why only shunt Component is required for PCB Antenna Tuning

[SaiTeja]

Below linked blogpost says, if using a PCB antenna, the matching network should consist of one shunt component. devzone.nordicsemi.com/.../ . And in the reference design 1.2 pF is used. So I am assuming shunt capacitor is need to be added for tuning PCB antenna.

If my assumption is correct, the real value of admittance of PCB antenna should always be 0.02 mho to match impedence to 50 ohm. How can the admittance of any PCB antenna can always be 0.02 ohm? Where am I thinking wrong?Why only one shunt component is sufficient for tuning PCB antenna? For antenna tuning procedure, I referred below linked white paper. infocenter.nordicsemi.com/.../nwp_017.pdf

[AmbystomaLabs]

Most of the DK's use a monopole which has a theoretical input impedance of 36.5 +j21.25ohms. If you plot this on a smith chart you will see how you can use a shunt C right at the feed point of the antenna to pull it in pretty close to 50ohms real. Or another way to look at it is: Assuming 1.2pF @ 2.45GHz is (0 -j54.13) Zin = ((36.5+j21.25)x(0-j54.13))/((36.5+j21.25)+(0-j54.13)) = 44.13 -j14.2 ohms

Of course the farther that shunt is away from the feedpoint the impedance will transform and you will shunt to higher and higher impedances. This is why we always speak at length about the importance of the distances between RF components.