RE: How to measure rf impedance

FormerMember — Thu, 02 Oct 2014 11:37:13 GMT

From the top layer image, I assume that you are going to measure the impedance of/tune the PCB antenna.

Measure the impedance of the PCB antenna:

Remove R3.
Connect a network analyzer to the "R3 pad" that is closest to the antenna. Since R3 is removed, there is no connection between the antenna and the rest of the circuit, and that's what you want! When connecting the network analyzer to the "R3 pad", remember to ground the network analyzer cable in the ground plane, see attached image
You can now measure the antenna impedance.

If you consider tuning the antenna: From your schematic, C17 is the shunt tuning component. However, in order for C17 to work as a tuning component, it has to be placed "next to"/"in front of" the antenna, where R3 is placed. If you take a look at at our nRF51822 evaluation/development kit, you will see that the shunt tuning component is placed right "in front of" the antenna. C17 will not serve its purpose unless it's placed right "in front of" the antenna. It can also be useful to take a look at the "Antenna tuning" whitepaper.