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Antennas RF Exposure

See Also:
Greg N9GL presentation
Paul VP9KF RF Exposure Calculator
N7OH RF Exposure Calculator - Note, click on the underlined title near the top to add items

Greg Lapin, PhD, PE, licensed as N9GL, has given an excellent presentation on the new FCC rules for amateur radio about RF exposure. He is Chairman of the ARRL RF Safety Committee and has been in direct contact with the FCC about the rules.

What follows is my summary of the new rules ONLY AS THEY PERTAIN TO A TARPN NODE. If you are interested in how the rules apply to other aspects of amateur radio such as HF transmitters or VHF/UHF HTs, you'll have to read the full presentation. I'll put the links at the end of this message. I have made a reasonable effort to understand N9GL's presentation and to apply it to the TARPN context. However, I disclaim any responsibility for this material. "Human beings" in this context means anyone, including the amateur radio operator himself or herself.

If a human being can be within the following distances of the antenna for a TARPN node, calculation of SAR (Specific Absorption Rate) must be performed. This is the most burdensome possibility of the new rules because SAR modelling or measurement is very difficult.
VHF, 7.9 inches
420-450 MHz, 4.3 inches
902-928 MHz, 2.0 inches

Except for bench testing (when you should be using a resistive dummy load instead of an antenna anyway), these distances are so short that they won't come into play for a TARPN node -- with the possible exception of a public demonstration of a TARPN node. There is no exclusion from the SAR requirement as a function of transmitter power. Even a 100 mW transmitter requires a SAR calculation if a person is within the critical distance. I doubt we will ever be in a position to do SAR modeling for TARPN transmitters and antennas. Bottom line, to be certain that you avoid the SAR requirement, keep all human beings more than 8 inches away from a VHF antenna and more than 4 inches away from a UHF antenna whenever the TARPN node is active, regardless of power and antenna gain.

If a human being can be within the following distances of an antenna of a TARPN node, you must perform a "full evaluation". The full evaluation is basically what amateur radio operators have had to perform in the past, when using more power than the FCC exclusions allowed. All those FCC exclusions have been repealed.
50-54 MHz, 3.1 feet
144-148 MHz, 1.0 feet

These distances are the boundaries between near-field and far-field antenna effects. Again, I believe these distances are an issue mainly for bench testing and public TARPN demonstrations. I will comment on full evaluations later. Note that 1.0 feet for a 2 m TARPN node is more than the 7.9 inches required to avoid SAR. The problem is that most tools for full evaluation that have been available to amateur radio operators are predicated on far-field assumptions. Therefore, to avoid complex calculations, keep all human beings more than 3 feet away from a 6 m antenna and more than 1 foot away from a 2 m antenna whenever the TARPN node is active.

But that's not the only possibility. For VHF, if the Effective Radiated Power (ERP) of the TARPN node in any direction exceeds 3.83 times R^2, where R is the distance in meters between the human being and the antenna in that direction, you will have to perform a full evaluation. Suppose that a person is 1 meter from the antenna. A full evaluation will have to be performed if the ERP exceeds 3.83 watts. Almost all VHF TARPN nodes routinely exceed this value. If a person is 3 meters away from the antenna, a full evaluation will have to be performed if the ERP exceeds 34.4 watts. ERP of a VHF TARPN node might exceed this value for the antennas and radios that we often use. If a person is 8 meters away from the antenna, a full evaluation will have to be performed if the ERP exceeds 245 watts. Remember, this is ERP. If you have a 10-element Yagi connected to a Tait radio on 2 meters and you're in the center of the Yagi's radiation pattern -- looking down the bore, in other words -- your ERP could exceed 245 watts.

For UHF, the formula is different: For UHF, if the ERP in any direction exceeds 0.0128 times R^2 times f, where f is the frequency in megahertz, you will have to perform a full evaluation. Suppose that a person is 1 meter from the antenna at the top of the 70 cm band. A full evaluation will have to be performed if the ERP exceeds 5.76 watts. Almost all UHF TARPN nodes routinely exceed this value. If a person is 3 meters away, a full evaluation will have to be performed if the ERP exceeds 51.8 watts. ERP of a 70 cm TARPN node might exceed this value for the antennas and radios that we often use. If a person is 8 meters away, a full evaluation will have to be performed if the ERP exceeds 369 watts. If you have a 15-element Yagi connected to a Tait radio and you're in the center of the Yagi's radiation pattern, your ERP could exceed 369 watts.

What does a full evaluation require? It's a calculation of frequency, power, feed line losses, antenna gain, distance from the antenna, modulation type (FM as we use in TARPN is the worst case!), duty cycle, and whether persons are considered to be "controlled" (the amateur radio operator and his/her family) or "uncontrolled" (everyone else). The RF safety calculators that we have been using for years are still applicable in far-field conditions. In general, these RF safety calculators give a minimum distance from the antenna to be observed by controlled persons and a much larger minimum distance to be observed by uncontrolled persons.

I ran a sample calculation for TARPN at 2 meters, using 30 watts, a 2 dBi antenna (a dipole), 20 feet of LMR 400 Ultraflex coax, and a 20% duty cycle. The minimum distance for a controlled person is 1.4 feet. The minimum distance for an uncontrolled person is 3.15 feet. Again, this sounds like bench testing and public demonstration scenarios. But if you are on 6 meters, or running more power, or using a higher-gain antenna, the minimum distances will be greater. If the calculations in a full evaluation show that the minimum distances are violated, you must reduce ERP. As general practice we should run our TARPN radios at the minimum power needed to have a link budget that provides a low bit error rate. Besides, FCC Part 97 requires that we use the minimum power needed to communicate.

Now, a few words about the applicability of the new rules. You will be expected to conduct your own analysis of how the rules apply to your TARPN node. You do not have to submit the results to the FCC unless you are asked. However, in case you are asked, you must have the results of your analysis on file so that you can provide them immediately to the FCC.

For stations that are placed on the air on or after May 3, 2021, the new rules apply immediately. For stations already on the air that are modified on or after May 3, the new rules apply immediately. For stations already on the air as of May 3 that are left unchanged, you have until May 3, 2023 (two years) to perform the analysis.

To see the video of N9GL's two-hour presentation, go to https://vimeo.com/540342124. To download the presentation, go to https://www.dropbox.com/sh/mq3djoyqppeucwg/AACjeZjnNjFcDINtEI_B6xPpa?dl=0.

73, Chuck, K4RGN

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