Traveling-Wave Tubes

The world of RF amplifiers is fascinating because of the wide range of methods in use. Today, many are constructed from solid-state components, but there are still situations where vacuum tubes are the only suitable choice. Let’s look at the traveling-wave tube amplifier, another unsung electronic hero.

One of the most fascinating things about traveling-wave tube amplifiers is how they work. They feature a heater, cathode, and acceleration electrodes to form an electron gun, much like a cathode-ray tube, that beams a stream of electrons to the collector (Figure 1).

Design of traveling-wave tubes — Figure 1: Basic design of a traveling-wave tube.

This stream is focused by an external magnetic field that is usually made of permanent magnets. Using velocity modulation, which mixes the electron stream with the incoming RF electrons, the tube amplifies the applied RF input signal.Since the streaming electrons travel much more slowly than the RF electrons, the RF signal is fed through a spiral wound wire, called the helix. This slows the RF signal down to match that of the electron stream.As the RF electrons proceed down the helix, they modulate the velocity of the electrons in the stream because the in-phase electrons speed up, and the out-of-phase electrons slow down. These modulated electrons then bunch together, inducing an amplified signal back into the helix that is then picked off the end of the helix using a directional coupler.

When compared to klystrons, traveling-wave tubes have the advantage of wider bandwidths. Additionally, they don’t require resonant components, making them ideal for lower power microwave applications like radar or even spacecraft and satellites. A great example is the Collins Radio S-Band amplifier (Figure 2 and Figure 3) used in the Apollo space program.

Figure 2: Collins Radio S-Band traveling-wave tube amplifier used for communication with earth during the Apollo mission. (Source: Ken Shirriff)

Tightly-packed tangle of coaxial cables. — Figure 3: Operating at several thousand volts, the Collins Radio amplifier was a tightly-packed tangle of coaxial cables. (Source: Ken Shirriff)

It was a compact, 32-pound, 20-W amplifier that transmitted all of the voice, data, and television back to NASA’s network of 26-m, earth-based dish antennas. By comparison, the ground station used a focused 10,000-W signal to communicate back to the craft.

Even though traveling-wave tubes are mostly the domain of commercial applications, a small group of enthusiasts still like experimenting with these wonderful little amplifiers in amateur microwave transmitters. Their biggest challenge, however, is finding the tubes!