Build a quality RF power sampler

A quality RF power sampler can be built quite easily. This device can be used to sample RF from your transmitter, power amplifier etc in order to accurately measure output power, intermodulation etc.

In general a spectrum analyzer or power meter can accept up to + 20dBm (100 mW) maximum of RF power, however to stay on the safe side, we will limit this to 0 dBm (1 mW).  So, if we want to measure power up to 1 kW, we have to attenuate by 60 dB !

I found a very good description by Don Jackson W5QN, download it as PDF here.

Don's sampler was designed for -50dB, however for -60dB I used following component values:

This provided  -60dB sampling +/_ 0.5dB from 4 MHz up to 50 MHz , but is still useable up to 145 MHz taking into account the actual attenuation at that frequency.

Important remarks :

RF sampler (the compensation capacitor not yet in place)



One picture says more than thousand words ... measured with FIELDFOX VNA and  lab-grade calibrated  termination 50 Ohm.

RF sampler output vs frequency (S21 measurement on VNA)

RF sampler SWR vs frequency - before compensation (up to 1.2 : 1 on 150 MHz)

RF sampler SWR vs frequency - after compensation ( better than 1.05 : 1 up to 150 MHz)

RF sampler output with 'OUT port'  open (see W5QN comments)
Upper trace is output with termination 50 Ohm

RF sampler output with 'OUT port'  shorted (see W5QN comments)
Lower trace is output with termination 50 Ohm