Electromagnetic energy is classified as either ionizing or non-ionizing radiation. Ionizing radiation has enough energy per photon to remove electrons from atoms and create ions. Radio signals occupy the low‑frequency end of the electromagnetic spectrum (including all amateur radio bands) and are non‑ionizing — their photon energy is far too low to dislodge electrons, regardless of transmitter power. Higher‑energy radiation such as gamma rays and alpha particles is ionizing.

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The human body absorbs radiofrequency energy most efficiently in the range of about 30 MHz to 300 MHz. Because absorption is greatest in that range, regulatory maximum permissible exposure (MPE) limits are most restrictive (i.e., lowest) there. Of the frequencies listed, 50 MHz falls within that 30–300 MHz range, so it has the lowest MPE. The other frequencies are either well below or above that absorption band and therefore have less restrictive (higher) MPE values.

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Duty cycle is the ratio of time a transmitter is ON compared to the total time (ON + OFF). If the transmitter is ON only 50 percent of the time, the time-averaged exposure is half what it would be at continuous (100 percent) operation for the same peak power.

RF exposure limits are based on time-averaged power (or SAR — Specific Absorption Rate). Because average power = peak power × duty cycle, halving the duty cycle means you can double the peak power while keeping the same time-averaged exposure. Therefore, when duty cycle goes from 100% to 50%, the allowable (peak) power density increases by a factor of 2.

Memory aids:

• 50% duty cycle → allowed power doubles (×2)
• Formula: Average power = Peak power × Duty cycle

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All of the listed factors influence how much RF energy a person near an amateur antenna will be exposed to.

Frequency: The frequency of the RF signal affects how the body absorbs the energy (penetration depth and absorption characteristics vary with frequency), and therefore affects the specific absorption rate (SAR).

Power level: The transmitter power determines how much energy is available to be radiated. Higher power generally produces higher power density near the antenna and increases potential exposure.

Distance: RF power density falls off with distance from the antenna. In the far field it decreases roughly with the square of the distance; even in the near field increasing separation reduces exposure.

Radiation pattern: The antennas radiation pattern shows where energy is concentrated (main lobes) and where it is weaker (nulls). A directional antenna can produce much higher exposure in the directions of its main beams than an omnidirectional one.

Because frequency, power, distance, and the antennas radiation pattern all affect how much RF energy reaches a person, all of these factors are relevant to RF exposure.

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The human body absorbs RF energy differently at different frequencies because tissues and body parts couple to electromagnetic fields in a frequency-dependent way. At some frequencies the body (or parts of it) is more “resonant” or couples more efficiently, so more energy is absorbed for the same external field strength. Exposure limits are set to account for those variations in how much energy the body actually absorbs.

Other proposed reasons are incorrect: the amount of energy absorbed depends on frequency-dependent coupling rather than a simple statement that lower frequencies have more energy; lower-frequency fields can penetrate the body; and higher-frequency fields are not inherently more transient than other frequencies.

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All of the listed methods are acceptable ways to determine whether a station complies with FCC RF exposure regulations. The FCC provides guidance for performing exposure calculations in OET Bulletin 65, which gives formulas and limits to use for hand calculations or conservative estimates. Computer modeling is also an acceptable way to predict RF fields and exposure levels, and can be especially useful for complex antenna installations or when accounting for nearby structures. Direct measurement of field strength using properly calibrated equipment is a valid method as well, and is commonly used to verify calculations or when measurements are required to demonstrate compliance.

When using measurements, be sure the measurement equipment is appropriate for the frequency and field type being assessed and is calibrated. Calculations (manual or computer-modeled) are often sufficient for many typical amateur installations, while measurements are used when calculations are uncertain or when physical verification is desired.

Memory aids:

• OET65 = FCC guidance document for RF exposure calculations

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The human body absorbs radio‑frequency (RF) energy and behaves like a resistor, converting that energy into heat. Excessive RF absorption near an antenna can produce localized heating and painful RF burns to the skin and underlying tissue. Touching an antenna during transmission therefore risks RF burns.

RF from radio transmitters is non‑ionizing, so it cannot cause the kind of chemical or DNA damage associated with ionizing radiation. Electrocution implies a dangerous low‑frequency or DC current flow through the body; while energized feedlines or high voltages associated with some antenna systems can present shock hazards, the primary hazard specifically from touching an antenna during transmission is the heating effect (RF burn).

Memory aids:

• RF causes heating (think microwave) — non‑ionizing
• "Burns, not poisoning" — RF = thermal effect

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RF energy is emitted from the antenna, so changing the antenna location is what changes the fields people are exposed to. Moving the transmitter itself does not reduce exposure as long as it remains connected to the same antenna, because the radiating source (the antenna) hasn't changed. "Duty cycle" is the fraction of time the transmitter is actually radiating; increasing the duty cycle means the antenna is transmitting more often and therefore increases RF exposure rather than reducing it.

Memory aids:

• RF radiation "radiates" outward from the antenna — move the antenna to change exposure
• Moving the transmitter won’t help if the antenna and feedline stay the same
• Higher duty cycle = more transmit time = greater exposure

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Re-evaluate the station whenever an item in the transmitter or antenna system is changed. Any modification to the transmitter, antenna, feedline, or their placement can change the RF field strengths around your station and therefore affect compliance with exposure limits. The amateur service is generally self-policing, so you are expected to check compliance yourself rather than notify the FCC of every change. Having a low SWR helps with efficient power transfer and protecting your equipment, but it does not guarantee that RF exposure limits are being met. Similarly, product safety listings (such as UL) relate to electrical/fire safety of equipment, not RF exposure compliance.

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Duty cycle influences the average RF energy a person receives, and exposure limits (Maximum Permissible Exposure, MPE) are based on average exposure over time. A very short, infrequent high-power pulse produces a high peak but low average exposure, while continuous transmission at a lower power can produce a higher average exposure. Duty cycle is the ratio of transmit time to total time, so it directly determines average exposure. Factors like antenna feed line loss or final-amplifier heating do not determine human RF exposure limits.

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Duty cycle is the ratio of on-air time to the total operating time during the averaging period used for RF exposure. For example, a 40% duty cycle means the station is transmitting 40% of the time and not transmitting 60% of the time.

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RF (radiofrequency) energy is non‑ionizing. Its photons do not have enough energy to remove tightly bound electrons from atoms or molecules, so RF cannot break chemical bonds or directly damage DNA. By contrast, ionizing radiation (for example X-rays or gamma rays) has sufficient energy to ionize atoms, break chemical bonds, and cause genetic damage. RF exposure at high levels can produce heating, but it does not produce the chemical changes associated with radioactivity.

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A good rule of thumb for Amateur Radio is that you are responsible for your own emissions. Any time you are transmitting you are responsible for the consequences.

This includes compliance with FCC RF exposure limits. Whether the issue is RF exposure, causing interference to other stations or devices, or producing unwanted harmonics on other bands, the station licensee must ensure the station operates within the rules. Even if you don't realize your station is causing a problem, the responsibility rests with you as the licensee to prevent unsafe or illegal emissions.

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