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RF-exposure safety (was: microwave absorption)



Rex VK8RH wrote:

	And twenty years ago cigarettes didn't cause cancer - we know where that
	stands now.

	I think caution is warranted until the facts are proven - and they are a
	long way from that.

	We as amateurs, some running high power should also be careful with exposure
	limits not only from antenna's but from the equipment within the shack.  (eg
	a 1000 watt plus amplifier on 400Mhz not properly shielded)


Wayne W9AE replies:

The U.S. does have RF exposure safety regulations for amateur stations.  I imagine that Australia does, too, considering that the Australian government is a world leader in RF safety regulations.  U.S. Amateur operators are required to keep their stations safe for themselves and others, within the guidelines specified by the FCC.  That is, the antennas must be located in a manner that humans can't accidentally get in the path of the RF energy within a certain safe distance (determined by the frequency and ERP).  For instance, it's not legal to run a kilowatt into a 15 dB-gain Yagi aimed directly at your neighbor's house 20 feet away.  There are many links to more information on this subject on the ARRL WWW site (regulations, articles, RF-exposure calculator programs, etc.)

http://www.arrl.org/tis/info/rfexpose.html <http://www.arrl.org/tis/info/rfexpose.html> 

Here are "Some Sensible Guidelines" from the ARRL safety committee:

1.      Although antennas on towers (well away from people) pose no exposure problem, make certain that the RF radiation is confined to the antennas' radiating elements themselves. Provide a single, good station ground (earth), and eliminate radiation from transmission lines. Use good coaxial cable, not open-wire lines or end-fed antennas that come directly into the transmitter area. 

2.      No person should ever be near any transmitting antenna while it is in use. This is especially true for mobile or ground-mounted vertical antennas. Avoid transmitting with more than 25 W in a VHF mobile installation unless it is possible to first measure the RF fields inside the vehicle. At the 1-kW level, both HF and VHF directional antennas should be at least 35 ft above inhabited areas. Avoid using indoor and attic-mounted antennas if at all possible. 

3.      Don't operate high-power amplifiers with the covers removed, especially at VHF/UHF. 

4.      In the UHF/SHF region, never look into the open end of an activated length of waveguide or point it toward anyone. Never point a high-gain, narrow-bandwidth antenna (a paraboloid, for instance) toward people. Use caution in aiming an EME (moonbounce) array toward the horizon; EME arrays may deliver an effective radiated power of 250,000 W or more. 

5.      With hand-held transceivers, keep the antenna away from your head and use the lowest power possible to maintain communications. Use a separate microphone and hold the rig as far away from you as possible. 

6.      Don't work on antennas that have RF power applied. 

7.      Don't stand or sit close to a power supply or linear amplifier when the ac power is turned on. Stay at least 24 inches away from power transformers, electrical fans and other sources of high-level 60-Hz magnetic fields.

Wayne Estes W9AE
Mundelein, IL, USA

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