High-power microwave systems still depend on a variety of vacuum electronic devices to reach their required output levels. While vacuum tubes may have long ago departed from consumer radios, they are alive and well in many satellite communications (satcom) and military electronics systems. In fact, in deep space where reliability is essential, microwave traveling-wave tubes (TWTs) and traveling-wave-tube amplifiers (TWTAs) are often the signal-boosting subsystems of choice at microwave through millimeter-wave frequencies.
TWTs and TWTAs are both associated with high-output-power levels and gain. But in recent years, the performance levels of these devices has improved in terms of a number of performance parameters, including gain flatness with frequency, linearity, and phase distortion. At one time, “linearity” in a TWT or TWTA was considered equivalent to its continuous-wave (CW) output power level backed off by at least 3 dB. But with more and more TWT and TWTA manufacturers complying with the requirements of MIL-STD-188-164A interoperability and performance standards for C-band, X-band, and Ku-band satcom earth terminals, an increasing amount of data are available on the performance levels of TWTAs under a variety of operating conditions.
In addition to the motivation to save weight and volume on satellites that has helped reduce the size of newer TWTAs, military customers have pushed TWT and TWTA designers and suppliers for smaller products as part of military size, weight, and power (SWaP) initiatives. and the requirements of modern conflicts have demonstrated the need to transfer an increasing amount of data quickly and securely from the tactical edge of a conflict to command centers. hence, TWTs and TWTAs have gotten smaller while maintaining high output-power levels.
As an example, Fig. 1 shows a US army satcom trailer that was deployed in Iraq, incorporating a Ku-band TWTA from MCL. The high-frequency tube amplifier is one of a wide range of TWTAs designed and produced by the company for satcom applications, meeting both MIL-STD-188-164AB (proposed) and Army Forces Strategic Command (AR-STRAT) requirements.
In keeping with the need for increased linear power in smaller packages,MCL has developed lines of antenna-mountable TWTAs, including its outdoor amplifier models MT2300 and MT3600, for Kaband satcom uplinks (Fig. 2). The MT2300 TWTAs weigh just 33 lbs while the MT3600 TWTAs weigh 47 lbs. mT2300 TWTAs can be specified for Ku-band frequencies from 13.75 to 14.50 GHz as well as for Ka-band frequencies from 27.5 to 31.0 GHz. The Ku-band models deliver 100 w linear output power and 175 W peak output power. The Ka-band models provide 60 W linear output power and 150 W peak output power, or 120 w linear output power with 215 W peak output power. The MT3600 TWTA is available in Ka-band (27.5 to 31.0 GHz) and Q-band (43.5 to 45.5 GHz) versions, the latter with an integral frequency block upconverter (BUC) and solid-state driver amplifier. The Ka-band models offer either 8 W linear output power with 150 W peak output power, or 120 W linear output power with 215 W peak output power. The Q-band TWTAs can be specified for either 55 W linear output power or 80 W linear output power.
To simplify upgrades of the AN/USC- 60A fly-away triband satellite terminal (FTSAT), L-3 Communications Narda Satellite Networks has developed its model 1.2 Ka-60A upgrade system, which can be integrated with other military multiband super-highfrequency (SHF) satcom systems. The upgrade hardware is designed to meet ARSTRAT certification when integrated with the base system. It consists of a 1.2-m segmented carbon fiber reflector, adapter plate, and linearized 175-W TWTA.
Bosch Telecom GmbH emphasizes the importance of TWTAs for space-based communications in its brochure, “Traveling Wave Tube Amplifiers (TWTAs) for Space Applications,” available as a free PDF download from the firm’s website. The company’s TWTAs, which are available for use from 1.5 to 30.0 GHz at power levels from 10 to 450 W CW, have been integrated into a wide range of satellite systems, including PIONEER 1, PIONEER 2, the Ka-band and Ku-band Astra satellites, Eutelsat III, and Intelsat 9. The company’s TWTAs feature high-performance electronic power conditioner (EPC) subassemblies that provide the high voltages needed by TWTs.
Comtech Xicom Technology offers a number of TWTAs that cover more than one satcom band within a single unit, such as the model XTRD-2000CX TWTA with at least 500 W output power at C-band (5.850 to 6.425 GHz) and X-band (7.90 to 8.40 GHz) frequencies. It uses a TWT rated for 2 kW output power.
The firm’s model XTD-750KHE TWTA (Fig. 3) is designed for antenna mounting in Ku-band satcom links. It is based on a high-efficiency TWT with dual-stage collector. The TWTA delivers 270 W linear output power, 355 W maximum CW output power, and 750 W peak output power from 13.75 to 14.50 GHz with large- and small-signal gain of at least 70 dB. It weighs only 56 lbs and draws only 1450 W power at its full output rating. Level control is accomplished by means of a 30-dB attenuator circuit that can be adjusted in 0.1-dB steps. The amplifier is also available as model XTD-750KHE-B1 with a BUC that accepts input signals from 950 to 1700 MHz. Both amplifier versions are equipped with an Ethernet interface for remote control.
Communications & Power Industries offers a range of indoor and outdoor TWTAs for fixed and mobile satcom applications covering C-, Ku-, X-, and Ka-band frequencies at power levels from 200 to 750 W. For example, the TO- 1TO series of outdoor TWTAs is intended for use at C-band (5.85 to 6.425 GHz), Xband (7.9 to 8.4 GHz), and Ku-band (14.0 to 14.5 GHz) frequencies, with 85 W minimum output power at the amplifier flange at C-band, 120 W minimum output power at X-band, and 80 W minimum output power at Ku-band frequencies. The TWTA offers 41-dB small-signal gain at the lower frequencies and 45-dB small-signal gain at X- and Ku-band frequencies.
TMD Technologies Ltd. offers high-power tubes of different types, including TWTAs for laboratory test applications. Supplied in 19-in. rackmount enclosures, these TWTA-based systems include the model PTX7437 transmitter subsystem optimized for radar testing from 9.0 to 9.5 GHz. The subsystem generates 8000 W peak output power at a 2% pulse duty cycle and 20-µs pulse width. It boasts 60-dB gain to boost pulsed input signals, and measures 355 x 430 x 155 mm and weighs 20 kg.
Another well-known name in TWTAs for test applications is >AR RF/Microwave Instrumentation, with narrowband and broadband TWTAs for use at frequencies through 45 GHz, and with as much as 10 kW pulsed output power from 8 to 10 GHz for high-power RF/microwave test applications. For example, model 200T2Zz-5G40A is a rack-mount TWTA with forcedair cooling for test applications from 26.5 to 40.0 GHz. It provides 200 W CW minimum output power minimum with 50 W linear output power at 1-dB compression.
Finally, in addition to TWTs and TWTAs, some vacuum-tube manufacturers supply compact assemblies that include EPCs, linearizer circuits, and the TWTA circuits. Known as microwave power modules (MPMs), these compact units can not only save space in mobile and fixed satcom earth stations, but also support simplified connections for rapid installation.