Answer to Question #10951 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
I plan to install a marine VHF (very high frequency) radio on my boat. The frequency is 156–158 MHz. The installation guide notes that the antenna should be at least 0.305 m from people and the unit itself should also be 0.305 m from the antenna.
My boat is small, and mounting options are limited. What are the health risks to individuals on my boat and are the risks only present when I am transmitting?
If the risks are principally during transmission, is it reasonable to presume that individuals would be safe if we are all at least 0.305 m from the antenna during transmission?
Manufacturer's radiofrequency (RF) safety requirements are generally based on compliance with Federal Communications Commission (FCC) guidelines for human exposure to RF energy. At the frequency mentioned, in the VHF range, the FCC exposure limits are most restrictive for whole-body human exposure. The typical effective radiated power (ERP) for one of these radios is around 100 W. So the 0.305 m separation distance was likely based on the calculation for how far away one should be to be in compliance with the limits. You are correct, of course, that RF is only of concern when the antenna is transmitting. So if the 0.305 m distance is maintained during that period, there should not be any concern over potentially hazardous exposure. In fact, there are safety factors built into the limits themselves anyway, so the 0.305 m distance is probably conservative. In addition, time-averaging and averaging over the whole body are factors that would add to the conservatism of the requirement. More details can be found at the FCC website dealing with RF safety.