There are many types of waveguides. The most common waveguide cross-section "shapes" are rectangular and circular/elliptical. The waveguide can carry electromagnetic energy through various modes, although these modes are usually designated as basic modes with the most ideal loss and bandwidth characteristics. This mode is usually the transverse electric (TE) wave mode, in which the electric field direction is perpendicular to the propagation direction. This mode has an electric field that points towards and away from the waveguide sidewalls.
Adding internal ridges to the waveguide changes the behavior of the electric field inside the waveguide because the walls of the waveguide are grounded. Therefore, adding internal ridges moves the ground plane and limits the distance that the electric field must travel within the waveguide. It also increases the capacitance between the walls compared to a ridgeless waveguide. Like the single-ridge waveguide, the double-ridged waveguide uses this concept to further "squeeze" the electric field inside the waveguide.
The result is a decrease in impedance and a lowering of the waveguide's low-frequency cutoff point. This basically allows smaller-sized waveguides to accommodate lower frequencies compared to waveguides without ridges.
In addition, adding ridges to the waveguide also creates higher-order waveguide modes that may have new internal structures. By appropriately designing and manufacturing the ridges' size and depth, very specific behaviors can be achieved, including in some cases pushing unwanted modes beyond the frequency of interest and reducing the need for filtering.
The ability to control waveguide impedance using ridges also allows impedance matching without the need for additional components or equipment or reducing the margin of matching impedance to a more easily manageable level. This can greatly reduce design costs and complexity. In many applications with physical space and weight limitations, smaller physical-sized waveguides that can still operate at low frequencies may be desirable.
(1) A lower cutoff frequency compared to non-ridged waveguides of similar sizes.
(2) Extending the high-order mode frequency may be advantageous for waveguide filter design.
(3) Replacing planar transmission lines when power handling needs to be enhanced in a compact space.
(4) Wider bandwidth than rectangular waveguides allowing for more widespread use as a "transmission line".
Another advantage of double-ridged waveguides over non-ridged or even single-ridged waveguides is the ability to place switch elements in the ridge gaps, making it easier to create switches.
Double-ridged waveguides have smaller gap areas between the electric field ground planes, making them easier to bridge with smaller, more reliable, and faster actuators. In some cases, even micro-electromechanical (MEMS) switch technology can be used to create relatively compact and fast waveguide switches.