Traveling-Wave Antenna

a directional antenna in which a traveling wave of electromagnetic oscillations is propagated along its geometric axis. Traveling-wave antennas are made either with discrete radiators placed along the axis at a certain distance from one another or in the form of a continuous radiator that extends in the direction of the axis. (The latter is considered as the sum of discrete radiators adjoining one another.) The Yagi antenna and the helical antenna belong to the first category; the dielectric rod antenna and the Beverage antenna belong to the second. There are also traveling-wave antennas consisting of several elements, each of which is a traveling-wave antenna of the second type (the rhombic antenna and others). This type of antenna is used in receiving and transmitting installations for all wavelengths of the radio band.

The traveling-wave antenna has its maximum radiation (reception) in the direction of its axis. The directivity is D = kl/λ where / is the length of the antenna, λ is the wavelength, and k is a coefficient that depends on the directivity of the individual radiating element, the phase velocity of the traveling wave, the relationships of the current amplitudes in the radiating elements, and other factors. The value of k usually lies in a range from 4 to 8. The directivity reaches a maximum when the phase velocity v of the traveling wave is somewhat less than the velocity of light c and equal to

v = c • 2l/(2/ + λ)

The typical characteristics of a traveling-wave antenna are the axially symmetrical shape of its three-dimensional radiation pattern (that is, the shape of the pattern is the same in any plane passing through the antenna’s axis) and the maintenance of adequate directivity (in the majority of traveling-wave antennas) over a broad wavelength range. The first characteristic becomes increasingly evident with an increase in the ratio l/λ and the axial symmetry of the radiation pattern of each radiating element.