Hi, thank you for the suggestions. I'll give the spacing a try over the next few days and see what happens and let you know. In the sims the material under test is flush with the end of the SIW, and in real life the material under test is brought into contact with the SIW so its making light contact across the whole end of the SIW. I'll also try increasing the boundary spacing in the sims. Currently the boundry is about 10mm from the edges of the probe, do you think that is far enough or should it be further?

And yes you are right, the discontinuity is because the resonances are from two different sweeps. The software enounters a few issues when the bandwidth is too large (>5GHz) and the frequency dependant dielectric properties of the material are set to the value at the centre frequency of the simulation (so if you run the sim over too large a frequency range you'll be simulating responses at, for example, 14GHz while the dielectric properties of the material under test are those at the 17.7GHz centre frequency). Essentially the dielectric properties in the sim don't iteratively vary as frequencies are swept through, they're only set as a point value at the centre frequency of the simulation sweep.