You probably recall the regulation, or at least the exam questions about it:
Stations transmitting more than 50 watts PEP at the antenna with VHF must conduct an RF exposure evaluation.
If an evaluation of your station shows radiated RF energy exceeds permissible limits, you must take action to prevent human exposure to the excessive RF fields.
But how do you practically conduct an RF exposure evaluation of your station? What information do you need and what values must you compute? What are the tools and techniques readily available to the casual operator to ensure compliance with the FCC regulations about RF exposure? This article guides you through the answers to these vexing questions for the vast majority of single-transmitter amateur stations.
The Regs: The requirement to conduct an RF exposure evaluation is described in FCC regulation §97.13. This regulation provides a table of all amateur radio bands with a transmitted power level in watts for each band (recreated below). The regulation states that an RF environmental evaluation must be performed if the station’s power into the antenna exceeds the table value for the band of operation. The power into the antenna is specified as peak envelope power (PEP).
If an evaluation is required, §97.13 states that it must be performed as prescribed by §1.1307(b), but for amateur radio stations the FCC Office of Engineering and Technology (OET) Bulletin 65 provides the best guidance for complying with §1.1307(b) evaluation methods. We will come back to OET Bulletin 65 and evaluation methods, but first let’s see whether or not an evaluation is required at all.
Evaluation Required? Determining whether or not you need to conduct an evaluation is pretty straight-forward. Reference the §97.13 table by amateur band and calculate whether or not your PEP into the antenna on any band can exceed the listed power value. This is a rather simple computation by these steps:
- Ascertain the maximum power output of your transmitter for the band, in units of watts PEP.
- Determine the power loss in decibels (dB) imposed by your transmission line.
- Calculate the resulting maximum power input to the antenna, PEP.
- Compare the antenna input PEP with the table maximum value.
PEP – You may use the stated maximum power output specification for your transmitter, but attend closely to the manufacturer’s specification to make sure values are PEP ratings. Alternatively, you can use a power meter in PEP measurement mode at the transmitter output to get actual values of transmitter output. Take care to provide an appropriate driving audio or other signal to reach maximum output values.
Loss – Your transmission line causes some power loss over the length of the line, and this loss should be subtracted from the transmitter’s output PEP value to determine the PEP input to the antenna. Coaxial cable manufacturers provide loss values that you can use along with a measure of your feedline length to estimate the loss at a specific transmitting frequency. (See this Question of the Week article for more on transmission line loss. Also see this handy Times Microwave Systems coaxial loss calculator.) Loss is often characterized in units of decibels per 100 feet, and you can scale the loss linearly with your feedline length – for instance, a 50-foot feedline will impose a loss that is one half the stated value for 100 feet of cable. With online calculators like the reference above, you can simply enter the type and total length of your feedline and let the calculating engine handle the rest.
Example: Suppose my station has 500 watts PEP maximum output on all HF bands according to manufacturer specifications, but my antenna system allows operation only on 10-meter, 20-meter, 40-meter, and 80-meter bands. Additionally, my station provides 100 watts output on the 2-meter band. My HF feedline is 125 feet of standard RG-8, while my VHF feedline is 75 feet of LMR-400-UltraFlex coaxial line. Using the online calculator I determined the following loss values for each operable band, and resulting input power to the antenna:
Comparing the resultant PEP into the antenna values with the §97.13 table we see that an evaluation is required for 2-meters since the PEP at the antenna exceeds the 50-watt limit for all VHF bands. Also, both 10-meter and 20-meter HF bands require evaluation since the power into the antenna exceeds the 50-watt limit of the 10-meter band and the 225-watt limit of the 20-meter band. However, no evaluation is required for the 40-meter and 80-meter bands since the limit for each is 500 watts PEP.
Making the Evaluation: After you have determined that an evaluation of RF exposure is required for your station, it’s time to dig into OET Bulletin 65 to see just how to do that. And it’s not really difficult to comply! The regs indicate that you can determine compliance of your station by any of these three methods:
- By calculation based on FCC OET Bulletin 65
- By calculation based on computer modeling
- By measurement of field strength using calibrated equipment
Since calibrated field strength measurement equipment is quite expensive and unavailable to most hams, it is not the practical solution. While computer modelling of antenna RF fields is not out of the question for many with the technical expertise and patience to implement the software, simple calculations based on FCC OET Bulletin 65 is usually much more practical. That is the method that we will explore here.
Maximum Permissible Exposure (MPE) Limits: The human body absorbs RF energy with variable efficiency based upon frequency. Absorbed RF is transformed to heat, so RF energy heats bodily tissues and can cause health hazards if the exposure level is too great. The OET Bulletin 65 provides a definition of the Maximum Permissible Exposure (MPE) for humans by frequency. The unit of RF exposure is one of power density, or power per unit area. The bulletin specifies the maximum allowable power density of any given frequency in milliwatts per square centimeter (mW/cm2). Any calculation of RF exposure produced by your station should result in a value of power density in this specific unit of measure.
Notice from the chart reproduced here from OET Bulletin 65 that the lowest MPE is in the VHF range. The MPE for VHF exposure is 1 mW/cm2 for “occupational/controlled” individuals. That’s you and me, the station operators who are voluntarily exposing ourselves to these energies. However, the MPE for the general population – our families, visitors, and neighbors – is just 0.2 mW/cm2 in the VHF range. A similar difference in MPE between these two populations is defined for all amateur bands, 160-meters and above.
From our previous example we established that we should be interested in complying with the MPE for the 2-meter, 10-meter, and 20-meter bands. The VHF limits are clear from the bulletin chart, and a calculation of the specific MPE for the HF frequencies is provided by a table in the bulletin by frequency ranges. For the entire HF range the calculation of MPE power density for the general population is made as
MPE = 180 ÷ f2
where frequency (f) is in MHz. (To compute HF MPE for occupational/controlled personnel, substitute 900 for 180 in the equation above.)
Since the higher frequencies in a band have the lowest MPE for the band, it is good practice to make band calculations of MPE using the highest frequency in the band on which your station will operate. Thus, we will use 29.7 MHz for the 10-meter band and 14.350 MHz for the 20-meter band calculation, as follows:
10-meter band MPE = 180 ÷ 29.72= 0.204 mW/cm2
20-meter band MPE = 180 ÷ 14.3502 = 0.874 mW/cm2
Now we must determine if our station exposes anyone to power density values greater than the values we have computed for each evaluated band, 2-meters, 10-meters, and 20-meters.
Factors Affecting Exposure: Many factors determine the exact RF power density in an area, but the vast majority of single-transmitter cases can be narrowed to a few critical numbers that are easy to determine. We will elaborate a bit on each of the values necessary to compute an estimate of power density by the formulae provided in OET Bulletin 65. However, instead of directly computing values by these formulae, we will employ a convenient online calculating engine that uses these same bulletin formulae. All we must do is ensure that the values we enter into the calculating engine are accurate. The factors we need are as follows.
Average Power at Antenna – The average power at the antenna may be estimated using the PEP values computed earlier for each band as a starting point. However, we must adjust the PEP for the typical duty cycle of the mode used, and we must further adjust PEP for the typical transmit percentage expected over a standard 30-minute period (the “averaging period”). Typical transmitter duty cycles for various modes are:
FM 100% (1.0) CW 40% (0.4)
SSB Phone 20% (0.2) AM 100% (1.0)
RTTY/Digital 100% (1.0)
We will assume for purposes of example a 60% transmitter on time through the 30-minute averaging period. That means you are transmitting or calling somewhat more than you are listening, and this is a conservative estimate since more transmit time equates to greater total exposure. If the actual transmit percentage is less, we are still within the bounds of our computations for exposure safety.
Antenna Gain (dBi) – Virtually all antennas exhibit directional gain over the model isotropic antenna that radiates equally in all spherical directions. The unit designation dBi means that the isotropic antenna model is used as the basis for comparison in defining the gain of an antenna (See article: dBi vs dBd). For example, a horizontal wire dipole exhibits gain in the main lobes of approximately 2.15 dBi – gain as compared to the isotropic radiator. You should obtain the dBi gain figures for the main lobe of your antenna, usually as indicated by the manufacturer. A beam antenna such as a Yagi may have significant gain that can greatly impact the RF power density in the main lobe pointing direction.
Operating Frequency – As noted above, using the highest frequency in a band for computation of power density is a conservative technique since MPE generally lowers with frequency in the HF range. For our calculator, frequency should be indicated in megahertz (MHz).
Distance to Area of Interest – The distance from the center of the antenna to the location at which people will be positioned should be measured in units of feet for use in the calculating engine. The RF field power density falls off with distance, specifically as 1/d2. Thus, a greater distance to the location at which exposure may occur will result in lower power density calculations.
Example of Exposure Factors: Continuing from the previous example in the case of 2-meter band, assume we have the following exposure factors that apply:
PEP at Antenna = 72.9 watts
FM mode (1.0 duty cycle factor)
60% (0.6) transmit on-time percentage
Average Power = 72.9 x 1.0 x 0.6 = 43.74 watts
Antenna Gain = 9.8 dBi (2m Yagi, multiple element)
Operating Frequency = 148 MHz
Distance to Area of Interest = 20 feet
Just for illustrative purposes, suppose the area of interest is your child’s bedroom on the second floor of your home, and covenants restrict your antenna height such that it cannot extend above roofline. As a result, the Yagi can be pointed quite directly into the nearby bedroom area. (Poor planning, or a highly restrictive scenario, yes!)
Calculating Engine: We can now enter these values into a calculator that uses the OET Bulletin 65 formulae to compute power density in the area of interest. You can download the HamRadioSchool.com RF Exposure Calculator spreadsheet (MS Excel format) and simply enter your station values to quickly compute Average Power Into the Antenna, MPE, Controlled & Uncontrolled Power Density, and a comparison determining compliance with regulations.
Another such online calculating engine created by Paul Evans, VP9KF, may be found at: http://hintlink.com/power_density.htm . The entry fields and interface are shown in this image. After clicking “Calculate” the results are presented as in the lower screen capture.
Notice in our 2-meter example, the estimated RF power density is 0.229 mW/cm2. This is also noted to comply with the MPE for the controlled environment case (MPE = 1.0 mW/cm2), but not for the uncontrolled (general population) case (MPE = 0.2 mW/cm2).
So, if we are going to have friends and family within 20 feet of the 2-meter antenna, we must take some action to avoid exposing those individuals to the RF radiation from this antenna. In this particular case of a 2-meter Yagi, that might include locking out the rotation of the antenna such that it is impossible to point the main lobe at this occupied area. Alternatively, we might elevate the antenna so that the vertical extent of the main lobe does not extend into the occupied area with such high gain, thereby limiting the power density to values complying with the MPE. Reducing power when pointing in the direction of this area is also an option, although a less secure one.
We must also evaluate for the other bands and their respective HF antenna. Let’s assume horizontal wire dipoles in each case, such as a fan dipole configuration might provide. The gain in both cases is then 2.15 dBi.
Other factors for the HF cases of 10-meters and 20-meters may be:
CW Mode highest duty factor used: 40%
10m highest frequency: 29.7 MHz
20m highest frequency: 14.350 MHz
10m Avg. Power = 373.5 x 0.4 x 0.6 = 89.64 watts
20m Avg. Power = 407.5 x 0.4 x 0.6 = 97.8 watts
Distance to Area of Interest = 35 feet
Using the calculating engine, the power densities compute as:
10m: 0.0264 mW/cm2 (fully compliant, MPE = 0.204 mW/cm2)
20m: 0.0288 mW/cm2 (fully compliant, MPE = 0.874 mW/cm2)
In fact, the HF cases are not even close to the MPE values computed earlier for those bands. This makes sense because the greatest human absorption rates are in the VHF range, such as the 2-meter band, and absorption by body tissues of HF is much less.
Step-by-Step Summary: The process is simple. Follow this sequence of steps discussed in this article.
- Use your transmitter PEP and feedline loss to calculate PEP into the antenna for each band used.
- Compare PEP into the antenna with 97.13 table power values to determine if an evaluation is required.
- If an evaluation is required for any band, collect these data for the band:
- PEP into antenna
- Highest frequency used in band
- Highest duty cycle of modes used in band
- Estimated percent of transmit on-time per 30-minute period
- Antenna gain in dBi for the band’s antenna
- Calculate average power into antenna
- Measure distance in feet to exposure area of interest
- Enter these values into the online calculating engine:
- Average Power into Antenna
- Antenna gain (dBi)
- Distance to area of interest
- Frequency of operation
- Examine your output results for both controlled and uncontrolled exposure vs. MPE.
- Take steps to mitigate the possibility of exceeding MPE, as necessary:
- Relocate or elevate antenna
- Restrict antenna pointing directions
- Restrict frequency/band use
- Restrict mode use
- Reduce PEP into the antenna
- Restrict station operation times
Multiple Transmitters: If your station operates more than one transmitter simultaneously, such as a field day operation, the power density of all transmitters operating simultaneously must be considered. The computations become trickier, but conservative estimates can be made by simply adding individually computed power densities in the area of interest and comparing with the MPE for the highest frequency used in the group of transmitters. Of course, some common sense should be used as well in the application of that recommendation, especially when a single, lower-powered VHF transmitter is used among multiple, higher-powered HF transmitters. Applying the VHF 0.2 mW/cm2 uncontrolled MPE to larger power densities produced by HF may unnecessarily hinder your field day operation.
Calculator Nuances: The online calculator referenced in this article utilizes FCC OET Bulletin formulae to estimate power density with the use of ground reflection factor as an option. It is intended for far-field calculations, at distances at least several wavelengths from the antenna. It may overestimate near-field and high-gain antenna densities (conservative calculations), and it may underestimate densities within ‘hot spots’ of the near field created by some antenna configurations. It is an estimator only, and again common sense in receiving RF exposure is warranted.
Wrap Up: The process of evaluating your station is not too difficult, so give it a shot. You can see from the examples provided in this article that the most relevant exposure evaluation is likely to be with VHF transmissions, and for exposure to become a problem outside of VHF you will have to be running some serious power, multiple transmitters, or high gain antennas near to occupied areas. But if your station exceeds the PEP to the antenna as listed in §97.13, do your due diligence and make the simple calculations for all potential areas of occupation by humans around your station. Be thorough. It’s good engineering and operating practice.