Q Lou, KB6JLI, asks, “While reading an advertisement for a vertical antenna, I noticed the ad mentioned that no radials are needed, but it also says that you need to use an 80- foot counterpoise. Please clarify for me the difference between a radial and counterpoise. Aren’t they basically the same?”
A The difference between the terms “radial” and “counterpoise” is subtle, but significant. Radials usually consist of multiple bare wires either buried in, or laid upon, the ground and are not tuned to a specific frequency. Such wires don’t really show a resonance because they are coupled so heavily to the lossy Earth. Their purpose is solely to reduce ground losses (very good information on this topic can be found in the 19th edition of The ARRL Antenna Book, starting on page 6-24).
A counterpoise is a wire or group of wires mounted close to ground, but insulated from ground, to form a low-impedance, high-capacitance path to ground. The purpose of a counterpoise is to provide an RF ground for the antenna.
From QST March 2001
Showing posts with label counterpoise. Show all posts
Showing posts with label counterpoise. Show all posts
Friday, April 15, 2011
Thursday, July 8, 2010
What is the safe distance one should be from a counterpoise? ...
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
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
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