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Efficient Design and Analysis of Airborne Radomes
Building on its strong background in the development and production of Helix travelling wave tubes (TWT) for military airborne applications, e2v has developed a ‘new generation’ of TWTs that expands upon established airborne decoy applications and anticipates future system requirements. Over the past 20 years the company has manufactured broadband mini-TWTs (6 kV operating voltage) for use in airborne towed decoy systems. In the late 1990s, it developed a mini-TWT (4.5 kV operating voltage) for use in airborne towed decoy systems, which addressed major European and US military programs.
The standard mini-TWT range was based upon e2v’s original broadband (4.5 to 18 GHz), 100 W, 4.5 kV device, designed primarily for high altitude (70,000 ft) and high temperature operation (> 140°C measured at TWT base plate). The new range of tubes also introduces custom narrow-band mini-TWTs, the goal being to supply a range of devices to meet future applications, including Electronic Counter Measures (ECM), Electronic Warfare (EW), Microwave Power Modules (MPM), Synthetic Aperture Radar (SAR) and microwave data links. Building upon present-day rugged and proven technology, these new tubes offer enhanced output power, frequency range and efficiency.
Design modeling of key areas of the TWT, coupled with rapid prototyping has seen a new family of mini-TWT tubes evolve, using the standard mini-TWT as the host design vehicle. The result is three new mini-TWT ranges:
• The N20173 6 to 18 GHz TWT range, achieving a minimum 100 W
• The N20160 4.5 to 18 GHz TWT range, 140 W typical
• The N20154 13.75 to 14.5 GHz (Ku-band) TWT range, achieving a minimum 120 W
Key performance data, attributes and operating parameters for these three new devices are covered in this article.
Current standard mini-TWTs typically operate over this 6 to 18 GHz bandwidth, but fall short of output power in the upper frequency range. The design driver for the N20173 (shown in Figure 1) was to increase the efficiency of the new device at higher frequencies without degrading performance at the lower end of the operating band. This was achieved by the use of a Slow Wave Structure (SWS) with a novel phase velocity taper.
Consequently, the new N20173 tube achieves >100 W over the full operating frequency band and the new RF structure reduces the second-harmonic output to < –9 dBc at 6 GHz. Figure 2 shows the power to frequency performance of the N20173 compared to e2v’s standard mini-TWT.
Also, an optimized coaxial TNC output achieves better than 2.2:1 VSWR and variants within the range include two- and three-stage collectors and a focus electrode switched option for pulsed or CW operation. The N20173 maintains the compact design of the standard e2v mini-TWT, being 220 mm long, 27 mm wide and 29 mm high, with a mass of less than 320 g. It can operate at base plate temperatures up to 140°C and altitudes up to 70,000 ft, under severe shock and vibration levels.
There has recently been increasing demand for higher-powered mini-TWTs capable of operation over the extended 4.5 to 18 GHz bandwidth. Thus, the N20160 device (shown in Figure 3) has been designed as a drop-in replacement for the standard mini-TWT, capable of operation at the same voltages and maintaining the same space envelope.
The increased performance has been achieved through a combination of efficiency enhancement, the utilization of velocity tapers in the slow wave structure and increased beam current. The N20160 device also offers a minimum of 140 W between 6 and 13 GHz, more than 100 W at 18 GHz and more than 50 W at 4.5 GHz. The maximum RF drive required for saturation is 22 dBm, and the small-signal gain achieved mid-band is 59 dB.
With optimization of the slow wave structure it has been possible to enhance the second-harmonic performance, which is a dominant characteristic of multi-octave devices. The device typically achieves second-harmonic levels of 1 dBc at 4.5 GHz and –10 dBc at 6 GHz. Figure 4 shows the power versus frequency performance of the N20160 compared to e2v’s standard mini-TWT.
Pre-production N20160 models have been tested using both dual- and three-stage collectors, achieving prime powers of less than 460 W. This increased performance has been achieved while maintaining compact size and weight—220 mm long, 27 mm wide and 29 mm high, with a mass of 320 g.
The N20154 mini-TWT (shown in Figure 5) has been developed to satisfy the ever-increasing demand for low-cost, lightweight, high-efficiency, compact TWTs for communication and data link applications. The mini-TWT’s design moves away from broadband convention and is optimized specifically for operation over the narrow communications band. This gives the advantage over broadband devices of improved linearity and gain flatness.
The RF circuit has been optimized around a negative velocity taper in order to achieve high circuit efficiency, combined with a multi-stage collector to realize an overall tube efficiency of greater than 33 percent. Output power across the band is 120 W minimum, with a small-signal gain of typically 52 dB. The linearity parameters are vital to communication tubes; the third-order IP performance at 4 dB Output Back-Off (OBO) is typically –22 dBc and –28 dBc at 7 dB OBO.
Other enhancements include optimization of the input and output VSWR to achieve better than 1.5:1 across the operating band. In order to meet the long operational life requirements of communications devices, the N20154 cathode has been designed to operate at less than 1.5 A/cm2 current density. The packaged N20154 device weighs less than 280 g, is 171 mm long, 30 mm wide and 25 mm high, making it ideal for microwave power module applications. Mechanical design of the N20154 shares many component parts and manufacturing processes with the standard e2v broadband mini-TWTs. This has resulted in a low-cost, high-reliability product that offers system manufacturers a serious alternative to solid-state amplifiers. Table 1 shows the performance characteristics of the new mini-TWT series.
New generation mini-TWTs from e2v address the requirements of proven rugged design and enhanced RF performance characteristics. The latest three mini-TWT devices can be customized with dual- or three-stage collectors, SMA or TNC RF terminations, focus electrode switching and reduced gain power-booster versions. e2v will also consider bespoke packaging to meet specific customer requirements.
Looking forward, further design development and enhancement of the TWT product range continues, with planned efficiency enhancement through continued collector optimization. Further design targets include increased output power to 200 W, and the inclusion of a grid switched device. In addition, a narrow-band mini-TWT is under development, which will offer the (linearity, flatness) benefits seen in the Ku-band product but at X-band. This will complement the range and offer a compact lightweight device suitable for MPMs, radar and SAR applications.
RS No. 302
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