file: ridge.wp6 11-16-00 jbh

Intuitive explanation for operation of ridged waveguide - why the cutoff frequency can be arbitrarily low.

The top of the ridge and the the part of the top wall immediately above it form a parallel strip transmission line. The capacitance per unit length of this line is 0w/d. The rest of the walls are shorted stubs, in parallel with the capacitance per unit length. These stubs, at low frequencies, are inductors that shunt the capacitance per unit length. If the capacitance is high enough, the net susceptance will remain positive; the capacitance per unit length is reduced, but it is still capacitive, and energy can propagate down the guide.

The high loss of ridged waveguide at low frequencies is due to the high currents circulating in the shunt inductance paths.