Minimum-Loss Matching Pad

In my last post I promised to write about the minimum-loss matching pad that I’m using to couple my signal generator to the device I’m testing. The source impedance of the generator is 600 ohms and the output is intended to be terminated in a 600 ohm load, but the device I’m testing is only 228 ohms. The way to match this with the lowest loss is with a transformer, but it is inconvenient and unnecessary to come up with a transformer for every mismatch this piece of test-equipment will face.

Thanks to advice from the ham who is guiding me in this project, I’m using a minimum-loss matching pad, also known as an “L-pad,” to match these two impedances. (I’d tell you who this fine fellow is, but to keep you in suspense about my project I’ll wait until my final write-up. If I name him now, the cat will be out of the bag!) This quick, cheap, and easy match requires only two resistors:

Courtesy of http://www.microwaves101.com/encyclopedia/attenuatorL-pad.cfm#minloss

To calculate the value of the resistors and to calculate the loss of the matching pad, use these formulas (A spreadsheet that uses these formulas is available through this webpage.):

In my case R1=472, R2=290, and the loss is -9.25 dB. That loss is pretty significant, but it is acceptable for this application. Remember this is a minimum-loss matching pad, not a no-loss matching pad. Using what resistors I had on hand to come as close as I could to the required values, I soldered this pad on a generic PC board from Radio Shack that I cut in half using my Dremel tool with a cutting wheel:


For more on this topic, I commend to you this webpage on “Impedance and Impedance Matching.”

Todd Mitchell, NØIP, is a regular contributor to AmateurRadio.com and writes from Minnesota, USA. He can be contacted at [email protected].

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