Inside a 20-Watt Traveling Wave Tube Amplifier from Apollo #Space @kenshirriff

Ken Shirriff’s blog discusses how the Apollo astronauts communicated on their trip to the Moon, 240,000 miles back to Earth. They used a 32-pound amplifier, built around a special kind of vacuum tube called a traveling-wave tube. He looks inside this amplifier and explain how the traveling-wave tube works.

Surprisingly, this amplifier only produced 20 watts of power, not much more than a handheld walkie-talkie.1 You might wonder how a 20-watt signal could be received all the way from the Moon. To pick up the weak signal, NASA built a network of 26-meter (85-foot) dish antennas that spanned the globe, with ground stations in Spain, Australia, and California. For the signal to the spacecraft, the ground stations broadcast a strong, focused 10,000-watt signal that could be picked up by the spacecraft’s small antennas. Additional ground stations with smaller 9-meter (30 foot) antennas filled in coverage gaps, along with tracking ships and airplanes.

The amplifier was built by Collins Radio, a company that had a large role in the space program. (Collins claims that from Mercury and Gemini to Apollo, every American voice transmitted from space was via Collins Radio equipment.) The photo below shows the amplifier with the cover removed, showing the circuitry inside. Note the tangles of coaxial cables for the high-frequency RF signals. The “Danger High Voltage” warning is due to the thousands of volts required by the traveling-wave tubes.

The traveling-wave tube (TWT) is the heart of the amplifier. TWT systems have been popular for satellites because they are compact and provide high amplification with very wide bandwidth. They are still widely used in satellites, radar, and other systems.

See the detailed article for more on this historic piece of equipment.