Simple calculation of the equivalent circuit of loss-free obstacles

The paper deals specifically with the equivalent circuits corresponding to dielectric and metallic rods of circular cross-section placed in rectangular waveguides. If such an obstacle is placed in a resonant length of waveguide, the resonant frequency is changed. Both the shunt and series elements of the equivalent T network can be calculated if the change in resonant frequency is known for two suitably chosen resonant lengths. The changes in resonant frequency are calculated using simple perturbation techniques.