GaN v. TWTA for a UAV SAR application

Would it be worthwhile to replace the TWTA Transmitter power amplifier in the Lynx RF front end? If one were designing a replacement radar would GaN be the best solution? The Lynx Radar requires a long pulse high duty factor  with flat response over a wide band at Ku band in order to achieve high resolution imagery at moderate range. Sandia designed the radar in the mid 1990's, when TWTA was the only solution. In the UAV application space, weight, power, and reliability are at a premium, which make a solid state solution attractive. It is not clear that GaN is up to the challenge yet.

Sandia spie99_paper Lynx SAR/GMTI Radar

Lynx is a high resolution, synthetic aperture radar (SAR) that has been designed and built by Sandia National Laboratories in collaboration with General Atomics (GA). Although Lynx may be operated on a wide variety of manned and unmanned platforms, it is primarily intended to be fielded on unmanned aerial vehicles. In particular, it may be operated on the Predator, I-GNAT, or Prowler II platforms manufactured by GA Aeronautical Systems, Inc.

The Lynx production weight is less than 120 lb. and has a slant range of 30 km (in 4 mm/hr rain). It has operator selectable resolution and is capable of 0 .1 m resolution in spotlight mode and 0.3 m resolution in strip map mode. In ground moving target indicator mode, the minimum detectable velocity is 6 knots with a minimum target cross-section of 10 dBsm. In coherent change detection mode, Lynx makes registered, complex image comparisons either of 0.1 m resolution (minimum) spotlight images or of 0.3 m resolution (minimum) strip images.

The front-end microwave components include a TWTA capable of outputting  peak power of 320 W at 35% duty factor (105 W average) averaged over the Lynx frequency band (16.7 GHz +/- 1.5 GHz) and a Low Noise Amplifier LNA that allows an overall system noise figure of about 4.5 dB.

GaN Amplifiers Challenge TWTAs | Active components content from Microwaves & RF
For fans of solid-state power, API Technologies Corp. is unveiling a new line of gallium-nitride (GaN) power amplifiers at Booth No. 1515 at the 2014 IMS. With frequency coverage to 18 GHz and power levels to 1 kW, these pulsed amplifiers offer the power levels of traveling-wave-tube amplifiers (TWTAs) at a fraction of the size and weight. They are ideal for commercial and military applications, including radar, communication transmitters, and jamming systems. The amplifiers are supplied in hermetic housings and available with a variety of options, include sleep mode, blanking, forward/reverse power detection, discrete power supply designs for wide DC input voltage ranges, as well as microprocessor-based control features for bias optimization, temperature compensation, and fault monitoring.

GaN Amps Replace TWTAs | Active components content from Microwaves & RF
Designing and building a line of solid-state power amplifiers based on gallium-nitride GaN) transistors and active devices, Diamond Microwave offers amplifiers at X- and Ku-band frequencies as alternatives for traveling-wave-tube amplifiers (TWTAs) in demanding defense, aerospace, and communications applications. Certified to ISO 9001:2008, the firm believes that its amplifiers provide excellent pulsed-power performance with power-to-volume ratio that is among the highest in the industry. The GaN power amplifiers offer pulsed output-power levels to 150 W and ae flexible in layout and architecture for a wide range of applications. The company displayed a Ku-band amplifier at the recent International Microwave Symposium with peak pulsed output power of 125 W at 16.5 GHz and 1-dB bandwidth of 1.5 GHz. 

Comparison of High Power Amplifier Technologies: TWTAs vs SSPAs

Designers of high power communication systems are constantly evaluating and debating the merits of using Traveling Wave Tube Amplifiers (TWTA) and Solid State Power Amplifiers (SSPA) in their transmitting systems. Most decisions are made on personal preferences for one technology over another. The reality is that both technologies have advantages and disadvantages, with higher power levels and higher frequencies leaning toward the TWTA as a better choice.
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SSPA and TWTA are both viable alternatives to consider for power amplification in a satellite earth station, with SSPA the more advantageous at lower power, i.e. less than 20 watts. At greater power levels, size, weight, and efficiency become important characteristics that skew the preferred choice to the TWTA. The following table summarizes the comparison of a 200 watt SSPA and a 665 watt TWTA, but this comparison typically can be scaled to other power levels in the range of 50 watts to 2.5k watts. The TWTA advantage increases at higher power levels and higher frequencies, where SSPA efficiency decreases.
200 Watt SSPA vs. MCL MT4000 700 Watt TWTA

SSPA
• 200 Watt P1dB (250 Watts Psat)
• 2800 Watts AC Power
• 7.1% Efficient
MCL MT4000
• 665 Watt Psat (Equivalent to SSPA 222 Watts)
• 2400 Watts AC Power at Psat (27.8% Efficient)
• 28% Efficient at Psat
• Back-Off (Usually Mode of Operation)
• 1650 Watts TWT vs. 2800 Watts SSPA
• TWTA uses 40% less power
TWT vs. SSPA Comparison
• MT4000 is 10% higher equivalent power
• Typically uses 41% less power
• 266% more available power
• Smaller size (24" vs. 28" length)

New and Exciting GaN-based Solid State Power Amplifiers & Converters | Advantech Wireless
GaN based SSPAs are opening  completely new market opportunities for Satellite based Communications with unmatched performance, high reliability and low OPEX. Within the same footprint, GaN allows us to double the RF power and reduce energy consumption by up to 70%! In addition, GaN exceeds by several orders of magnitude the performance of previous generation of SSPAs based on LDMOS or GaAs technology.

GaN The TWT Killer Not So Fast
Gallium nitride’s (GaN) meteoric rise to darling of the RF design industry has been accompanied by some lofty predictions. One such claim is that the emergence of GaN solid-state semiconductors means certain death for traveling wave tube (TWT) technology. As it turns out, GaN might not replace  TWTs after all, at least not for the foreseeable future. Although GaN has captured market share from TWTs in some lower-power, lower-frequency applications, there is much evidence suggesting TWT technology is alive and well.