Technician: VHF Exposure Limits

Technician: VHF Exposure Limits

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

The answer to this question is C. 50 watts PEP at the antenna.  The answer lies in a FCC publication called OET Bulletin 65.  In this document is a lot of engineering and math, but most important is a table of frequencies and minimum power levels for which a station should be evaluated.

 

The reason for such a wide variance in the limit, is that the human body is affected by RF differently for different frequencies.  The same power level at one frequency may be completely harmless, while at a higher frequency, dangerous.

Think microwave ovens.  They simply use RF in a certain frequency range, and we all know what it does to stuff put in the oven, and not at a particularly high power.

Now, notice that this is not 50W PEP output from your radio.  This is at the antenna.  Depending on your station, you very likely see losses in your feedline and antenna which will make the actual output lower.   In practice, since most VHF radios are limited to 50W transmit power or lower anyway, you probably won’t have to worry about this.  If you run an amplifier or a radio capable of more than 50W output on VHF, then you should consider performing an RF evalution.

[T0C03]

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