Waveguide to microstrip transition
At higher frequencies coaxial to microstrip transition offers higher losses and hence not used. At such high frequencies waveguide offers less losses and hence are preferred to couple with microstrip circuits. The waveguide to microstrip transition operates using dominant mode bandwidth. TE mode is supported in the waveguide.
Impedance of each mode in both waveguide and microstrip changes with frequency and hence it is impossible to obtain full dominant mode because of dispersion in Waveguide to microstrip transition.
There are two types of waveguide to microstrip transition viz. orthogonal transition and end launch transition.
In orthogonal transition, microstrip is placed inside the waveguide through a narrow opening called slot. Slot is part of waveguide with which only microstrip mode will be excited in the frequency band of operation and all the other waveguide modes will be cut off. Here ground plane of the microstrip will be attached with the waveguide walls. This makes it possible for microstrip probe to radiate as an antenna.
End launch type of transition is used in parallel plate waveguide to microstrip transition as shown in the figure. Here both the waveguide and microstrip support TEM mode and hence broader bandwidth can be obtained. Here parallel plate waveguide is balanced type and microstrip line is an unbalanced type. Balun is needed to match both of these transmission lines. As shown in the figure, ground plane of microstrip is connected to one plate and strip line is connected to the other plate of the waveguide.
Balun used in this Waveguide to microstrip transition is a flared structure which helps ground and signal currents to get diverged from each other.