Q Ray, AH6LT, asks, “I have an antenna mounted on my apartment balcony with a counterpoise wire laying on the floor. The RF safety regulations don’t say anything about counterpoises, but I presume that they radiate. What is the safe distance one should be from a counterpoise?”
A You are correct—a counterpoise is a radiating part of the antenna, especially if it is located significantly above ground. There are several ways you can evaluate it. If you had accurate field-strength measurement equipment, you could measure the fields. This is, however, usually beyond the scope of most amateurs.
Most evaluations will be done by calculation. You can use the
rather straightforward calculation methods outlined at the RF
Safety Calculator Web page at the University of Texas at: http://
n5xu.ae.utexas.edu/rfsafety/. The calculator will help you estimate the distance people need to be from any part of your antenna system (including the counterpoise). This is generally a conservative estimate; fields are usually less than the calculation would indicate. You could also use one of the antenna modeling software packages to get a more accurate estimate.
See http://www.arrl.org/news/rfsafety for ARRL info on the subject, plus links to the FCC.
Figure 2—You can find an RF safety calculator on the Web at: http://n5xu.ae.utexas.edu/rfsafety/.
From QST February 2000