Traveling-wave antenna is a class of antenna that use a traveling wave on a guiding structure as the main radiating mechanism. Traveling-wave antennas fall into two general categories: slow-wave antennas, and fast-wave antennas. Fast-wave antennas are often referred to as leaky-wave antennas.
Slow-wave antennas
In slow-wave antennas, the guided wave is a slow wave, meaning a wave that propagates with a phase velocity vph that is less than the speed of light in free space. Such a wave does not fundamentally radiate by its nature, and radiation occurs only at discontinuities (typically the feed and the termination regions). The propagation wavenumber of the traveling wave is therefore a real number (ignoring conductor or other losses). Because the wave radiates only at the discontinuities, the radiation pattern is emitted from two sources, one at the beginning and one at the end of the structure. This makes it difficult to obtain highly-directive single-beam radiation patterns. However, moderately-directive patterns having a main beam near endfire can be achieved, although with a significant sidelobe level. For these antennas there is an optimum length depending on the desired location of the main beam. Examples include wires in free space or over a ground plane, helixes, dielectric slabs or rods, corrugated conductors. An independent control of the beam angle and of the beam width is not possible.
Leaky-wave antennas
The wave on a leaky wave antenna (LWA) may be a fast wave, with a phase velocity greater than the speed of light. This type of wave radiates continuously along its length, and hence the propagation wavenumber kz is complex, consisting of both a phase and an attenuation constant. Highly-directive beams at an arbitrary specified angle can be achieved with this type of antenna, with a low sidelobe level. The phase constant β of the wave controls the beam angle (and this can be varied changing the frequency), while the attenuation constant α controls the beamwidth. The aperture distribution can also be easily tapered to control the sidelobe level or beam shape. Leaky-wave antennas can be divided into two important categories, uniform and periodic, depending on the type of guiding structure.
References
- C. H. Walter, Traveling Wave Antennas, McGraw-Hill, 1965, Dover, 1970, reprinted by Peninsula Publishing, Los Altos, California, 1990.
- T. Rozzi and M. Mongiardo, Open Electromagnetic Waveguides, The Institution of Electrical Engineers(IEE), London, 1997.
- M. J. Ablowitz and A. S. Fokas, Complex variables: Introduction and Applications, second edition, Cambridge University Press, 2003.
- A. A. Oliner and T. Tamir, "Guided complex wave, part I: field at an interface", Proc. IEE, Vol. 110, pp. 310-324, February 1963.
- A. A. Oliner and T. Tamir, "Guided complex wave, part II: relation to radiation pattern", Proc. IEE, Vol. 110, pp. 325–334, February 1963.
- A. Hessel, "General characteristics of traveling-wave antennas", Chapter 19 in Antenna Theory, R. E. Collin and F. J. Zucker, Editors, McGraw-Hill, New York, 1969.