Design vs. manufacturing: A failed directional coupler

Article By : John Dunn

How would you react if a sample of your design came back with a major change?

Confession time: I have always been somewhat intimidated by and in awe of microwave electronics. Experts in that field have always struck me as being spookily skilled in the conjuring arts.

In the summer of 1966, I was employed at Sanders Associates in Nashua, NH. One day, I got put on loan from my regular department to work in the Simon Street facility where all kinds of microwave projects were underway. Sanders had a proprietary form of what I saw as stripline, but it was politically incorrect to use that word there. The only acceptable noun was “tri-plate.”

There were efforts going on to develop directional couplers for X-band or 9-GHz service. High-attenuation couplers, loosely-coupled couplers if you will, were already well-established products. Essentially, they looked like this.

loosely coupled directional coupler designFigure 1 This was the structure of a loosely-coupled directional coupler.

The goal was to make much tighter directional couplers. Instead of the coupled signal being 20 dB down from the main line signal, the goal was 6 dB. The structure was to look more like this.

tightly coupled directional coupler designFigure 2 This was the design for a tightly-coupled directional coupler.

This one engineer set about devising a design for a 6 dB coupling. He did all kinds of mathematics; he specified what kind of dielectric board would be used, he specified the copper foil’s dimensions and thickness, he specified the aluminum block in which this structure would be contained, and so on. He really thought he had it aced.

He sent his papers over to manufacturing, who shortly afterward delivered a first piece sample to him for evaluation. The measured coupling was pretty close to the intended 6 dB and the bandwidth of the coupler was better than he had anticipated. That was one really happy engineer, but not for long.

After running all kinds of exhaustive tests and being very pleased with everything he saw, he opened the unit up to inspect the physical innards. He then discovered that the dielectric board was only half as thick as he had called for, and upon making that discovery, this guy went berserk. Shouting expletives at the top of his lungs, he slammed the coupler prototype down to the floor where it bounced around and came to rest under someone’s desk.

This was in August of 1966, approximately a month before the premiere airing of the first episode of Star Trek with the sci-fi popularization of the word “warp,” but this guy had thrown that coupler to the floor at warp speed.

I don’t know if that coupler project was ever a success or not because I was shortly afterward returned to my original department. Frankly, I was quite happy to get back home to Analog Land.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

Related articles:

Leave a comment