Kerry Wong did a teardown of an HP 436A power meter:
HP 436A is a power meter capable of measuring RF power between -70 dBm and 44 dBm (roughly between 0.1 nW to 25 W) with a frequency range from 100 kHz all the way upwards to 110 GHz depending on the sensor module selected. This power meter was made in the early 70′s and has long been discontinued.
When I saw one at a local auction selling for almost nothing, I could not resist to pick it up. In fact, if you take a look on eBay you will see that HP 436A’s are abundant and most of them can be bought rather inexpensively. But only get one if you really need a RF power meter or you already have a power sensor and cable to go with it as the cable and sensor can easily set you back ten times more than a base unit itself.