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Like all waveguides, ridged waveguides carry electromagnetic radiation in the form of a transverse wave. The fundamental mode of most waveguides used in interconnect and RF devices is the transverse electric (TE) mode with no electric field in the direction of propagation. This means that the electric field is pointing from one of the side-walls to the opposite sidewall, or perpendicular to the waveguide. By changing the distance between the sidewalls with the electric field present, the wave propagation can be adjusted with different characteristics compared to a typical waveguide with flat walls on each side.
The main effect of reducing the internal dimensions of the waveguide walls effectively lowers the low frequency cut-off of the waveguide in respect to its fundamental mode compared to a smooth wall waveguide. Moreover, the impedance of a waveguide with a ridge on both the top and bottom face of the wider wall (with the E-field), otherwise known as a double ridge waveguide, is significantly lower than that of a standard waveguide. A reduction in impedance can aid with impedance matching to certain devices and components with lower impedance than standard waveguide.
Moreover, the exact dimensions of the ridges in a rigid waveguide allow for design of the higher order waveguide modes, which may be advantageous in certain applications that require wide bandwidths. For instance, in the case of waveguide filters, the spurious pass-bands present at higher order modes can be mitigated by designing the ridge gap dimensions that push the higher order mode pass-bands far enough way in frequency to not affect desired operation. This could eliminate the need for a low-pass filter to attenuate the spurious pass-bands at higher order modes, which may otherwise be necessary for certain applications.
In some cases ridged waveguides are used in place of striplines or microstrip lines which may not exhibit the necessary power handling and typical rectangular waveguides may not have adequate bandwidth. Though using ridged waveguide instead of conventional rectangular waveguide means a comparative reduction in power handling and increase in loss. Double ridged waveguides also tend to be slightly heavier than standard waveguides due to the additional conductive material throughout the structure.
Another beneficial use for rigid waveguides are with microwave switch applications. For a waveguide electromechanical switch to function, the gap between the walls must be bridged with a quality conductive connection in order to realize a short circuit. With double-ridged waveguide, a nearly complete short circuit can be made by merely bridging the reduced distance between the ridges. This feature may result in the ability to design higher performing switch mechanisms for waveguide switches.
Learn more about Pasternack’s line of Double Ridged Waveguides.
Sources
-Closed-Form Expressions for the Parameters of Finned and Ridged Waveguides
– Ridge Waveguides and Passive Microwave Components, by J. Helszajn, IET Electromagnetic Waves Series 49
