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Analysis of the Advantages of Double-Ridged Waveguide

In radio frequency and microwave systems, the main types of transmission lines include coaxial cable, waveguide, and stripline. Among them, the waveguide effectively transmits RF signal energy within a given frequency band. Its primary structure is made of metal conductive materials, capable of handling extremely high power levels. There are many types of waveguides; today, we will learn about single-ridged and double-ridged waveguides.


What Is a Ridged Waveguide


Waveguides can be categorized into various types based on the shape of their internal cross-sections. Commonly used types in RF systems include rectangular, circular, elliptical, single-ridged, and double-ridged waveguides, with the rectangular waveguide being the most common.


Waveguides carry electromagnetic energy through multiple modes, although these modes are typically designated as fundamental modes with the most desirable loss and bandwidth characteristics. This mode is usually the transverse electric (TE) mode, where the electric field direction is perpendicular to the direction of propagation. This mode has an electric field pointing towards and away from the sidewalls of the waveguide.


Because the walls of a waveguide are grounded, adding internal ridges alters the electric field within the waveguide. Therefore, a waveguide with conductive ridges added from the top wall, bottom wall, or both walls is referred to as a ridged waveguide. Adding internal ridges moves the ground plane and restricts the distance the electric field must propagate within the waveguide, increasing the capacitance between the walls compared to a waveguide without ridges.


The ability of ridges to control waveguide impedance also allows for impedance matching without the need for additional components or devices, or reducing the impedance matching margin to a more manageable degree. This can greatly reduce design costs and complexity. In many applications where there are constraints on physical space and weight, a waveguide with a smaller physical size capable of operating at lower frequencies may be desirable.


Advantages of Ridged Waveguides


Compared to similar-sized non-ridged waveguides, the cutoff frequency is lower. This not only reduces impedance but also the low-frequency cutoff point of the waveguide.


Extending higher-order mode frequencies can be beneficial for waveguide filter design. Adding ridges within the waveguide also creates higher-order waveguide modes that may have new internal structures. With proper design and manufacturing, the size and depth of the ridges can be controlled to achieve very specific behaviors, including, in some cases, pushing unwanted modes beyond the frequency of interest and reducing the need for filtering.


When enhanced power handling is needed in compact spaces, it can replace planar transmission lines.


Allowing for a wider bandwidth than rectangular waveguides, they can be used in a wide range of applications like transmission lines.


Single-Ridged VS Double-Ridged Waveguides


Ridged waveguides can be categorized into single-ridged and double-ridged. Both are rectangular waveguides that have capacitive loads at the center of the wide wall.


A single-ridged waveguide is a rectangular waveguide with a single ridge protruding from the top or bottom wall, but the wide wall has a greater capacitive load capability. Compared to a rectangular waveguide, a single-ridged waveguide has a lower cutoff frequency and a smaller cross-section. However, compared to double-ridged waveguides, single-ridged waveguides have higher losses and lower power handling capabilities.


Double-ridged waveguides are rectangular waveguides with ridges from both the top and bottom walls. The ridges in this waveguide design are used to increase bandwidth but come with the downside of increased attenuation and decreased power handling capability.


Another advantage of double-ridged waveguides over non-ridged and even single-ridged waveguides is the ability to place switching elements within the gap of the ridges, making it easier to create switches. Double-ridged waveguides have smaller gap areas between the grounded planes of the electric field, making them easier to bridge with smaller, more reliable, and faster actuators. In some cases, it is even possible to use microelectromechanical systems (MEMS) switch technology to create relatively compact and fast waveguide switches.

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