Measure the impedance:

[JK5]

PCB : thickness 1 t(1mn), 1/2oz, 2 layer

The antenna was removed and the VNA was connected between L3 and C6.

The impedance was about 100 ohms.

The part value is shown in the picture.

Question :

1. Should I connect the power and do TX mode or RX mode? (The result was the same whether or not the power was connected. about 100 ohms.)

2. L1 = balun, L2,C3 = ?, C5 = DC Blocking Capcitor, C4,L3,C6 = PI Network. What is l2 and c3?

3. It is currently about 100 ohms. Should the PI Network (C4, L3, C6) be adjusted to 50 ohms?

[ketiljo]

You shouldn't evaluate the matching network by the impedance. Instead, measure the output power and the harmonics. Take a look at this whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_013.pdf. The radio in the nRF51-series is the same as in the nRF24L-series. 

[JK5]

hello.

nwp_013 thank you;

What is the x in nrf51822?

[ketiljo]

X = 2 for all nRF24Lxx and nRF51-series radios. Note that this does not apply for nRF52 or later radios as they are completely different.