Fall 2004 Issue

MICROWAVE

Above and Beyond, 1296 MHz and Up

A Single-Thermistor Power-Meter Adapter

 By Chuck Houghton,* WB6IGP
 

The original HP-432-KO5 adapter.

My last column covered something I found while trying to resurrect an old military-surplus microwave power meter, the USM-174. The reason I put my efforts into repairing this old-world unit was because it came with a single, unbalanced thermistor power-meter head for 24 GHz, and I was trying to upgrade my test equipment for 24 GHz.

Power-Meter Head Operation

First let’s cover some common power-meter head operation.

Most power-meter heads in operation today use a temperature-compensated power-meter head with two sets of thermistors, one for RF and one for temperature balance, making for a very stable-reading-RF power-meter head. There are other power-meter heads which use diodes and other elements in the RF head. They are very good, even superior, but they are very expensive.
Finding the USM-174 and its compartment of various single-thermistor power-meter heads was quite a thrill. There are four RF heads—100 MHz to 8 GHz, 8 to 12 GHz, 12 to 18 GHz, and 18 to 25 GHz. The experimentation that followed as I was trying to repair the instrument gave me new respect for these power-meter heads. The RF heads are from the 1950s era of microwave test equipment, and they still seem to be available at swap meets and such. They require the older tube-type power meters to function like the Hewlett Packard 430 series of meters (vacuum-tube units). Most of the older power meters are just not available, but the heads keep turning up.

Knowing that simple resistance testing of the more popular HP-478 series dual thermistor (used with 431 and 432 power meters) was a simple resistance test of the two thermistors internal to the head, I tested the old, single power-meter head’s thermistor out of curiosity to see if the head was functional. It had a similar resistance of a few thousand ohms. The test was to first determine if it was alive.

 

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