Overview
Ernst Frederik Werner Alexanderson (1878–1975) built the high-frequency alternator that made continuous-wave radio practical at power, at Fessenden's prompting. It was a rotating machine generating radio frequencies directly, and it carried transatlantic communication before valve transmitters matured. He held around 340 patents and later worked on early television at GE.
What You Need
- Lived: 1878–1975, Sweden, worked in the United States
- Anchor year: 1906 — the high-frequency alternator
- Strand: canon
Steps
The problem as it stood
Fessenden had established that voice needs a continuous carrier. Nobody could generate one at useful power. Spark transmitters produced damped bursts; arc converters were unstable and noisy.
What he actually did
Alexanderson built an alternator — in principle an ordinary AC generator — engineered to spin fast enough, with enough poles, to output tens of kilohertz directly at high power. Machines of 200 kW were eventually built and installed at stations including Grimeton in Sweden, which still exists and is operated occasionally.
How it worked
Output frequency is rotor speed multiplied by pole count. Reaching radio frequencies means extreme rotational speed and very fine pole spacing, with the rotor under enormous mechanical stress. It is a brute-force mechanical answer to what became an electronic problem, and it worked reliably for decades.
What it made possible
Reliable long-wave transatlantic radio communication and the first serious voice transmissions. It also demonstrated the commercial value of continuous-wave working, which valve transmitters then delivered more cheaply.
What happened to him
He remained at General Electric for a very long career, contributing to power transmission, railway electrification and early television — he was involved in GE's television demonstrations of the late 1920s. He died in 1975 at 97.
Where the credit landed
Alexanderson is reasonably credited in radio engineering history and almost unknown outside it, partly because his machine was superseded within twenty years by valve transmitters. Being the last and best implementation of an approach that is about to be replaced is a reliable route to obscurity — the same shape as Baird's mechanical television.
Pro Tips
- Output frequency is rotor speed times pole count — hence the extreme engineering.
- The Grimeton alternator survives and is still occasionally run.
- Being the best version of a soon-obsolete approach is a route to obscurity.
Knowledge Base
What You'll Learn
Sometimes the mechanical answer arrives first and works for twenty years.
Why valves killed the alternator
An Alexanderson alternator is a building-sized machine with a rotor under severe stress, tied to one frequency band, requiring constant maintenance. A valve oscillator is a component: cheap, small, tunable across wide ranges, with no moving parts. Once de Forest's grid made valve oscillators practical at power, the mechanical approach had no future — but for two decades it was the only thing that worked.
Where This Fits
This guide covers one specific part of the history of media technology. The wider picture — how each link in the chain from capture through transmission to display was actually built, who built it, and why the credit so often landed somewhere else — is in A History of Broadcast Technology: The Chain From Capture to Screen, which frames the discipline as a whole and links out to the detailed guides underneath it, including this one. If you are starting from scratch rather than solving a specific problem, read that first and come back here.
FAQ
Q: What is an Alexanderson alternator?
A: A high-speed rotating machine generating radio-frequency alternating current directly, used for long-wave transmission before valve transmitters became practical. Machines of up to 200 kW were built.
Q: Are any still working?
A: The alternator at Grimeton in Sweden survives as a UNESCO World Heritage Site and is operated on special occasions.