This topic is on the dead track?? It is one very interesting topic, considering there is one Hi number of HP 435, 436, 437 models for a low price is a good opportunity for small lab and student on the RF world to grab one of these simple instrument for 50/100 pounds, the big problem the second-hand market is on the hand of surplus scavengers they sell the hold and rusty powerhead at Hi price.
Way no make one at a low price and nice fun to handmade one with some good result making a one-hybrid system like es: detector AD8318/17 the chopper, and the amplifier using the 8484A diagram using the chopper FET circuit and the NPN Transistor amplifier.
If you need one power meter to make money BUY one 12.000 meters, but for hobby student and small lab the approach of Tobias whos the good one.
Francesco