# Patch Antenna Design Frequency Shift

1. ## Patch Antenna Design Frequency Shift

I am currently designing a patch antenna to operate at a resonant frequency of 312.5 MHz with coaxial probe center feeding. Using a 25mm air substrate, and using the design equations from Balanis' book, I obtain the length and width of the patch as 444.8mm and 480mm respectively. However, after designing and simulating the antenna in HFSS, the frequency shifts to around 560 MHz. It's only after re-designing the antenna for a frequency of 200 MHz that I get it operating at close to the 312.5 MHz mark. Can anyone kindly help me get to the bottom of this mystery, for I'm really baffled?

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2. ## Re: 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.

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3. ## Re: Patch Antenna Design Frequency Shift

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.

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4. ## Re: Patch Antenna Design Frequency Shift

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.

5. ## Re: Patch Antenna Design Frequency Shift

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.

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6. ## Re: Patch Antenna Design Frequency Shift

Originally Posted by FvM
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.
I agree. From my experience, patch antenna equations should give reasonable accuracy. Something might be wrong with your HFSS testcase.

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