Capable of high gain and as much as 160 W output power over its frequency range, this single tube can be used as an output device in broadband systems from 2 to 18 GHz.
Such applications as radar and electronic-warfare (EW) systems in unmanned aerial vehicles (UAVs) require highpower amplifiers that can deliver generous amounts of output power in relatively small packages. One of the firms that has answered this challenge is e2v with its miniature traveling-wave tubes (TWTs). Its latest TWT product, model N20180, is a mini helix TWT capable of continuous operation from 2 to 18 GHz and better than 40-percent efficiency in a package measuring 220 x 26.8 mm and weighing no more than 400 g.
Model N20180 (Fig. 1) is a compact TWT that represents an evolution of the company's efforts to achieve full 2-to-18-GHz frequency coverage in a single design. Modern TWTs in general feature rugged metal/ceramic construction and good broadband performance compared to solid-state devices, with design approaches such as coupledcavity and helix TWTs delivering high output power densities in relatively small housings. Coupled-cavity TWTs are typically larger than helix-type TWTs, built upon a series of coupled cavities that are arranged axially along the electron-beam path. A helix TWT employs a helical coil configuration formed of wire attached in place by ceramic insulators. The electron beam is held in a magnetic containment field formed by strong permanent magnets. An RF/microwave input signal is fed into the start of the helix by means of an input coupler, inducing current flow in the coil. An output coupler, positioned just before the collector at the end of the tube, provides access to the amplified output RF/microwave signals.
Model N20180 is a helix-type TWT with a dual-stage collector for high efficiency. The tube includes conduction cooling of the collector for long-term reliability, with a convergent electron gun and periodic-permanent-magnet (PPM) beam focusing. It provides minimum RF output power of 10 W at 2 GHz and 100 W at 18 GHz, although the numbers in between are more impressive. The minimum RF output power from 6 to 13 GHz, for example, is 140 W, with minimum output power of 100 W at 5 GHz, 130 W at 14 GHz, 120 W at 15 GHz, and 100 W from 16 to 18 GHz. The small-signal gain ranges from 15 to 21 dB at 2 GHz and from 43 to 49 dB at 18 GHz. The maximum small-signal gain reaches as high as 71 dB at 9 GHz and 70 dB at 11 GHz.
Second-harmonic outputs are specified at +2 dBc or less at 2 GHz, -9 dBc or less at 6 GHz, and -14 dBc or less at 9 GHz. Spurious outputs are -30 dBc or less relative to the saturated fundamental- frequency output level. The model N20180 helix TWT is equipped with an SMA male input connector and a TNC male output connector. The maximum input and output VSWR is 2.50:1.
The TWT is designed to operate with typical prime power of 500 W. It requires typical helix voltage of 4.8 kV at 6 mA with heater voltage of -6.3 V at less than 1 A. The dual collector design requires two supplies, with collector 1 voltage of typically 2.7 kV at 2 to 160 mA and collector 2 voltage of typically 1.68 kV at typically 220 mA. The tube is designed to operate with the helix fully grounded and the collectors depressed. The firm also offers variants of this device for portions of the total 2-to- 18-GHz bandwidth. e2v Technologies plc, 106 Waterhouse Lane, Chelmsford, Essex CM1 2QU, UK; +44 (0) 1245 493493, +44 (0) 1245 493492, Internet: www.e2v.com.