Waveguide is a technology uniquely synonymous with high-frequency RF/microwave electronics. Described simply, it is a form of hollow conductive metal tube that carries energy from one point to another. For RF/microwave use, it handles electromagnetic (EM) energy; forms of waveguide have also been used to carry acoustical and optical energy. Waveguide can be used to fashion a wide range of RF/microwave and even millimeter-wave components, including transmission lines, couplers, power dividers, and filters.
For RF, microwave, and millimeter-wave applications, waveguide can be constructed of flexible and rigid conductive materials, and can be rectangular, elliptical, and circular in shape. In contrast to coaxial cables, where signals pass through the cables and electric and magnetic fields propagate through the inside and around the outside of the cables, propagating electric and magnetic fields in waveguide are confined to the space within the conductive guides, so power is not lost due to radiation as in coaxial cables. Waveguide are normally filled with air, which serves as the dielectric material for the propagation of energy, so dielectric losses are also minimal. Some transmitted power is lost as heat within the walls of the metal waveguide, but this loss is minimal compared to losses from coaxial cables, such as insertion loss and radiation loss.
The most common forms of waveguide for EM applications are rectangular waveguide, denoted by a model number such as WR62 which represents the shape and a specific guide size, double ridge rectangular waveguide, with sizes identified by the WRD letters and a number combination, such as WRD650, and circular waveguide, with sizes identified by the WRC letter prefix and a number for the relative size of the guide. Circular waveguide are often used in applications requiring rotation, such as for a rotary joint in a radar antenna, although the greater number of applications for low-loss waveguide in commercial, industrial, and military systems call for rectangular or double ridge rectangular waveguide. The use of ridges in the double ridge rectangular waveguide configuration helps boost bandwidth albeit with somewhat higher attenuation and lower power-handling capability than a comparable size of rectangular waveguide.
The lower-frequency limit, or cutoff frequency, of a waveguide component is determined by the physical dimensions of the waveguide. Signal energy below the cutoff frequency will be significantly attenuated in a short distance by a given waveguide structure. (It should be noted that the cutoff frequency of a coaxial cable refers to the other frequency direction, or its highest frequency of operation.) As the required operating frequencies for different waveguide structures increases, the dimensions of the waveguide diminish. A waveguide transmission line can propagate energy in a number of different modes, including in transverse electric (TE), transverse magnetic (TM), and transverse electromagnetic (TEM) modes. To differentiate, a waveguide operating in TE mode has no electric field in the direction of propagation; a waveguide in TM mode, has no magnetic field in the direction of propagation; and a waveguide in TEM mode has no electric or magnetic fields in the direction of propagation. For a particular waveguide component or transmission line, the dominant mode is that mode which exhibits the lowest cutoff frequency; for rectangular waveguide, this is the TE mode. Waveguide are usually designed and fabricated to support only one mode.
As an example, WRD650 is a size of double ridge rectangular waveguide with outer width and height dimensions of 0.720 x 0.421 in. Typically formed of aluminum, brass, and copper, this particular waveguide has an operating frequency range of 6.50 to 18.00 GHz. A similar rectangular waveguide size, WR62, has outside width and height dimensions of 0.702 x 0.39 in. and is usually made of copper and aluminum. It has an operating frequency range of 12.4 to 18.0 GHz. A circular waveguide size for TE-mode operation from 13.2 to 18.9 GHz, designated as WRC621D1, has an inside diameter of 1.281 in. and an outside diameter of 1.441 in. At higher, millimeter-wave frequencies, where metals tend to suffer higher losses, dielectric rod and slab waveguide are typically used to minimize signal losses.
A flexible version of rectangular waveguide, with a size designation of WR62 and typically fabricated from brass, will also provide low-loss performance from 12.4 to 18.0 GHz, although trading some electrical performance for mechanical flexibility compared to the standard WR62 waveguide. Similarly, a flexible version of WRD650 double ridge rectangular waveguide will also cover a frequency range of 6.50 to 18.00 GHz, although with different insertion loss, power-handling capability, and other characteristics than the standard double ridge rectangular waveguide for that size.
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