By Ed Hare, W1RFI
Excerpted from January 1998 QST
Copyright © 1997, American Radio Relay League
All Rights Reserved

It's been a long road, but Amateur Radio now has a clear light at the end of the tunnel leading us toward the implementation date of the RF exposure rules. The FCC has released "Supplement B," the Amateur Radio supplement to "OET Bulletin 65."  This answers our questions about the "routine environmental evaluation" required by the rules. The actual requirements are not nearly as onerous as they sound!

The Sky Is Not Falling! 

Most hams will not have difficulty meeting the requirements. In fact, most hams are already in compliance with the maximum permissible exposure (MPE) levels. Some fear, however, that they'll have to do difficult measurements, perform extensive calculations or file paperwork with the FCC. Wrong on all counts. The evaluation is often as easy as using tables to determine that your antenna is far enough away from people.

The rules set limits on the RF exposure levels people may be subjected to. The MPE limits vary with frequency. The MPE levels represent the amount of energy that can be present where and when people are being exposed. They do not limit the permitted radiated strength from a radio station and do not change the maximum power levels permitted to Amateur Radio operators. The actual MPE limits were explained in the January 1997 QST article.

The rules define two exposure environments, each with different MPE levels. The uncontrolled environment applies to areas where people would not normally know they are being exposed. This includes "public" areas such as your property line or a neighboring apartment.

Controlled environments apply where people are aware of their exposure and have the ability and knowledge to control it. Greater MPE levels are permitted in controlled areas. A good rule of thumb is that the controlled exposure limit can be applied to those areas in which you can control access. An example of this is your fenced-in backyard. Your own household can also be a controlled environment if your family or guests have been given instruction about RF exposure and safety.

The rules also require that some amateur stations be evaluated to verify that they are in compliance with the MPE levels. It's this aspect of the rules that raises eyebrows among hams.

The FCC is relying on amateurs to perform their own station evaluations. Other than a simple statement on Form 610, the FCC does not require any paperwork from amateurs; once the evaluation is complete, the amateur can begin operation

Let's take a look at what is found in Bulletin 65 and Supplement B. This article can't reprint the bulletin in its entirety, but it is available for download from the FCC. The most applicable parts will be reprinted in ARRL's upcoming book, RF Exposure and You. The bulletin outlines several ways that hams can evaluate their stations. However, hams may use any other technically appropriate methods. Many hams envision complicated measurements when they think about evaluating their stations. While precise, scientific measurements could be used, most hams will probably "pass" using one of the easier methods.

You can estimate compliance by using:

• Tables developed from power-density and field-strength formulas.
• Tables derived from antenna modeling.
• Antenna modeling software (NEC, MININEC, etc.)
• Power-density and field-strength formulas
• Software developed from power-density and field-strength formulas.
• Calibrated field-strength measurements.

FCC rules define amateur power in PEP. (PEP is the average power of a single RF cycle at the peak of a modulation envelope.) The MPE limits, however, are based on average exposure, not peak exposure. This means that the total exposure for the averaging period must be below the limits. One way of factoring in average exposure could be to determine the average transmitter power.

To calculate average power, multiply PEP by the duty factor for the mode being used. The duty factors for various modes are shown in Table 2. Multiply that result by the percentage of time the transmitter could be in use during the appropriate averaging period -- 6 minutes for controlled exposure, 30 minutes for uncontrolled.

Most amateurs will use the tables in Supplement B to estimate their compliance with the MPE levels. These tables show the distances people must be from any part of the antenna to avoid being exposed at levels exceeding the MPE limit. They have been calculated with a ground-reflection factor, which includes the ground gain of an antenna over typical ground. This allows hams to use manufacturer's antenna gain figures in dBi with confidence that the result represents a conservative real-world estimate. This model, although simplified, has been verified by the ARRL Laboratory staff using NEC antenna-modeling software with a number of antennas modeled over ground. These tables do not necessarily apply to all antenna types, though. NEC models of small HF loops, for example, give fields near the antenna that are much higher than the power-density formula predicts. A more accurate method was used to develop the small-loop table in the supplement.

The tables derived from formulas do have advantages: they generally offer conservative estimates and they are easy to use. If a ham "passes" using these conservative tables, the evaluation is complete. Doing a station evaluation can be just that simple!

Supplement B contains a number of these tables. Select the ones that best apply to your station, calculate your average power, then determine if your antenna is far enough away for each band and mode that you use. (One shortcut is to use the highest PEP you use on each band.) Figure 1 shows how this worst-case estimate applies to the main beam of the antenna. Figure 2 shows how to calculate the actual distance to the points you are evaluating.

Table 4 was prepared for the FCC Bulletin by the W5YI Group, working in cooperation with the ARRL. It shows the distances required to meet the power-density limits for different amateur bands, power and antenna gain, for occupational/controlled exposures ("con"), or for general population/uncontrolled exposures ("unc"). (All FCC tables give the all distances in meters; the tables in this article have been converted to feet.)

This table probably represents the easiest approach to doing a station evaluation. It can be conservatively applied to most antenna types. Select the appropriate band and "round up" antenna gain and power to match the table. The distances are the minimum separation that must be maintained between any part of the antenna and any area where people will be exposed. If the station passes, the evaluation is done. If not, a more precise evaluation can be done by using average power and exposure and the tables developed by ARRL, and discussed later in this article.

Supplement B also contains tables for specific antenna types. Table 5 is an example of those supplied by Wayne Overbeck, N6NB. It shows the estimated distances to meet RF power density guidelines for a typical 3-element "triband" Yagi HF antenna. These tables are also based on the power-density equations.

All of these simple tables are easy to use. They almost always give conservative estimates of compliance. They estimate the required distance one needs to be from the antenna in the main beam of the antenna (see Figure 1). In many cases, however, exposure below an antenna can be much less than that indicated by the far-field tables. If a station "passes" using the simple tables, this could be a moot point. Even so, some hams may find it useful to use other methods to demonstrate that the exposure from their station is much less than what the rules allow.

Like many tables, the ones shown in this article and Supplement B paint with a broad brush. They provide conservative answers to generalized conditions. If you need more precise evaluation methods, those are certainly available to you as well.

A number of antenna modeling programs will give estimates of field strength in the near field of an antenna. Using NEC4 the ARRL Lab staff modeled a number of antennas, heights and power levels and calculated the compliance distances at ground level, first story and second story exposure points.

The results were distilled into Tables, showing a 10M triband Yagi modeled 30 feet and 60 feet, providing more accurate estimates of actual exposure than the derived-from-formula tables. However, the antenna and its height must match the table to be applicable. (If the antenna is located higher than the heights in these tables, in general, the exposure should be less than the predicted values.) The ARRL offered a number of these tables to the FCC for inclusion in Supplement B. The remainder will be made available in the ARRL's new book, RF Exposure and You. Supplement B features a number of these antennas at heights of both 30 feet and 60 feet, helping us demonstrate that "higher is better!"

The software used to create our tables can model virtually any antenna system. Hams sometimes use some exotic antennas and it is not practical to create a table for each one. Some hams may want to evaluate the effect of multiple antennas or other conductors in proximity to their antennas to have a more accurate answer than can be derived from any other calculation method.

Many of these modeling programs are based on NEC or MININEC versions. They can give good results if the model is accurate, which usually means modeling all nearby conductors. These programs can calculate the near-field electric and magnetic-field strengths.

The subject of making field-strength measurements has been treated in Bulletin 65. In general, however, most hams will not have the accurate calibrated equipment to do actual field-strength measurements. The relative field-strength meters in common use by hams do not have the necessary accuracy, especially when dealing with fields of different frequencies. Even in the most qualified hands, repeatability of more than a few dB is difficult to achieve.

Some amateur stations use multiple transmitters, such as an HF DX or contest station that also accesses a VHF PacketCluster. Other stations might be located at sites also occupied by transmitters in other radio services. Two or more transmitters could be operating at the same time, each adding to the exposure level. In these cases, the operators must take steps to ensure that the total exposure does not exceed the MPE level.

In most cases, multi-transmitter amateur station operators are responsible for the total exposure. In cases where an amateur station location is shared with other transmitters, all operators may be jointly responsible for the site compliance.

It is relatively easy to calculate total exposure at multitransmitter sites. For any point being evaluated, determine what percentage of the permitted MPE will actually be contributed by each transmitter. Then, add up the percentages for any transmitters that could be in operation simultaneously. If the total percentage exceeds 100%, the site is not in compliance. For example, if a 2-meter transmitter creates exposure at 40% of what is permitted on that frequency, and a simultaneous transmission is occurring by a 1.5-GHz commercial transmitter at the same at 70% of the limit, the total is 110% and the site is out of compliance, even though each transmitter is being operated below its own limit.

The FCC has determined that any transmitter that operates at an exposure level greater than 5% of the power density permitted to its own operation is jointly responsible with all the other operators within its exposure area who are also exceeding 5% for site compliance. In those areas where the exposure from the transmitter is less than 5% of the MPE level for the repeater, the operator is not jointly responsible.

In addition, however, those stations that are not required to be evaluated generally are presumed not to be responsible for site compliance. The FCC can require any operator to conduct an evaluation if they believe that there could be a problem.

The antenna tables elsewhere in this article cannot be used to determine actual power-density levels. The field-strength formulas in this article and in Bulletin 65 or various antenna-modeling programs can be used instead.

In some cases, amateurs may not be able to obtain full information about the other transmitters on the site. If you find yourself in this situation, you should attempt to secure information from the site owner. If that isn't available, make the best estimates possible of other transmitter powers and antenna gains on the site to determine compliance.

As described in the FCC rules, there is no specific requirement that mobile and portable (hand-held) devices used under Part 97 (Amateur Radio) be evaluated. Bulletin 65 explained that this applies to amateur mobile operation using push-to-talk operation. This is because of the low power, low operating duty cycles usually employed and the expected shielding of the vehicle occupants by the vehicle body. Most Amateur Radio mobile or portable stations that meet these general criteria do not need to be evaluated.

If the regulations do not specifically require you to perform an evaluation, there could be a number of reasons to do one anyway. If nothing else, doing an evaluation now would be good practice for the day when you upgrade your station (by adding an amplifier or antenna, for instance) in such a way that makes an evaluation necessary. More importantly, the results of your evaluation will certainly demonstrate to yourself, and possibly your neighbors, that your station is operating well within FCC guidelines and is no cause for concern. Finally, if you have an antenna that is located close to people, you may be operating in excess of the MPEs. It's a good idea to evaluate and be on the safe side, just in case.

The vast majority of stations will pass their evaluations handily. But some stations whose antennas are close to areas of exposure may not meet the MPE limits. The FCC gives amateurs considerable flexibility in correcting problems. You can choose another frequency band where the MPE limits are higher, or another operating mode that results in a lower average power. You could also adjust your operating power to produce less exposure, or reduce the percentage of time the station is on the air during the averaging period. Those with directional antennas can simply ensure that they are not pointing toward areas that would be out of compliance if people are present in the areas.

As this article was going to press, the FCC sent a draft of an evaluation worksheet included in Supplement B. This optional work-sheet has instructions on how to include the various factors necessary to do a station evaluation and provides a handy way to maintain a record of the evaluation. It runs step by step through the procedures outlined in this article, using the methods outlined in Supplement B. The worksheet describes the methods to calculate power to the antenna using feed line losses, and how to calculate ERP using both feed line losses and antenna gain. This is another example of how the FCC has made the evaluation process as easy as possible for the Amateur Radio Service.

The biggest impact of the new rules is that many hams will have to learn something new. Learning new things is what attracted most hams to the avocation anyway, so this is not a major problem. The FCC has worked closely with the amateur community to fine tune both the rules and the FCC's own materials. All that remains is for us to put all of this information to good use in our own stations!

[1] The FCC Office of Engineering and Technology Web page has all the FCC information on RF exposure. See http://www.fcc.gov/oet/rfsafety/.

[2] The FCC defines "portable" devices as those generally operated with the antenna located within 20 centimeters of the body. This is different from taking a base station "portable" to a remote location.

No doubt many of you are shaking your heads and muttering, "Why should I even bother to do an evaluation? The FCC will never enforce these rules. This is a waste of time!"

There are a number of important reasons why amateurs should follow all FCC rules, including these. The Amateur Radio Service has a tradition of compliance with FCC regulations; Part 97 is our bible! The ARRL has worked hard to help the FCC fine tune these rules for the Amateur Radio Service. If we hope for more cooperation in the future, we must set the best example possible. The FCC (and our Amateur Radio supporters on Capitol Hill) must be assured that the majority of hams follow all the rules "by the book."

Safety is also a concern. While RF energy isn't known to cause major health problems, the research is still continuing. The levels that have been set by various standards bodies and the FCC are our best assurance that no ill effects on human health are expected from the normal operation of radio transmitters. Being in compliance buys peace of mind for you and your family. As the old saying goes, "better safe than sorry."

Your neighbors may also have questions and concerns. (The ARRL has already received quite a few questions on this subject from neighbors of hams.) Many of these concerns can be easily addressed by explaining the requirements to your neighbors and showing them the results of your station evaluation. The new rules even offer us a significant advantage; if our neighbors do have concerns, we are much better off being able to demonstrate that there are rules governing our conduct and that we have done what the rules require.

In most cases, these evaluations are not hard! They can usually be done by looking at a table, or spending a few minutes with some free software or a calculator. There is not much to lose, and a lot to gain. So, hams should complete their station evaluations and point to them with pride! -- Chris D. Imlay, W3KD, ARRL General Counsel

Let's look at a hypothetical example of an amateur station and run through the evaluation steps. Assume that Al, N9AT, has the following station configuration:

· 80 meters, 100 W and 1000 W CW and SSB with a half wavelength dipole antenna 10 feet above ground. (I know this would be a terrible height for an 80-meter dipole, but bear with me!)

· 40 meters 100 W and 1000 W CW and SSB with a half wavelength dipole antenna 10 feet above ground. (Ditto my height comments above!)

· 10 meters, 100 W and 1500 W CW and SSB with a 3-element Yagi beam 30 feet above ground.

· 2 meters, 35 W FM, 100 W CW and SSB 60 feet above ground.

Al first looks at Table 1 to see which operation requires a station evaluation. In this case, his 100-W 80- and 40-meter operation and his 35-W 2-meter FM operation do not need to be evaluated. (Al intends to evaluate them anyway, just to learn more about the subject.)

He could calculate his average power for the remaining operation, but this may not be necessary. Al first tries his evaluation with PEP, using Table 3 in conjunction with Table 4. Rounding up to 3 dBi for the antenna gain, Table 4 estimates that on 80 meters at 1000 W his antenna needs to be located 2.8 feet from areas of controlled exposure and 6.2 feet from uncontrolled exposure. The antenna is located about 10 feet from the property line and is attached to the house with 5-feet of rope, so this band would be in compliance for operation at a 1000-W continuous carrier level.

On 40 meters at 1000 W, Al first rounds his dipole gain up to 3 dBi. Table 4 shows 5.1 feet for controlled exposure and 11.4 feet for uncontrolled exposure. On this band the end of his antenna is located 5 feet from the property line and tied to the house with a 4-foot rope. It doesn't quite pass with full power. Al has a few choices. He can relocate the antenna, reduce power, or calculate his average power and try again or use the antenna-specific table at the same height. In this case, he calculates his average power and determines that he is using 133 W average power on SSB and 266 W average power on CW. Rounding up, he selects 500 W in Table 4 and determines that his antenna needs to be 3.6 feet from controlled exposure and 8.0 feet from uncontrolled exposure. He passes for controlled exposure, but the antenna would be located 6.4 feet from a person standing on the property line, so the station may still not be in compliance. Al decides to move the antenna 10 feet from the property line sometime next week. In the meantime, he will reduce his power on 40 meters.

On 10 meters, he is using a 3-element Yagi 30 feet in the air. Rounding his gain up to 9 dBi, he determines that his antenna needs to be 50.6 feet from controlled exposure and 113.2 feet from uncontrolled exposure. The tower is located 40 feet from the house, and solving for the hypotenuse of the distance between his residence and the tower (his one-floor house has the top of the first floor 12 feet above ground), he calculates that the antenna is located 43.9 feet from areas of controlled exposure. This does not pass for full power, but passes easily when he calculates his average power. The tower is 50 feet from the property line, for a total distance of 55.5 feet from ground level exposure on the property line. This does not pass for uncontrolled exposure. Al doesn't give up, though, he goes to Table 6 and determines that at ground level, the NEC model shows that the compliance distance needs to be 57.1 feet from the center of the antenna at 1500 W average power. He clearly cannot do 30 minutes of tuneup if his neighbor is on the property line. At 500 W average power, however, Al notes that his antenna could be built on the property line and ground-level exposure would be below the limits. He has passed and does not need to make any changes to his station except to limit his tuneup time.

On 2 meters, his antenna has 8 dBi of gain. Rounding up to 9 dBi, he determines that at 100 W his antenna needs to be 13.2 feet from controlled exposure and 29.5 feet from uncontrolled. This antenna is at the top of his 45 foot tower, so he can run continuous power on 2 meters. Al gathers all of the papers containing these calculations (along with his notes) and files them with his station records. Within 20 minutes he has completed his station evaluation!

The power levels shown in Table 1 are in peak-envelope power (PEP) input to the antenna. FCC rules specify amateur power in PEP and most transmitters are rated in PEP. However, you must consider feed line losses to determine power to the antenna.

There is a little wrinkle in the rules when it comes to repeaters. The evaluation exemption for amateur repeater operation is determined by the effective radiated power (ERP) of the repeater. ERP is referenced to the gain of a half-wave dipole in free space (unlike equivalent isotropically radiated power, EIRP, which is referenced to an isotropic source). Supplement B describes how to calculate feed line losses and determine ERP for an amateur repeater.

All amateur repeaters using 500 W ERP or less generally do not need to be evaluated. Those that operate with more than 500 W ERP need to be evaluated if they have an antenna mounted on a building, or if any part of a nonbuilding-mounted antenna is less than 10 meters (32.8 feet) above ground. (This is another example that higher antennas generally create less field strength on the ground than lower antennas!)

The calculations used to create the far-field tables have been written into BASIC by Wayne Overbeck,N6NB. This software has been written into a Web-page calculator by Paul Evans, VP9KF.

Brian Beezley, K6STI, has made a scaled-down version of his Antenna Optimizer software available. Download NF.ZIP. These programs are based on MININEC and will generally give the same results as you can obtain from using Table 6 or Table 7. Contact Brian Beezley, K6STI, 3532 Linda Vista Drive, San Marcos, CA 92069; tel 760-599-4962, e-mail k6sti@n2.net.

Roy Lewellan, W7EL, offers EZNEC antenna modeling software.