Excitation of modes in Circular waveguides
As we know, TE modes have no z component of an electric field, and TM modes have no z component of magnetic intensity. If a device is inserted in a circular guide in such a way that it excites only a z component of electric intensity, the wave propagating through the guide will be the TM mode; on the other hand, if a device is placed in a circular guide in such a manner that only the z component of magnetic intensity exists, the traveling wave will be the TE mode. The methods of excitation for various modes in circular waveguides are shown in Figure :
Figure : Methods of excitation of various modes in a circular aveguide
A common way to excite TM modes in a circular guide is by a coaxial line as shown in Figure. At the end of the coaxial line a large magnetic intensity exists in the φ direction of wave propagation. The magnetic field from the coaxial line will excite the TM modes in the guide. However, when the guide is connected to the source by a coaxial line, a discontinuity problem at the junction will increase the standing-wave ratio on the line and eventually decrease the power transmission. It is often necessary to place a turning device around the junction in order to suppress the reflection.
Extra care to be taken while excitation of circular waveguides , so that the desired mode can be exciated in the waveguide accuarately.