Spring 2006 Issue

MICROWAVE

Test Equipment for Power Measurements

 By Chuck Houghton, WB6IGP

 

 

 

Pads, including high-wattage versions

(the large black pads). At the far right is

a box of 1-dB step value SMA connectors.

Testing power at microwave frequencies can be somewhat perplexing if you donít have some basic tools. Some of the tools we will describe here might seem somewhat strange to those who are on the microwave frequencies. An example is the Bird Corporation Thruline wattmeters, and particularly the Bird 43 series of wattmeters.

Most amateurs who use Bird wattmeters assume they are good in frequency and power to about 450 MHz. Well, thatís not true. They are capable of higher frequency operation and top out at 2304 MHz. I happen to have a selection of slugs that are rated at 5 watts for the Bird 43 meter. The slug for 400 to 1000 MHz performs fairly well at 1296 MHz and can be used at 1296 MHz with good results. You can obtain slugs that cover 1296 and 2304 MHz with better results.

A better method, however, can be brought into play. That is an AC-powered meter. The answer here is to use a low-cost microwave power meter from Hewlett Packard, such as the HP-432 power meter. To measure power above the +10 dB maximum all you need is an external attenuator to limit expected power to no more than +10 dBm. The HP 432A microwave power meter requires a connection cable between the thermistor power detection head and the meter itself. The power head is capable of a maximum power input to the meter of +10 dBm. If you exceed this limit, expect it to blow the thermistor internal to the power-meter head. Once gone or over-stressed with excess power, repair is possible if the thermistor is way out of balance. However, if it is open on one thermistor, itís gone forever and itís toast.

I like the 432 power meter because itís inexpensive, and the 478A power head does have a few chances of repair if it is over-stressed. Open heads still have a good temperature thermistor and can be used in combination with a single older thermistor head to achieve an operational condition. Itís tricky but it can be done.

I over-stressed a good 478A power-meter head measuring power with what I thought was a 30-dB pad that should have protected the thermistor. When I applied RF in the range of 5 watts, it pegged the power meter. I shut down the RF source of power as soon as possible and checked things out. I found out that the Bird 30-dB pad I was using was actually model xxx3C. I got bitten by the 3C part number; it was not ď30Ē as I had thought.You have to check things out to be sure it is a 3-dB pad, not a 30-dB attenuator!

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