The dominant mode in a waveguide is the propagation mode with the lowest cut-off frequency. To calculate the cut-off frequency f c of the rectangular waveguide, use the following equation, where c is the speed of the light inside the waveguide and m and n are the numbers that define the mode of propagation.ĭominant and Degenerate Modes Dominant Mode Let TE mn be the mode active in the waveguide. The Equation for Rectangular Waveguide Cut-Off FrequencyĬonsider a rectangular waveguide with width ‘a’ and thickness ‘b’.
In the electronics market, waveguides are available in standard sizes however, if you wish to use waveguides for specific applications, they should be custom made. As the waveguide gets larger, it lowers its cut-off frequency. During waveguide construction, it is recommended to keep the width of a waveguide in the same order of magnitude as the wavelength of the signal being transmitted. When trying to pass signals of lower frequency than the cut-off frequency, the waveguide develops mechanical constraints. To avoid signal attenuation and power loss from multiple active modes, waveguides should be constructed with their cut-off frequency in mind.
How to Avoid Signal Attenuation and Power Loss Only one mode should be active and waveguide dimensions should be selected so that the waveguide only supports one active mode. However, if all modes are active in a waveguide, it results in attenuation of the signal. There can be various modes in a waveguide, such as TE 10, TE 20, TE 30, or TM modes. The geometry of a waveguide is an important factor in determining the cut-off frequency. When a wavelength is too long, the waveguide stops carrying signals and becomes inoperative. The signal propagation through a waveguide is dependent on the signal wavelength as well. Frequencies below the cut-off frequency are attenuated by the waveguide. Below the cut-off frequency, waveguides fail to transfer wave energy or propagate waves.Ĭut-off frequency can also be described as the frequency above which the waveguide offers minimum attenuation to the propagation of the signal. All the signals that propagate through a waveguide are above a certain frequency, called the cut-off frequency. Waveguides are hollow metallic structures that carry signals from one end to another. A similar condition applies to circular waveguides, however, in this article, we will focus on the rectangular waveguide cut-off frequency. In a rectangular waveguide being used to transfer energy, the rectangular waveguide cut-off frequency determines when the waveguide becomes obsolete, which is whenever it reaches below the cut-off frequency.
In RF and microwave circuits, the cut-off frequency is a significant specification for waveguides. While there are many specifications to consider, there are a few key ones to focus on in each component being used in a circuit. When selecting an electrical component, designers should focus on the component’s specification or rating. The cut-off frequency is the frequency above which the waveguide offers minimum attenuation to the propagation of the signal.įrequencies below the cut-off frequency are attenuated by the waveguide.