation There are 4 categorical exemptions for amateur stations: –Amateur stations whose PEP antenna input power is less than that shown in the table on the next page. –Amateur repeaters whose ERP is 500 W or less. –Amateur repeaters with antennas not mounted on buildings, if the antenna is located more than 10 m (32 ft 9 5/8 in) above ground. –Amateur mobile and portable hand-held stations using push-to-talk of equivalent operation.

Power Limits for Amateur RFE Evaluation Exemption * Input power to the antenna BandsUpper Power Limit (PEP)* 160m – 40m500 W 30m425 W 20m225 W 17m125 W 15m100 W 12m75 W 10m – 1.25 m50 W 70cm70 W 33cm150 W 23 cm200 W 13 cm and above250 W

Amateur Station RFE Evaluation

The Evaluation Process Determine whether the station meets the standard for a categorical exemption on any of the bands of operation, and exclude these bands. Determine which evaluation technique will be used: –Tables developed from far-field power density formulas –Tables developed from antenna modeling –Modeling of the actual antenna farm –Graphs made from power-density formulas –Field strength software –Calibrated field strength methods Select a method for determining RF power –PEP –Time weighted average

RFE Evaluation Example We will make an evaluation of station N4UFP using tables developed from the far-field power density formulas. Evaluation will be done using PEP power and average power. The PEP power/far-field table approach is very conservative and is the easiest to do, thus it should be done first.

Station N4UFP 160m/80m/75m/40m –vertical loops 40 feet from shack, 40 feet from property line –Max power = 1000 W SSB/500 CW 20,15,10m –tribander at 50 ft, located 45 feet from the shack, 55 feet from property line –Max power = 1000 W SSB/500 CW 6m –5 element yagi up 54 feet, located 45 feet from shack, 55 feet from property line –Max power = 300 W 2m –13 element yagi up 57 feet, located 45 feet from shack, 55 feet from property line –Max power = 100 W 70cm –25 element yagi up 60 feet, located 45 feet from shack, 55 feet from property line –Max power = 55 W

Step 1: Check for Exemptions Referring to the table on slide 14 of this presentation, we see that only N4UFP’s 70cm operation is exempt from evaluation. slide 14

PEP/Far Field Evaluation Method The Far-field tables (Table 4a,Table 4b- VHF, and Table 4b-UHF of supplement B to OET Bulletin 65) show the minimum compliance distance as a function of antenna gain, frequency and antenna input power and the type of environment.Table 4a Table 4b- VHFTable 4b-UHF The minimum compliance distance is the minimum distance at which the E and H fields fall below the RFE limits shown in slide 7.slide 7

Measuring Distances The diagram to the right shows a typical wire antenna installation For RFE compliance purposes, the distance to the antenna is the shortest distance from the point of exposure to the any part of the antenna. Minimum distance from house to antenna

Measuring Distances The diagram to the right shows a typical tower mounted antenna For RFE compliance purposes, the distance to the antenna is the shortest distance from the point of exposure to the any part of the antenna. The Pythagorean theorem can be used to compute the distance D

160m Evaluation Antenna Gain = 3 dBi (single quad loop) Max input power = 1000 W PEP. Operating frequency ~ 2 MHz From Table 4a, the compliance distances are:Table 4a –Controlled environment: 0.6m = 2.0 feet –Uncontrolled environment:1.06m = 3.5 feet Since this antenna is 40 feet from the shack and 40 feet from the property line, N4UFP’s 160m operation is in compliance

80/75m Evaluation Antenna Gain = 3 dBi (single quad loop) Max input power = 1000 W PEP. Operating frequency ~ 4 MHz From Table 4a, the compliance distances are:Table 4a –Controlled environment: 0.9m = 3.0 feet –Uncontrolled environment:1.9m = 6.2 feet Since this antenna is 40 feet from the shack and 40 feet from the property line, N4UFP’s 80/75m operation is in compliance

40m Evaluation Antenna Gain = 3 dBi (single quad loop) Max input power = 1000 W PEP. Operating frequency ~ 7.3 MHz From Table 4a, the compliance distances are:Table 4a –Controlled environment: 1.6m = 5.2 feet –Uncontrolled environment:3.5m = 11.5 feet Since this antenna is 40 feet from the shack and 40 feet from the property line, N4UFP’s 40m operation is in compliance

20m Evaluation Antenna Gain = 9 dBi (3-el Yagi) Max input power = 1000 W PEP. Operating frequency ~ MHz From Table 4a, the compliance distances are:Table 4a –Controlled environment: 6.1m = 20 feet –Uncontrolled environment:13.6m = 44.6 feet This antenna is 50 ft above ground and the tower is 45 ft from the shack, so the distance to the shack is sqrt( ) = sqrt( ) = sqrt(4525)= 67 ft. By similar reasoning, the distance to the property line is 74 ft N4UFP’s 20 m operation is in compliance.

15m Evaluation Antenna Gain = 9 dBi (3-el Yagi) Max input power = 1000 W PEP. Operating frequency ~ MHz From Table 4a, the compliance distances are: Table 4a –Controlled environment: 9.1m = 29.8 feet –Uncontrolled environment:20.3m = 66.6 feet This antenna is 67 ft from the shack and 74 ft from the property line N4UFP’s 15 m operation is in compliance.

10m Evaluation Antenna Gain = 9 dBi (3-el Yagi) Max input power = 1000 W PEP. Operating frequency ~ 29.7 MHz From Table 4a, the compliance distances are: Table 4a –Controlled environment: 12.6m = 41.3 feet –Uncontrolled environment:28.2m = 92.5 feet This antenna is 67 ft from the shack and 74 ft from the property line. N4UFP’s 10 m operation may not be in compliance.

6m Evaluation Antenna Gain = 12 dBi (5-el Yagi) Max input power = 300 W PEP (use 500W entries). Operating frequency ~50 MHz From Table 4b, the compliance distances are:Table 4b –Controlled environment: 12.7m = 41.7 feet –Uncontrolled environment:28.4m = 93.2 feet This antenna is 70 ft from the shack and 77 ft from the property line. N4UFP’s 10 m operation may not be in compliance.

2m Evaluation Antenna Gain = 15 dBi (13-el Yagi) Max input power = 100 W PEP. Operating frequency ~ 144 MHz From Table 4b, the compliance distances are:Table 4b –Controlled environment: 8m = 26.2 feet –Uncontrolled environment:18m = 59.0 feet This antenna is 73 ft from the shack and 79 ft from the property line. N4UFP’s 2 m operation is in compliance.

Average Power/Far Field Evaluation Method N4UFP may not be compliant on the following bands: 10m and 6m. The FCC rules are based on time weighted averages, rather than PEP, so a more detailed analysis using average power will be made for these bands. Note that the simple evaluation using PEP is very conservative – a station compliant using PEP will always be compliant using the average power.

Determining Average Power Average power is given by: –Pavg = (PEP)*(Duty Factor)*(time)/(weighting period) To be conservative, we will assume: – That CW is the dominant mode of operation. Then the duty cycle = 0.4 –That transmissions are approximately 3 minutes in length with 3 minute listening periods in between.

10 m Avg. Power Evaluation The maximum PEP on 10m = 1000 W with gain = 9 dbi The average power for controlled environments is: –Pavg,c = (1000)*(0.4)*(3)/6 = 200 W The average power for uncontrolled environments is: –Pavg,uc = (1000)*(0.4)*(15)/30 = 200 W There is no 200 W column on Table 4a, so we must interpolate between the 100 and 500W columns. The compliance distances are:Table 4a –Controlled environment: 5.7m = 18.6 feet –Uncontrolled environment:12.6m = 41.3 feet This antenna is 67 ft from the shack and 74 ft from the property line, so N4UFP’s 10 m operation is in compliance.

6 m Avg. Power Evaluation The maximum PEP on 6m = 300 W with gain = 12 dBi The average power for controlled environments is: –Pavg,c = (300)*(0.4)*(3)/6 = 60 W The average power for uncontrolled environments is: –Pavg,uc = (1000)*(0.4)*(15)/30 = 60 W There is no 60 W column on Table 4b, so we must interpolate between the 50 and 100W columns. The compliance distances are:Table 4b –Controlled environment: 4.4m = 14.4 feet –Uncontrolled environment:9.9m = 32.3 feet This antenna is 67 ft from the shack and 74 ft from the property line, so N4UFP’s 6 m operation is in compliance.

What if I am not compliant? If an amateur station is not compliant on a particular band/mode combination, the following actions can be taken: –Reduce transmitter power. –Relocate the antenna to place inhabited areas beyond the minimum compliance distance. –Cease operation on that band/mode

Recordkeeping The FCC does not require amateurs to keep a written record of their RFE evaluation However, an amateur must be able to demonstrate that his/her station is compliant with RFE rules, should the FCC request it.

Summary You should now be able to make a simple evaluation of your amateur station for RFE using either PEP or average power techniques. More detailed information can be obtained from FCC OET Bulletin 65, or “RF Exposure and You” available from ARRL. I will be happy to assist in your RFE evaluation.

RFE Tables from OET Bulletin 65

TABLE 4a. (MF/HF Bands) (Developed by Fred Maia, W5YI Group, working in cooperation with the ARRL.) Estimated distances in meters from transmitting antennas necessary to meet FCC power density limits for Maximum Permissible Exposure (MPE) for either occupational/ controlled exposures (“Con”) or general population/uncontrolled exposures (“Unc”) using typical antenna gains for the amateur service and assuming 100% duty cycle and maximum surface reflection. Chart represents worst case scenario. Freq. Antenna (MF/HF)Gain Peak Envelope Power (watts) (MHz/Band) (dBi) 100 watts 500 watts 1000 watts 1500 watts Con. Unc. Con. Unc Con. Unc. Con. Unc. 2.0 (160m) (160m) (75/80m) (75/80m) (40m) (40m) (40m) (30m) (30m) (30m) (20m) (20m) (20m) (20m) (17m) (17m) (17m) (17m) (15m) (15m) (15m) (15m) (12m) (12m) (12m) (12m) (10m) (10m) (10m) (10m) Note: Multiply above distances by if duty cycle is 50% - such as during a typical back and forth communications exchange. To convert from meters to feet multiply meters by Distance indicated is shortest line-of-sight distance to point where MPE limit for appropriate exposure tier is predicted to occur.

Table 4b. (VHF Bands) (Developed by Fred Maia, W5YI Group, working in cooperation with the ARRL.) Estimated distances in meters from transmitting antennas necessary to meet FCC power density limits for Maximum Permissible Exposure (MPE) for either occupational/ controlled exposures (“Con”) or general population/uncontrolled exposures (“Unc”) using typical antenna gains for the amateur service and assuming 100% duty cycle and maximum surface reflection. Chart represents worst case scenario. Freq Antenna (VHF/UHF) Gain Peak Envelope Power (watts) (MHz/Band) (dBi) 50 watts 100 watts 500 watts 1000 watts Con. Unc. Con. Unc Con. Unc. Con. Unc. 50 (6m) (6m) (6m) (6m) (6m) (6m) (2m) (2m) (2m) (2m) (2m) (2m) (2m) (1.25m) (1.25m) (1.25m) (1.25m) (1.25m) (1.25m) Note: Multiply above distances by if duty cycle is 50% - such as during a typical back and forth communications exchange. To convert from meters to feet multiply meters by Distance indicated is shortest line-ofsight distance to point where MPE limit for appropriate exposure tier is predicted to occur.

Table 4b. (UHF Bands) (Developed by Fred Maia, W5YI Group, working in cooperation with the ARRL.) Estimated distances in meters from transmitting antennas necessary to meet FCC power density limits for Maximum Permissible Exposure (MPE) for either occupational/ controlled exposures (“Con”) or general population/uncontrolled exposures (“Unc”) using typical antenna gains for the amateur service and assuming 100% duty cycle and maximum surface reflection. Chart represents worst case scenario. Freq Antenna (VHF/UHF) Gain Peak Envelope Power (watts) (MHz/Band) (dBi) 50 watts 100 watts 500 watts 1000 watts Con. Unc. Con. Unc Con. Unc. Con. Unc. 450 (70cm) (70cm) (70cm) (70cm) (33cm) (33cm) (33cm) (33cm) (33cm) (23cm) (23cm) (23cm) (23cm) (23cm) Note: Multiply above distances by if duty cycle is 50% - such as during a typical back and forth communications exchange. To convert from meters to feet multiply meters by Distance indicated is shortest line-of-sight distance to point where MPE limit for appropriate exposure tier is predicted to occur.

TABLE 4a. (Developed by Fred Maia, W5YI Group, working in cooperation with the ARRL.) Estimated distances in meters from transmitting antennas necessary to meet FCC power density limits for Maximum Permissible Exposure (MPE) for either occupational/ controlled exposures (“Con”) or general population/uncontrolled exposures (“Unc”) using typical antenna gains for the amateur service and assuming 100% duty cycle and maximum surface reflection. Chart represents worst case scenario. Freq. Antenna (MF/HF)Gain Peak Envelope Power (watts) (MHz/Band) (dBi) 100 watts 500 watts 1000 watts 1500 watts Con. Unc. Con. Unc Con. Unc. Con. Unc. 2.0 (160m) (160m) (75/80m) (75/80m) (40m) (40m) (40m) (30m) (30m) (30m) (20m) (20m) (20m) (20m) (17m) (17m) (17m) (17m) (15m) (15m) (15m) (15m) (12m) (12m) (12m) (12m) (10m) (10m) (10m) (10m) Note: Multiply above distances by if duty cycle is 50% - such as during a typical back and forth communications exchange. To convert from meters to feet multiply meters by Distance indicated is shortest line-of-sight distance to point where MPE limit for appropriate exposure tier is predicted to occur.

Table 4b. (VHF Bands) (Developed by Fred Maia, W5YI Group, working in cooperation with the ARRL.) Estimated distances in meters from transmitting antennas necessary to meet FCC power density limits for Maximum Permissible Exposure (MPE) for either occupational/ controlled exposures (“Con”) or general population/uncontrolled exposures (“Unc”) using typical antenna gains for the amateur service and assuming 100% duty cycle and maximum surface reflection. Chart represents worst case scenario. Freq Antenna (VHF/UHF) Gain Peak Envelope Power (watts) (MHz/Band) (dBi) 50 watts 100 watts 500 watts 1000 watts Con. Unc. Con. Unc Con. Unc. Con. Unc. 50 (6m) (6m) (6m) (6m) (6m) (6m) (2m) (2m) (2m) (2m) (2m) (2m) (2m) (1.25m) (1.25m) (1.25m) (1.25m) (1.25m) (1.25m) Note: Multiply above distances by if duty cycle is 50% - such as during a typical back and forth communications exchange. To convert from meters to feet multiply meters by Distance indicated is shortest line-ofsight distance to point where MPE limit for appropriate exposure tier is predicted to occur.