Old Peter’s company has a legend. It has been passed down through generations of programmers and staff through an oral tradition. Oh, from time to time, someone would be inspired to record the tale for posterity, but inevitably, the hard copy was recycled, the digital copy was lost.

It was 1982, and the German tech industry was booming. Old Peter’s company manufactured a line of 8-bit computers that were targeted towards businesses. Their targets were generally larger companies and government organizations- like Frequenzhof Busgesellschaft.

2013 in Bonn. BYD ebus (electrical bus). Bus facing left

Frequenzhof Busgesellschaft - the bus company of Frequenzhof - served a bustling metropolis in the heart of Germany. They had a growing ridership and a growing need to automate their accounting processes. They bought one cabinet-sized 8-bit microcomputer and if they liked it, planned to buy another. With the addition of tape drives and other accessories, the Busgesellschaft was going to be a very valuable client.

So when the director of the bus company called support, people jumped to solve their problem. Unfortunately, it was easier said than done:

“When we use your computer to run our weekly batch process, all of our radios stop working. We cannot communicate with our drivers! This is unacceptable!”

The technician tried to clarify the problem. “I’m sorry, but… if you run an accounting job, the radios stop working? Our computer doesn’t have anything to do with your radios!”

“And yet, when we turn on your computer, the radios stop working! We think it must be interference.”

“That… that really can’t be.” A computer, of course, does throw off some electromagnetic fields- anything using electrical current did. But to kill a voice radio network? That seemed implausible.

The technician gathered more details, and then escalated. Management didn’t want to lose future sales, and got defensive about their system. It relatively well shielded, and the frequencies it generated- all harmonics of the 1MHz chip running in the system, or of the 50Hz mains power- were nowhere near common voice frequencies.

Over the next few months, a series of radio and electronics technicians examined the situation. They tested a computer right as it came off the assembly line, proving that it didn’t radiate any significant EM noise, especially not at the bands the radios used (≈ 26MHz - 27MHz). The company, eager to keep their customer happy, replaced the “defective” computer with a fresh one, confident that this would solve the problem.

It didn’t.

Frequenzhof Busgesellschaft grew increasingly concerned. This computer was an expensive capital asset, and they couldn’t use it without cutting communications with their drivers- what if there were an accident or an emergency? Fingers were pointed, blame was doled out, and the Busgesellschaft threatened to take their business elsewhere.

The computer company begged for one last opportunity to send in a technician, because obviously there was something extremely unusual going on. “This shouldn’t be happening,” they agreed, “but work with us to fix it.” So they sent Fritz out to the customer site.

Fritz was an expert in radio systems. Rumor had it that, before he entered the private sector, he had been working in signals intelligence, spying on the Russians. Whether or not there was any truth to the rumor, he was considered one of the best in his field. If he couldn’t solve the problem, no one could solve the problem.

When Fritz’s car pulled up to the bus company’s building, he had a suspicion as to what might be wrong. When he entered the computer room, and saw the computer was positioned against an exterior wall, he knew what was wrong. This was, after all, 1982. In Germany. Frequenzhof Busgesellschaft was housed in a late 1960s slab of brutalist concrete. Hidden inside of that concrete was structural rebar. The computer’s tiny EMF resonated with the rebar grid, creating a chain of harmonics that laid static over the radio system, killing communication.

Fritz’s solution was as elegant as his diagnosis: relocate the computer to the middle of the room, far enough away from the rebar that it couldn’t couple with it. They followed his instructions, and it worked perfectly- even when they did get around to adding that second computer.

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