I am simulating a 1/4 wavelength monopole antenna using COMSOL Multiphysics, a simulation software based on the finite element method. I am trying to simulate the S11 plots with the antenna submerged in water. However, I keep getting an inverted S11 plot. I am not sure how to interpret this. I am simulating over a range of lengths, and certain lengths within a small range seem to give the resonant dip. And this dip is at exactly the same frequency over the small length range. I uploaded the plots for anyone who is interested to check out, and if you can help me interpret this that would be great.
Thanks.

 

Looking at the three plots I'm not sure what you mean by inverted since a reflection coefficient is a complex number that can be positive, negative or 0, but the magnitude of a complex number is always positive. Anything expressed in dB means you are comparing two things by taking a ratio. In the case of S11 it is the ratio of the reflected wave to the incident wave. On the first plot for 72mm, you have 1.0 db at the center frequency and 0 db away from the center frequency. As you know 0 dB implies that the reflected wave has the same magnitude and phase as the incident wave away from the center frequency. 1 dB at the center frequency means the reflected wave is slightly larger than the incident wave. I'm not sure what is going on with the 55mm case, except that dipping below 0 dB indicates a phase inversion. In the 65mm case I see what looks like a normal reflection coefficient plot. You are down almost 2 dB at the center(resonant) frequency and you are unaffected away from the center(resonant) frequency. As far as antennas go they don't look very good or useful. I'd want to see something like the 65mm plot but with the notch at -30 dB or so. Can you transfer those plots to a Smith chart?