# How to find the Q factor for a resonant antenna?

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#### MrElec

Hi,

I want to reformulate my previous post by simple question: how to find the Q factor for a resonant antenna or a resonant Transmission Line using S parameters from ADS or HFSS simulation.
I know that Q is equal to fcenter/bandwith. How to calculate the bandwith, can i find this bandwidth from -10dB or VSWR(S11),VSWR(S21).

I find this in edaboard forum but unfortunatly is not clear specially for the loaded Q

From S11 measure - you matching Fc to best possible return loss via adjusting coupling grade or adjusting impedance from measure tools (port) to your antenna or resonator (easy to adjusting impedance on port in simulator and S-parameter block) - in smithchart must Fc passing to 50 Ohm centrum point. After this, you find bandwith as 7 dB point on your return loss curve and use above Q=fc/bw

calculated Q value is a unloaded Q-value of antenna/resonans circurit.

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I find this describe on paper to how measure Q-value in coaxial resonators (I think measure antenna is a simular problem) and after some testing in simulator find out this working and measure Q-value correctly on my test lossy resonators. Is clever thinking from people there....

measure as S21 and S12 give loaded Q-value measure depend of measure impedance on port (mostly 50 Ohm) also act as load, but measure S11 with matching for best possible return loss in Fc, and DUT:s other ports is shorted/gounded/left open - you measure unloaded Q!.

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But remember, measured impedance and calculated Q-value on antenna port can have low coupling to emitted radiation and radiated bandwith on antenna, all depend of internal loss in antenna structure and place of this loss in resonant structures.

- RF-terminator have fine return loss value and very wide bandwith - but radiate very small amount of electromagnetic wave... - ie. you cannot see antenna radiating effectivity and quality only via measuere S11 from antenna port...