A coaxial antenna is used to conduct radio frequency (RF) waves at a specific frequency between electronic devices, such as global positioning systems (GPS), cordless telephones, wireless local area network (WLAN) systems, and communication systems in buildings, tunnels, public transportation units, and for emergency services. The purpose of the antenna is to both shield and direct a specific RF signal so that it reaches its destination with the most strength and the least distortion or interference from other signals. The first patent for a coaxial antenna in the United States (US) was filed in 1937 by Arnold B. Bailey while working for Bell Telephone Laboratories.
Primary features of coaxial antennas are that they have low attenuation or loss of signal due to coupling; they are both strong and stable; and they are easy to install. Installation requirements include keeping the antenna at least 4 inches (about 10 cm) away from walls, using non-metallic and non-conducting clamps to hold the antenna in place, avoiding metallic structures in the vicinity of the antenna, and providing protection against dust and dirt. Anything that settles on the coaxial antenna sheath can eventually cause signal distortion or loss.
The tubular antenna most commonly in use is made up of an inner conductor, a concentric dielectric, outer conductor, a mica barrier tape, and an outer sheath. Inner conductors are made of solid copper or copper-plated aluminum wire, and outer conductors are usually copper tubing or overlapping, milled, and slotted copper tape. Between the inner and outer conductors, the concentric dielectric is a foam core that provides protection against accidents that could affect the signal reception and transmission. RF signals conducted through copper cause the copper to heat, so the mica barrier tape shields the antenna against overheating that can cause distortion of the signal. Outer sheathing is made of a flexible polyethylene thermoplastic that acts as a fire retardant.
Since the construction is very similar, coaxial cables may also be used as coaxial antennas. Coaxial antennas, however, have a few differences from basic coaxial cable characteristics. The transverse electric magnetic (TEM) mode wave that is created and grows inside the antenna radiates outward, unlike standard coaxial cables. Small gaps or apertures in the antenna's outer copper conductor can create the coupling mechanism between the antenna's interior and the environment external to the coaxial antenna.
Antennas may be configured as either coupled mode or radiating mode. The size, position, shape, and distance of the conductor's apertures determine the coaxial antenna coupling mechanisms and radiation characteristics. Coupled mode means that the apertures are spaced closer together than the operating frequency, and that the antenna works on a wide frequency range. Radiating mode means that the apertures are spaced about the same as the operating frequency and radiate with the required frequency.
