By Ed Hare, W1RFI
The FCC has answered some of our questions about the RF-exposure rules. Changes to the regulations have a few pleasant surprises for Amateur Radio.
Life seems to happen in spurts and jumps, and the FCC RF-exposure regulations have been no exception. In August of 1996, the FCC announced a new set of RF-exposure regulations. I described these in my January 1997 QST article, "The FCC's New RF-Exposure Regulations". The article discussed how the original standards were developed, the history of the rules, and included an explanation about what was, and was not, required of the Amateur Radio Service. It's must reading for anyone who wants to understand these rules.
The article you're reading now builds on that foundation, describing what is new, what rules changes and information the FCC has just announced, and what we can expect in the near future.
The January QST article didn't address one important issue: How to go about performing the evaluations. The FCC Office of Engineering and Technology offered to release information for all radio services to use -- Bulletin 65 -- which would offer instructions on routine station evaluation.
Again, we waited.
On August 25, 1997, the wait was partly over -- the FCC announced some helpful changes to the rules and released Bulletin 65, "Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields."
Things are not yet done -- the bulletin and rule changes do not offer complete solutions for hams. The FCC is still preparing an amateur supplement to this bulletin, expected to contain more information and easy-to-use charts and tables specific to the Amateur Radio Service. Most hams will use the tables in the supplement to complete their evaluations.
Most of these rules have not changed from what was announced in 1996:
- The MPE levels are the same.
- Hams can still conduct their own evaluations without having to file paperwork with the FCC.
- Push-to-talk-operated portable and mobile stations are still exempt from the routine evaluation requirement.
- Amateur Radio examinations for some classes of license will contain additional questions about RF safety.
All services were originally given a transition period until January 1, 1997, to be in compliance. A number of petitions asked for more time. In response, the FCC extended the transition period for the Amateur Radio Service until January 1, 1998.
The FCC has added an additional transition period for existing installations, establishing September 1, 2000 as a "date certain" by which existing stations must be in full compliance with the rules. Starting January 1, 1998, if a licensee must file a 610 form with the FCC, such as for renewal or change of station location, the station must be in compliance when the 610 form is signed.
The major change from the old rules is that the power level that triggers the need to do a station evaluation has been increased for the Amateur Radio Service. Under the rules announced in 1996, all amateur stations using more than 50-W PEP were required to perform a routine evaluation. The ARRL asked that the 50 W be scaled by frequency to match the MPEs in the regulations. The FCC agreed.
Stations that use more power than the levels shown in Table 1 must be evaluated; those using less power must still comply with the exposure limits, but do not need to be evaluated because they are presumed to be in compliance. For the majority of amateurs, this change has virtually eliminated the need to perform station evaluations. Most HF transceivers are rated at 100-W PEP output; on 15 meters and below, stations using this power level need not be evaluated. Most VHF transceivers are rated at 50-W PEP or less; stations using this power level on VHF need not be evaluated. Statistically, most HF operators use "barefoot" rigs, typically 100-W PEP. While this change doesn't cover all barefoot HF operation, operators who wish to use 12 and 10 meters could either perform an evaluation for those two bands, or they could even reduce power to the levels in Table 1 and forgo the evaluation altogether
Table 1 -- Power Thresholds for Routine Evaluation of Amateur Radio Stations
Band (Wavelength) Transmitter Power (W) 160 m 500 75 m 500 80 m 500 40 m 500 30 m 425 20 m 225 17 m 125 15 m 100 12 m 75 10 m 50 6 - 1.23 m 50 70 cm 70 33 cm 150 23 cm 200 13 cm and up 250
The new rules have a few provisions that will be helpful to stations located on sites shared with other transmitters. In the 1996 rules, stations at multitransmitter sites were jointly responsible for site compliance if their field exceeded 1% of the permitted MPEs. The new rules have been relaxed. They now exempt those stations whose exposure is less than 5% of that permitted. This actually covers a lot of small stations like amateur repeaters, although a station evaluation may be required to demonstrate that the exposure is below the 5% threshold. For VHF repeaters, though, this evaluation can be fairly straightforward, using the simple far-field formulas for field strength.
Although the regulations are firm requirements, the FCC intends that Bulletin 65 is advisory in nature. To quote directly from the bulletin:
The bulletin offers guidelines and suggestions for evaluating compliance. However, it is not intended to establish mandatory procedures, and other methods and procedures may be acceptable if based on sound engineering practice.
This flexibility applies especially to the Amateur Radio Service; the FCC is relying on the technical ability of hams to select an appropriate method of analysis for the station evaluations.
Although the FCC is preparing an Amateur Radio supplement to Bulletin 65, the "core" bulletin does contain a section on Amateur Radio. Many hams will want to read it to help get a more complete picture of what is expected of operators in all radio services.
Bulletin 65 was written primarily for commercial radio stations, although the information can be used by any radio service. While hams can use this existing bulletin to complete their station evaluations, they need to be careful. It is easy to get lost in the complex formulas and explanations intended to be most helpful to other radio services. Most hams will find the pending amateur supplement a lot easier to use.
Most amateur stations are already in compliance with the MPE levels. A few hams may need to make some changes to their stations. Bulletin 65 offers guidance and flexibilty on what the FCC considers acceptable. Hams can adjust their power, mode, frequency, antenna location, antenna pointing or on-and-off times to bring their operation into compliance. For example, if you discovered that you were not in compliance after 25 minutes of operation when you pointed your antenna in a particular direction, you could either not point your antenna in that direction, or take a break for 5 minutes after 25 minutes of operation.
It is not likely that many hams will make actual measurements. Even so, Bulletin 65 discusses measurement techniques. Many hams will be surprised at the difficulty of making near-field measurements.
Bulletin 65 describes how to use far-field formulas to obtain estimates of field strengths in the near field. NEC4 modeling done by the ARRL shows that the formula applies well to antennas like dipoles and Yagis. We found, however, that it does not apply well to some antenna types such as small loops, so these formulas should be used with some caution.
The formulas apply to the field-strength levels in the main beam of the antenna. For this reason, they may result in an overly conservative estimate for many actual installations. The ARRL has supplied the FCC with data tables illustrating real antennas over real grounds to offer realistic compliance distances. The FCC will include some of these tables in the amateur supplement to Bulletin 65, along with some simple tables based on the worst-case formulas.
You don't need to resort to complicated formulas to do the worst-case analysis. Check out the RF Safety Calculator. You'll find a "form" that allows you to enter transmitter power, antenna gain and distance. After you enter the information, it calculates the field strength and tells you if you are in compliance. (If you're not in compliance, it tells you at what distance you would be in compliance.)
These simple calculations can be a good tool because if you pass "worst-case," you pass. If you use peak-envelope power in these estimates, this is truly a worst-case; the regulations are specified in terms of average exposure, averaged over 30 minutes for uncontrolled exposure environments, 6 minutes for controlled environments. You also should use the ground-reflection options that are part of the formulas or programs if you want to ensure that you have a truly worst-case estimate.
This first part of Bulletin 65 and the rules changes have answered some of our questions. Many hams will find that they may not have to do station evaluations at all. Others can use the formulas to calculate their worst-case compliance. Some hams, though, will find it a lot easier to wait for the amateur supplement to use the simple lookup tables.
We can't say for certain what will appear in the supplement, but based on what we have learned from the FCC, we can offer an overview.
Most hams will probably use the worst-case lookup tables. Table 2 gives an example based on a 3-element Yagi at 28-MHz. If you pass with this table, you pass on 10 meters for this antenna. (Don't forget to use power averaged over the appropriate time period, not PEP for these calculations.)
For many antenna types, the antenna radiates more energy upward toward the ionosphere than it does downward toward people. The ARRL has developed a number of tables based on NEC4 analysis of real antennas over real ground.
Refer to Table 3 for an example of a 10-meter, 3-element Yagi at 40 feet above ground. It shows how far you must be from the antenna to meet the requirements if the exposure point is at either (a) ground level, (b) first-story height of 12 feet, or (c) second-story height of 20 feet. Note that these distances are smaller than for those of the worst-case scenario. In several cases, the table takes some pretty wild jumps, as noted between 1000 W and 1500 W at the 6-foot compliance point level. This is due to the distribution of fields under the antenna; the field strength is actually less right under the antenna than it is some distance away.
The FCC is including these tables in the amateur supplement. They also suggest that hams can use various software approaches to calculating compliance. When this supplement is released, we'll follow up with another article telling you how to use it. An ARRL book on the RF-exposure regulations will be available near the end of the year
Table 2 -- Estimated Distances to meet RF power density guidelines in the main beam of a typical 3-element Yagi for the 28-MHz Amateur Radio band. Calculations include the EPA ground-reflection factor of 2.56.
Frequency: 28 MHz
Antenna gain: 8 dBi
Controlled limit: 1.15 mw/cm2
Uncontrolled limit: 0.23 mw/cm2
Transmitter power (watts) Distance to controlled limited (feet) Distance to uncontrolled limit (feet) 100 11 24.5 500 24.5 54.9 1000 34.7 77.6 1500 42.5 95.1
Table 3 -- Compliance Distances in Feet -- Uncontrolled/General Public Environment.
10 meter three-element Yagi array Antenna Height = 40 feet
Height Above Ground Average Transmitter power (watts) 6 ft 12 ft 20 ft 100 0 0 0 500 0 0 28 1000 0 52 60 1500 54 76 93
The FCC has a number of pages devoted to the RF-exposure rules. Start with http://www.fcc.gov/oet/dockets/et93-62/ and go from there.