Conventional Rectangular Microstrip Patch Antenna

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Hi,

I designed a conventional rectangular microstrip patch antenna to operate at 3.1 Ghz with microstrip feed and quarter wave transformer.It resonates at 3.1 Ghz as it should but it also resonates around 4.55 Ghz without any modification ( slot,notch etc.).

So my question is that where this second resonant frequency is originated from? (I used hfss 13.1) . Is it possible something like this or im doing something wrong? Any ideas?
 
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Hi,
I'm still learning but I think its actually possible. It means your patch is now able to function in 2 different frequencies.
I did came across this situation where even though my calculations focuses on achieving a certain desired frequency, the design that I create for that particular frequency could also create another resonance which is not in my desired range. If you want to block completely other frequency range then I guess you need to design like a bandpass characteristic, probably with certain types of slots.
 

Hi again,

I found out that second resonant frequency's return loss changes with the length of quarterwave transformer, but still dont know why?
 

Your patch itself very likely doens't show a real impedance.

Your resonance is due to a combination of the wierd impedance of the patch at 4.55 GHz and the impedance transformation due to the quarter wave transformer. Of course at 4.55 GHz your quarter wave transformer no longer functions as a quarter wave transformer. It just rotates the reflection coefficient as seen by the quarter wave line at 3.1 GHz, over about 264 degrees when at 4.55 GHz.

If you want to do some reverse calculation, don't forget that your quarter wave line impedance isn't 50 Ohms, so use the right value for Zo when calculating impedances or reflection coefficients.
 

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