High-power amplification at RF and microwave frequencies still involves vacuum tubes in many military systems. Such devices as traveling wave tubes (TWTs) in TWT amplifiers (TWTAs) and crossfield amplifiers (CFAs) are capable of hundreds of watts of continuous-wave (CW) power and kilowatts of pulsed (peak) power in ground-based and airborne systems, and they have served as reliable RF/microwave amplifiers even in space-based applications. But in recent years, claims of “vacuum-tube replacements” from solid-state device manufacturers have been often bold and loud, touting newer device technologies such as silicon carbide (SiC) and gallium nitride (GaN) as the solution for producing the high power levels needed at high frequencies in electronic warfare (EW), electronic countermeasures (ECM), radar, and other military and aerospace systems. What is the truth about real RF/microwave power? Can transistors deliver the power levels at the same frequencies as their vacuum-tube counterparts? The answers can be found by comparing the technologies and the power levels available from each, along with related issues, such as power consumption, efficiency, and linearity.

Long before power transistors were being considered for military and space transmitter applications, TWTs were boosting signals in satellite communications and other systems in which reliability was of the utmost importance (see sidebar). The reliability of these devices has been impressive over the years, but military system designers have long sought amplification solutions that are smaller in size and lighter in weight, especially in space-based and airborne systems. Because of this search for more compact solutions, military research dollars over the last several decades have helped with the development of semiconductor materials that support higher-frequency, higher-power transistors, such as gallium arsenide (GaAs), gallium nitride (GaN), and silicon carbide (SiC). And, while TWT and TWTA suppliers may have grown weary of hearing about the phenomenal potential performance levels of these newer transistors, they have also benefitted from the military need for more compact forms of amplification, in their development of small but powerful microwave power modules (MPMs) based on miniature TWTs.

So what is the truth? Can transistors match the power levels of vacuum-tube devices such as TWTs and CFAs? As a wise man once said, “that depends.” It depends on many factors, including frequency range, instantaneous bandwidth, and how many devices are needed to reach a given power level. And this last factor is one of the chief differences in how transistors and tubes are used within a system because, at some frequencies, a solid-state amplifier can be designed and built with the same output power as a TWTA, although it will generally require multiple transistors to match the output power of a single TWT. While it is possible to sum the contributions of many power transistors to achieve relatively high power levels, it also requires sacrificing some of that power to the insertion loss of power combiners in multiple-transistor amplifiers.

TWTs are elegant in their simplicity and reliable because of the small number of parts. TWTs can be designed with different types of components, but common to all types are some form of electron gun, a slow-wave structure, such as a helix, high-power magnets to focus the emitted electron beam, a collector, and some form of input and output couplers to inject and collect an RF or microwave signal. In essence, the injected RF/microwave signal interacts with the electron beam in the slow-wave structure, with a resulting transfer of energy from the electron beam to the electromagnetic RF/microwave signal. The amount of energy transferred is characterized by several TWT parameters, such as output power, gain, and efficiency. As its name implies, the collector is the end point for the electron beam and is designed to effectively dissipate its remaining energy.

Two of the more popular TWTs in use in military systems are those with a helix slow-wave structure and coupled-cavity TWTs, which use a slow-wave structure formed of a series of cavities coupled by slots. Over the years, improvements in the cathodes used as electron guns have increased the reliability of TWTs and TWTAs, while also supporting higher current densities. Also, smaller, higher-powered magnetic circuits, such as periodic permanent magnetic (PPM) focusing structures, have resulted in smaller tubes without sacrifices in output power. These smaller tubes and tube amplifiers are particularly attractive for weight-sensitive airborne applications requiring high transmit power, including in unmanned aerial vehicles (UAVs). By applying three-dimensional electromagnetic (EM) simulation software, TWT designers have also been able to closely study the EM field interactions of different tube components in order to refine physical structures and improve output power and efficiency. In addition to TWTs and CFAs, highpower vacuum-electronic devices employed in military systems include klystrons (usually as amplifiers) and magnetrons (usually as oscillators).

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