It is possible to propagate several modes of electromagnetic waves within a waveguide. The physical dimensions of a waveguide determine the cutoff frequency for each mode. If the frequency of the impressed signal is above the cutoff frequency for a given mode, the electromagnetic energy can be transmitted through the guide for that particular mode with minimal attenuation. Otherwise the electromagnetic energy with a frequency below cutoff for that particular mode will be attenuated to a negligible value in a relatively short distance.
The dominant mode in a particular waveguide is the mode having the lowest cutoff frequency. For rectangular waveguide this is the TE10 mode. The TE (transverse electric) signifies that all electric fields are transverse to the direction of propagation and that no longitudinal electric field is present. There is a longitudinal component of magnetic field and for this reason the TEmn waves are also called Hmnwaves. The TE designation is usually preferred.
Figure 1 shows a graphical depiction of the E field variation in a waveguide for the TE10 TE20 , and TE30 modes. As can be seen, the first index indicates the number of half wave loops across the width of the guide and the second index, the number of loops across the height of the guide - which in this case is zero.
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It is advisable to choose the dimensions of a guide in such a way that, for a given input signal, only the energy of the dominant mode can be transmitted through the guide. For example, if for a particular frequency, the width of a rectangular guide is too large, then the TE20 mode can propagate causing a myriad of problems. For rectangular guides of low aspect ratio the TE20 mode is the next higher order mode and is harmonically related to the cutoff frequency of the TE10 mode. It is this relationship together with attenuation and propagation considerations that determine the normal operating range of rectangular waveguide.
Another type of waveguide commonly used in EW systems is the “double ridge” rectangular waveguide. The ridges in this waveguide increase the bandwidth of the guide at the expense of higher attenuation and lower power-handling capability.
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