Ensuring certain amplitude level of RF signal on a PCB board
In this situation, I'd build an approximately -20dB probe, and measure the voltage at the input directly.

The wavelength of 2.1GHz is 142mm (air), or \$\lambda/10\$ (plastic) is around 10mm. So it's straightforward to make 'short' connections with care, and small components.







simulate this circuit &ndash; Schematic created using CircuitLab

The 470 ohm resistor should be a good RF type. Its value is a compromise between giving an adequate signal, not loading the circuit too much, and having a good frequency response. The loading is reasonable compared to the 2x34ohms, and you can compute a small loading correction.

Although the coax appears to be mismatched, this is a flat probe. The 50 ohm input impedance of your power meter is transferred to the end of the coax, where it still looks like 50 ohm resistive. Then you get a (roughly) 10:1 pot-down of the probe resistor into that load.

You can calibrate the probe easily by dabbing the sharp end on the connector of  calibrated source, or across a well-driven load resistor.

The small size and the low cost of 470 ohm resistors means you can solder one to the measurement node, leave it there, and forget about it. When you come to attach and detach the probe coax (usually use RG178 or smaller), it's soldered to the resistor, protecting the terminals from wear.