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

[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.

[AmbystomaLabs]

Other antenna designs, such as a PIFA, have matching as part of their design. So on a PIFA the F bit shorts to ground to match the antenna. In this manner other antenna designs can give close to 50 ohm performance without external discrete matching components.

Always do your antenna characterization in the final product housing/mounting arrangement. Plastic, screws, hands, walls will all severely change the tuning of the antenna.