Waveguides Inside Out

Following from the name, a waveguide is a hollow conductive structure that guides an EM wave along a path. The conductive inner walls of the waveguide bounce EM energy along the path with low-loss and close to light speed.

Waveguides are commonly cylindrical, elliptical, or rectangular. In a rectangular waveguide, transverse electric (TE), or H-wave, can be formed when the magnetic field is in the direction of propagation. If the electric field is in the direction of propagation, it is known as transverse magnetic (TM), or E-wave.

The speed of the wavefront and the frequencies a waveguide can support depend upon the physical dimensions of the waveguide. Different frequencies will form various modes within a waveguide. The dominant mode of a waveguide is the one where EM energy is guided most efficiently. In rectangular waveguides, the TE10 mode is the most efficient, and in a cylindrical waveguide the TE11 mode is the dominant mode. These modes have lower-cutoff frequency and upper frequency boundaries where frequencies beyond this range operate in other modes. Multiple modes operating on the same waveguide can create unwanted interference to the dominant mode. Though, there are some applications which benefit from leveraging multiple modes within a waveguide.

For performance interconnect applications most waveguides are carefully designed to support only the dominant mode. These factors lead to a waveguides dimensions being directly tied to the lower frequency of the mode. This limits waveguide sizes to what dimensions can be supported by an application. For example, a 1MHz waveguide would be roughly 500 ft in the largest dimension. Though at microwave and millimeter wave frequencies, waveguides reduce dramatically in dimension and weight. Regardless, as the exact dimensions of a waveguide must be maintained throughout the structure, care must be taken during any bending or routing operation. Waveguides have limited flexibility as a function of the need for high mechanical precision.

The large conductive surface area of a waveguide leads to very little energy lost while channeling the EM energy. Plating the inside of a waveguide with a thin layer of greater conductivity material can also increase the efficiency of energy transfer and the power handling capability of a waveguide. Also, the dielectric within a waveguide is generally air, which has very low dispersion and loss compared to dielectric materials used in coaxial cables.

As all of the EM energy is contained within the waveguide, radiation losses are extremely low within a waveguide. Not surprisingly, the high level of isolation within a waveguide also prevents unwanted signal interference from external sources. On the other hand, the high performance of waveguide interconnects also comes at a high price. The precision machining, custom manufacture, and flange couplings at the joints tend to lead to an expensive final product.

To overcome these challenges, there are manufacturers that produce flexible waveguides that can be pre-molded or flexed multiple times. Additionally, creating high performance active devices and passive devices are relatively simple to manufacture mechanically. These devices also minimally impact the integrity of the signal path, enabling high performance interconnect and passives with waveguide technology.