Dielectric Antenna

an antenna in the form of a section of dielectric rod excited by a radio wave guide or the post of a coaxial cable. A wave of a specific shape (a so-called surface wave) is generated in the rod of the antenna and propagates along the axis of the rod. As a consequence, tangential components of electrical and magnetic fields whose phase varies according to the law of traveling waves appear on the surface of the rod. Dielectric antennas are essentially traveling-wave antennas, consisting of elementary electric and magnetic dipoles. The radiation maximum coincides with the axis of the rod, as does the maximum of any traveling-wave antenna. The type of radiation of a dielectric antenna depends on the phase velocity of propagation of the surface wave, which decreases with an increase in the diameter of the dielectric rod and in the dielectric constant of its material. The lower the phase velocity, the greater the length of the rod at which the front-to-rear factor is at a maximum (the so-called optimum length) and the greater the maximum possible front-to-rear factor. As the phase velocity decreases, or as it approaches the speed of light in the surrounding medium (air), the dielectric rod begins to lose its wave-guide properties. This leads to an abrupt decrease in the field intensity near the end of the rod, an increase of radiation into the medium surrounding the antenna (directly from the open end of the wave guide), and a decrease in the antenna’s efficiency. The diameter and material of the rod are usually chosen in such a way that the phase velocity is not too close to the speed of light (no more than 0.95-0.96 of it). At such phase velocities the optimum length equals 12 lengths of the radiated wave, and the front-to-rear factor is approximately 100. The rods of dielectric antennas are made from dielectric materials with low attenuation of electromagnetic waves—polystyrene and fluorine plastics. Dielectric antennas are used mainly in aircraft radio equipment, which operates on centimeter or decimeter wavelengths.