S-parameters of coax-to-waveguide transition

[Brian Frank <FrankB@EE.QueensU.CA>]

Hi,

I've been trying to simulate a coax-to-waveguide transition that uses a
half-waveguide-diameter probe that sticks directly into a 26 mm radius
circular waveguide. I've been having a problem with frequency-domain plots
of the reflected wave, in that it "spikes" to rather large values that are
much greater than the magnitude of the incident wave (which is a nice
Gaussian shape in the frequency domain). This occurs whether I look at the
voltage, current, or power.

I've exciting the coax section using a current source (modulated pulse
with center frequency of 6 GHz and maximum frequency of 9 GHz), and
measuring the voltage and current further up the coax (closer to the
waveguide). It's intended for 6 GHz operation, and I've tried different
cell sizes (0.25 and 0.50 mm) with different time steps (4e-13 and 8e-13
s). I've tried a "reference" simulation (using only the coax terminated in
absorbing boundary conditions) as the incident wave, similar to the
approach used in the "Analysis of a Coaxially-Fed Patch Antenna" example
on the LC web site, but it gives the same results.

Any ideas what I'm doing wrong?

Thanks,

Brian

-- 
Brian Frank
ECE Department
Queen's University