How does the capacitance of a patch antenna affect its resonance frequency

Hi everyone,

On Broadband Microstrip Antennas, pp.254, an open stub with length l and width w is coupled to the radiating edge of a rectangular patch antenna. When length l grows larger(in a certain range), its resonance frequency decreases. When the phenomenon is explained by the increment of effective resonant length, I feel it pretty easy to understand. However, the book then delivers another explanation which blows my mind. It says the decrease in the resonance frequency can also be explained in terms of an increase in the capacitance of the open-ended stub(due to increase in l). The total capacitance of the equivalent patch increases and consequently the resonance frequency decreases.

Could someone please tell me why should resonance frequency shift with patch capacitance?

The reason why I feel so lost is that every time I simulate an antenna, I fix the resonance frequency first and then apply appropriate matching networks to eliminate the reactance part (the part caused by capacitance/inductance). But that do not seem to change the center frequency I set up before matching the antenna. In other words, when I apply short stubs to match the patch antenna impedance, I definitely introduce a new capacitance part to the effective patch. But I do not suppose the matching network affect center freq. So how can the capacitance of the patch actually affect resonance freq., according to the textbook?

Thank you so much for reading. I will be so grateful if you would help me out.

Jeremy

A patch antenna have various current distribution on its perimeter.

The coupled stubs are the most effective changing the resonance frequency if they are coupled where the current is high (red color on the image above).
Generally the point where the patch is fed has lower current, so the matching stubs will not be such effective changing the resonant frequency.

vfone,

Thanks for the answer. The clarification you made really brought me back onto the track. I used this current distribution property in my previous design and coupled slots to alter the resonant frequency, but somehow I still got confused yeseterday.

Thanks again,
Jeremy

A matching network does not alter the antenna characteristic. An antenna is expected to have a real input impedance at it's intended operation or "resonance" frequency". If you look at the impedance locus (or S11 smith chart) you see a considerable real impedance component in a range of e.g. +/- 20 or 30 percent around the operation frequency. By matching the complex impedance with a transforming network, you can shift the operation frequency without changing the antenna characteristic.

The method is opposite to tuning the antenna itself.

JeremyGarry said:

It says the decrease in the resonance frequency can also be explained in terms of an increase in the capacitance of the open-ended stub(due to increase in l). The total capacitance of the equivalent patch increases and consequently the resonance frequency decreases.

Click to expand...

This is at the open end of the resonating patch. The effect of capacitance is different at the low(er) impedance feed point.