When designing slotted waveguide antenna, the most popular technique to feed the antenna is the use of a coax-waveguide transition.
A transition structure is required due to:
- Difference of guided modes: In the coaxial cable, the mode is TEM where both magnetic and electric fields are transverse (orthogonal to the propagation axe). Meanwhile in the waveguide, the mode is TE10.
- Difference of characteristic impedances: impedance in the coaxial cable is 50 Ohm and that of waveguide can be several times higher (depending on the structure), in the range of 300 – 400 Ohms. What does it mean “characteristic impedance”? Well, long story short, the geometry and the material of a transmission line defines a factor between the electric field strength and the magnetic field strength. That factor is “characteristic impedance”. It tells how the structure favor electric field compared to magnetic field.