A coaxial antenna is a variant of the dipole antenna, designed for use with an unbalanced feed line. One side of the antenna element consists of a hollow conducting tube through which a coaxial cable passes. The shield of the cable is connected to the end of the tube at the center of the radiating element. The center conductor of the cable is connected to the other half of the radiating element. The element can be oriented in any fashion, although it is usually vertical.
Coaxial antennas are used at frequencies above approximately 10 MHz, where the size is manageable. The design is especially popular among Class-D Citizens Band radio enthusiasts, who operate in a narrow band of frequencies near 27 MHz. Coaxial antennas are also used in short-range communications installations at VHF (very high frequencies)between 30 and 300 MHz.
If f is the frequency of operation in MHz, the overall length or height h of a coaxial antenna element in feet is given approximately by:
h = 460/f
For example, at 27 MHz:
h = 460/27 = 17 feet
This is a manageable size for a self-supporting vertical antenna. The tubing, through which the feed line passes, forms the lower half of the antenna.It must be insulated from ground and from metallic supporting structures.