coaxial line to waveguide transition
Almost all the waveguides are usually fed using coaxial line. The purpose of coaxial line to waveguide transition is to convert TEM mode of coaxial line into TE mode of the waveguide and vice versa. This can be done in two ways referred as E-plane coupling(probe coupling) and in-line coupling. This has been shown in the figure-1,2 and figure-3.
In E-plane transition referred as orthogonal transition, centre conductor of probe(coaxial line) is inserted into the waveguide. This excites the TE01 mode in the waveguide. The probe position is kept such that reflected EM fields will get combined in phase with the incident EM fields. The distance between backwall and probe centre is optimized to achieve good impedance matching. Typically it is kept at λc/4.
In H plane transition short circuit concept is employed which forms time varying magnetic field. These fields coupled with the waveguide. The short circuit is obtained using right angled metallic piece connected with the probe.
This type of coaxial line to waveguide transition can work at high powers but offer higher losses due to high VSWR.