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

[Jørn]

Hello SaiTeja

The capacitor with 1.2pF is for matching the specific PCB trace antenna that is used for the DK to 50 Ohm. When designing a PCB trace antenna you will typically design it longer than it should be, this way it will be more inductive than what you want. You then cut the tip of the antenna, shortening its length to reduce the inductance. The inductance of the antenna along with the capacitor will make the antenna resonate at the desired frequency.

The capacitor value may have to be changed depending on antenna design, ground plane geometry, product casing, PCB substrate etc. Antennas are very sensitive to their surroundings and the board layout. The safest way to properly match the antenna is to use a Vector Network Analyzer (VNA) and measure S11.

Best regards

Jørn Frøysa