Originally posted Jul 1, 2015
Admirable explanations and embarrassing memories
On a recent long-distance drive across western states, we encountered lightning and its usual transient interference across the AM radio band. That distinctive sound crackling from the receiver links two things in my mind: the first spark-gap wireless transmitters and my own unintentional transmissions from a spark gap driving a seven-foot Tesla coil.
Like many RF engineers, I’m fascinated by the history of radio, including the first steps on the path to where we are today. Unfortunately, I didn’t get much of this in school because our lectures only went as far back as the invention of negative feedback in the late 1920s. Practical spark-gap transmitters predated this by several decades.
That early history is enlightening, and I wanted to share an excellent—and underappreciated—explanation of it: a 1991 episode of the British Channel 4 series The Secret Life of Machines. It’s an understatement to call the series quirky and low-budget, but it’s also brilliant and entertaining. In here I do my best to create effective explanations of technical topics, but the hosts of this series have talent that I can only envy.
To see what I mean and get a glimpse of the earliest history of wireless, take a look at the series episode The Secret Life of the Radio Set. This YouTube link is one of several places where you can see the episode and others. You might want to look at the episode before reading the rest of this post. Go ahead. I’ll wait.
Welcome back. In the video, I was particularly struck by the sparks in both the transmitters and receivers. By the time I saw it, I was aware of the growing problems with spectral crowding and interference, and was working with the narrowband TDMA technologies that were being introduced to enable second-generation wireless phones. Videos of the spark-gap transmitters were an effective attention-getter in all kinds of talks about new and more spectrally efficient systems.
Early in my life as a practicing engineer my extracurricular activities included spark gaps and circuits that were the very opposite of spectrally efficient. In my defense, I didn’t come up with the design and, anyway, it was for a good cause. Here are a couple of pictures of the building process of that seven-foot Tesla coil.
Winding a mile of fine insulated wire on a Plexiglas tube to form the final stage of a Tesla coil. The completed winding is shown at right and, yes, that is a plumber’s toilet flange serving as a base anchor at the far end. Also yes, on the left that is your humble author as a younger, darker-haired practicing engineer.
The completed Tesla coil was inefficient by every measure. It was large and used high-voltage capacitors made from three-foot square panes of glass with heavy aluminum foil glued to each side. It was power hungry, driven by three neon-sign transformers that each produced 15 kV and 200 mA. I didn’t realize it at the time but it was a spectral monster, radiating power over a bandwidth that makes me cringe when I think about it now. It even made all 12 fluorescent tubes in the garage ceiling glow every time we switched it on.
Fortunately, we operated it for only a few seconds at a time, as part of a charity Halloween show. It was the centerpiece of our “Frankenstein laboratory,” sending bolts of lightning as the monster came to life and broke free to terrorize the crowd. Kids would run from the lab in a panic, only to get right back in line for the next show.
As with the radio industry of the last century, I quickly moved on to much more narrowband and civilized electromagnetic radiators. But every time I hear lightning crackle on the AM radio or the clattery, ringing buzz of a spark gap, I think of the true meaning of broadband and hope there is some sort of statute of limitations on my spectrum transgressions.