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Microwave Power Modules (MPMs) Miniature Microwave Amplifiers for Radars Tom Ninnis L-3 Communications Corporation Electron Devices Division

960 Industrial Road San Carlos, California,

94109 USA tom.

[email protected] Abstract A recent and dramatic shift with military aircraft has been the widespread use of UAVs (unmanned aerial vehicles). Most previous-generation UAVs do not carry radar and, therefore, do not have all-weather visibility. But now, nearly all large UAVs have radars as standard equipment. UAVs require miniature and lightweight radars with low power consumption. Previously, there was little demand for such radars and industry had few product offerings. However, several manufacturers are now developing new miniature airborne radar systems. This creates a need for miniature radar transmitters. Microwave Power Modules (MPMs), a new class of microwave amplifiers, is the enabling technology for these transmitters. MPMs are hybrids of solid state and vacuum tube technology. Within the MPM is a solid state driver amplifier, a TWT output amplifier, and an integral power supply. MPMs provide approximately

100 watts in narrow and wide band versions within the

2 to

18 GHz frequency range. To date, the primary use of MPMs has been in satellite uplink systems. But now, MPMs are being increasingly used in new miniature transmitters for radars. The requirements for radars are more stringent than for communication applications, particularly the phase noise and pulse characteristics. This paper describes MPMs and their performance characteristics for miniature radar transmitters. Keywords: Microwave Power Module, MPM, miniature radar, transmitter, SSPA, TWTA Introduction After decades of development, Unmanned Aerial Vehicles (UAVs) are now being used with great success in a variety of applications. UAVs perform tasks that would put lives at risk or would be more difficult and/or expensive if performed by piloted aircraft. In addition to their usefulness, UAVs are relatively affordable compared to similar piloted aircraft. As a result, the UAV market is growing at an accelerated rate. There are a wide variety of UAVs under development, from small hand-launched devices to full-sized aircraft. The larger of these, typically medium and high altitude long endurance aircraft, have over

100 kg available for sensors. Electro-optical and infrared systems have been standard equipment on these aircraft, but these technologies have limited visibility through poor weather and smoke. The solution to this problem has been the incorporation of radars, since they have the ability to see through these conditions. In order for radars to be useful for UAVs, they must be extremely small, lightweight, and efficient, consuming a limited portion of the overall space, weight, and prime power available for sensors. However, they must also be powerful enough to be effective from 5,000 to 15,000 meters in altitude. Several years ago, the best radars available for UAVs were those used on helicopters and maritime patrol aircraft, as these were the smallest of the high-resolution airborne radars. However, even these radars were too heavy and consumed too much power to be practical for most UAVs. Cleared by DOD/DFOISR for public release under DFOISR case number 06-S-0033 on October 31,

2005 As a result, there is now a significant and growing market for miniature, low power, high-resolution radars for UAVs. In response, several companies are developing synthetic aperture radar (SAR) systems specifically designed for UAVs at a fraction of the size and weight of their standard airborne radars. General Atomics in the U.S. and Sandia National Laboratories collaborated on a

52 kg. Lynx radar that has a range of

35 km1. Sandia recently demonstrated an

11 kg. miniSAR radar with a

6 km. range2. In Europe, EADS has a

5 kg. radar called MiSAR with a range of 5-10 km3 and Thales has the I-Master radar at

30 kg. with a range of

20 km4. A key determinant in the size, weight, power consumption, heat dissipation, range, and cost of a radar system is the output transmitter. Since these parameters are extremely important to UAVs, the transmitter takes on even greater importance for these applications. Transmitters include a high power microwave amplifier, a regulated power source for the amplifier including the control electronics, and the associated cooling. Thus, the new miniature radar market has created the need for miniature transmitters, much smaller, lighter, and more efficient than previous airborne transmitters. Enabling the development of these transmitters are Microwave Power Modules (MPMs). MPMs are a relatively new class of microwave amplifiers, with tremendous advantages over other technologies for UAV applications. This paper describes the performance achievements of standard MPMs and a new product C a pulsed MPM designed for miniature radar systems. Microwave Amplifiers for UAV Radars There is a tradeoff between the output power of the amplifier and the size of the antenna. For a given effective radiated power (ERP), an increase in the amplifier output power will result in a smaller and lower gain antenna. For UAVs, it is desirable to have a very small and lightweight microwave amplifier with relatively high output power. It is also important for the amplifier to have high efficiency in order to keep the UAV prime power source and the transmitter cooling as small and light as possible. For radars, the microwave amplifier must also have special performance characteristics. It must have excellent pulse characteristics and extremely low phase noise. The primary requirements for power microwave amplifiers used for UAV radars are summarized below: ? Low phase noise ? Small size and lightweight ? High power ? High efficiency There are three basic types of amplifier technologies suitable for modern airborne radars: ? Semiconductor technology (solid state power amplifiers, or SSPAs) ? Vacuum Electronics technology (traveling wave tube amplifiers, or TWTAs) ? Hybrid of semiconductor and vacuum electronics technology (MPM amplifiers) Each technology has advantages and disadvantages, with the primary discriminators being efficiency, weight, cost, risk, and performance.5 Figure

1 below shows where each technology excels. Figure

1 - Amplifier Output Power vs. Frequency For UAV radars, the primary frequency bands of interest are X, Ku, and to a lesser extent, Ka, and the desired output power is in the range of

25 to

100 watts average and up to 1,000 watts peak. Comparing these numbers with the graph in Figure 1, MPMs are indicated to be the optimum technology for UAV radars. This is proving true in practice. System developers are turning to the MPM for reduced system size, weight, and cost for new SAR/MTI systems and for upgrades to existing systems. MPM Description MPMs are hybrids of solid state and vacuum electronics technology that offer significant advantages over traditional high power amplifiers: a super component that takes advantage of the best features of both solid state and vacuum tube technology.6,7 Cleared by DOD/DFOISR for public release under DFOISR case number 06-S-0033 on October 31,

2005 A MPM includes a solid state driver amplifier (SSA), a traveling wave tube (TWT) output amplifier, and an integral high-density power supply as shown in Figure 2. Figure

2 C MPM Block Diagram The MPM concept surfaced at a US DOD Special Technology Review in 1988.8 The performance goals developed in 1989.9 ,10,11 are listed below in Table 1. Parameter Goal Frequency (GHz)

6 to

18 RF Output Power (Watts)

50 to

100 Gain (dB)

50 Duty Cycle 0% to CW Efficiency 33% Noise Power Density (dBm/MHz) -45 Noise Figure (dB)

10 Volume (cubic in / cc) 7.5 /

123 Thickness (in / cm) 0.31 / 0.79 Table

1 Most of the goals were subsequently achieved in the early 1990s as part of development programs, and now a variety of MPMs are available. An example is L-3 Communications Electron Devices Division'

s M1220 MPM, with the power versus frequency depicted in Figure

3 below.12 Figure

3 - L-3'

s M1220 MPM Power Output versus Frequency The MPM component........