Patch Antenna Design Frequency Shift
Like most design equations, Balanis' are stated under a given set of assumptions: for example, that there are minimal fringing fields due to the conductors being very close to each other. At 25mm, yours aren't very close.
There could also be simulation issues, for example, how did you feed the antenna -- is it identical to what Balanis assumes? Is the simulation appropriately meshed?
When working with antennas, it's usually a bad idea to assume that "design equations" will produce an exact result -- but they can be a good starting point.
We would expect roughly lambda/2 length and width, similar to your calculations. Getting nearly double resonance frequency seems to suggest a serious fault in the simulation setup, not with the calculation.
Thanks guys for the comments, they did help me a lot. From my experience with antennas, I never assume design equations to produce exact simulation results, but not the wide deviation I first saw. I fed the antenna by using a waveport at the bottom of the coaxial probe. I think the problem had to do with the positioning of the probe, I moved it from the exact center of the patch to nearer the edges and I am getting something closer to the intended resonant frequency.
I understand that you are not doing any simulations.
When is about microstrip and printed antennas is better to stay FAR AWAY from any equations, whatever books they came.
Just because there are so many things (or interactions of things) that can affect your design.
I agree. From my experience, patch antenna equations should give reasonable accuracy. Something might be wrong with your HFSS testcase.