T0C03 from the Technician License Course Section 13.3, RF Exposure Safety:
What is the maximum power level that an amateur radio station may use at VHF frequencies before an RF exposure evaluation is required?
A. 1500 watts PEP transmitter output
B. 1 watt forward power
C. 50 watts PEP at the antenna
D. 50 watts PEP reflected power
Radio frequency emissions can be harmful when the human body is excessively exposed to them. Excessive RF exposure causes damage to tissues by heating them. Human body tissues absorb RF and that energy is transformed into heat that must be dissipated by flowing blood, evaporating perspiration, and radiating skin, else tissues begin to get warm. The effect is identical to that of a microwave oven heating food with RF energy, although usually the exposure from amateur radio signals is not quite that intense!
In order to help keep human tissues safe and properly cooled, the FCC has provided recommendations for evaluating the RF exposure caused by your station emissions. The FCC Office of Engineering and Technology (OET) Bulletin 65 from August 1997 details how an amateur station’s RF exposure should be evaluated. You can find more information about OET Bulletin 65 in the HamRadioSchool.com Technician License Course book, section 13.3 RF Exposure Safety.
One interesting datum from the bulletin indicates that the lowest Maximum Permissible Exposure (MPE) limits apply to the VHF frequency range. That is, when it comes to exposing yourself to RF, your body most readily absorbs VHF emissions. Exposure to VHF will make you toasty more quickly than exposure to equivalent power densities of HF or UHF emissions because the human body is a better sponge for VHF absorption. Here’s a chart from OEM Bulletin 65 depicting the MPE across radio spectra.Frequency is depicted across the bottom, and RF power density is on the left vertical axis in milliwatts per square centimeter (mW/cm2). The plot depicts the maximum exposure that a human should receive by frequency. Notice how the plot drops in the VHF range of 30 to 300 MHz to the lowest values – about 1 mW/cm2 for us willingly exposed operators, and 0.2 mW/cm2 for the unwitting general public.
Based upon these values and typical operating scenarios, the FCC has implemented a rule to help station operators know when their emissions may begin to “get into the ballpark” of this VHF maximum exposure limit. The rule establishes a maximum power that a station may transmit without conducting an RF exposure evaluation. It establishes the safe zone above which you had better go to greater pains to make sure you are not cooking yourself, your family, or the neighbors.
Peak envelope power (PEP) is used to characterize this maximum power level. Peak envelope power is the average of the power peaks at the highest amplitude point of the RF signal envelope. (No worries, you’ll learn more about PEP in General Class studies.) Most power meters will provide a setting to measure PEP with the flip of a switch, reading directly from a display, making it a no-brainer to measure. Further, it makes sense that since the power is radiating from the antenna location, the PEP of interest is that emitted from the antenna. There will be some power loss through transmission lines, heating the coax cable instead of your tissues, so the antenna PEP will be somewhat less than that generated by your transmitter. (See topics on coaxial cable loss calculations with decibels to gauge the amount of loss by cable type and length.)
The answer is now evident, and the maximum antenna PEP allowed without conducting an RF exposure evaluation by the methods of OET Bulletin 65 is 50 watts.
The answer to Technician Class question T0C03, “What is the maximum power level that an amateur radio station may use at VHF frequencies before an RF exposure evaluation is required”is “C. 50 watts PEP at the antenna.”
Related Questions: T0C02, T0C05, T0C06, T0C09
Note: Simple-to-use RF exposure calculators may be found online that utilize the OEM Bulletin 65 methodologies. One notable calculator is by Paul Evans VP9KF and may be found at http://hintlink.com/power_density.htm.