RF Radiation: It's Out There

RF Safety Guidelines 

By Jonathan Imberi

Hand-held radios are very popular for VHF and UHF operation. Hand-helds transmit with less than 7 watts and are generally considered safe. The FCC classifies these radio as portables because they are designed to be operated with an antenna that is within 20 cm of a human body. Because of this, special considerations are in order to ensure safe operation. This is especially true since hand-held radios generally place the antenna so close to the operator's head. Amateurs need to position their hand-helds so the antenna is pointed away from the head and eyes. An external speaker microphone can be helpful in achieving this. 

A transceiver designed to be mounted in a vehicle is classified as a mobile device. Mobile devices are normally intended to be operated using an antenna that is at least 20 cm from any person. Mobile operations also require special considerations. You should try to mount the antenna in the center of the metal roof of your vehicle. This uses the metal body of the vehicle as an RF shield to protect people inside the car. Glass mounted antennas result in higher exposure levels, as do antennas mounted on a trunk lid or front fender. Glass is not a good form of RF shielding. 

Mobile units often produces less RF radiation exposure, even though they usually transmit with higher power levels than hand-held radios. This is because an antenna mounted on a metal vehicle roof is generally well shielded from vehicle occupants. 

Never operate RF power amplifiers or transmitters with the covers or shielding removed. This will help avoid both electrical shock hazards and RF safety hazards. A safety interlock prevents the gear from being turned on accidentally while the shielding is off. Safety interlocks are especially important for VHF and UHF equipment. Always replace all the screws that hold the RF compartment shielding in place when reassembling your transmitter. Make sure to tighten all the screws securely before applying power to the equipment. 

Another key area you should pay attention to is the feed line connecting your transmitter to your antenna. If you are using coax with shielding that is inadequate or if there are other causes leading to signals radiating from your feed line, corrective measures are an order. Improper grounding can also lead to a condition known as RF in the shack. This is a serious problem with stations installed in the second or third floor or higher, where the ground lead begins to act more like an antenna. If you notice that your SWR reading changes as you touch your equipment, or if you feel a tingling sensation in your fingers when you touch the radio or microphone, these may be indicators that you have RF in the shack. Some steps to correct these conditions to ensure a safe operating environment will have to be taken. 

If you are installing a repeater or other transmitter in a location that includes antennas and transmitters operating in other services be aware that the total site installation must meet the FCC RF radiation MPE limits. Remember that your signal is only one part of the total RF radiation from that location. You will have to cooperate with the licensees of the other transmitters to determine the total exposure.