Microwave Journal
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Defense Opportunities and Challenges in 2019

November 13, 2018

The 2019 defense budget seems to have something for everyone-including the first pay raise for troops in nine years-and sailed through the House with a 361 to 74 vote and was signed by the President. It is the first time in a decade this was achieved before the end of the fiscal year. By any unit of measure, 2019 should be a good year for the RF and microwave industry as the Defense Department's Third Offset Strategy requires a heavy dose of fields and waves.

 

Phased Array Radar

The U.S. has never faced the type and level of threats present today that require what only RF and microwave technology can deliver. For example, until recently, the U.S. and some NATO countries were the sole purveyors of active electronically-steered array (AESA) radars, but that is changing fast, as Russia and China have demonstrated their own formidable AESA systems. This is bad news for radar warning receivers (RWR) that are attempting to keep up with these shape-shifting electronic chameleons, whose lightning-fast reflexes and highly-developed brains can dispatch older RWRs with ease.

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Figure 1 An AESA radar is smaller, lighter and vastly superior to legacy radars.

Even though phased-array radars have been in the inventories of the most advanced countries for a decade or more, the latest crop can exploit advances in signal processing to deliver astonishing performance. They can provide almost-instantaneous 360 degree coverage, and are versatile enough to perform in any role, from fire-control to synthetic aperture radar (SAR) mapping, sea surface search, ground moving target indication and tracking and air-to-air search and track. An AESA radar (see Figure 1) can randomly change frequency with every pulse, rapidly vary its output power, change its pulse repetition frequency and waveform, use spread-spectrum techniques and suddenly become passive, using the RWR's own signal to defeat it. And that is the short list.

AESA radars are also rapidly expanding their range, thanks in no small measure to GaN MMICs that are poised to complement or perhaps replace GaAs in the T/R modules of most next-generation radars, and many electronic warfare (EW) systems, as they too may use the AESA architecture. In fairness, without GaAs MMICs, the military might still be using traveling-wave tube (TWT) powered amplifiers for want of a broadband, solid-state alternative. GaAs is a mature, cost-effective technology whose future remains somewhat secure, if not in AESA radars, than in dozens of other applications that it has either enabled or enhanced.


The Triumph of the TWT

The imminent demise of the traveling-wave tube (TWT) has been predicted since the first GaAs MMICs led to the AESA radar and the last nail in the TWT's coffin was supposed to be GaN. Neither of these pronouncements has occurred and is not likely to for many years. In fact, the vacuum electron devices (VED) that helped win World War II and make SATCOM and radio and television broadcasting possible are experiencing resurgence with interest at mmWave frequencies, and DoD is a driving force.

No solid-state device can produce as much power in the mmWave frequencies and above than a TWT. However, the small size of solid-state devices allows phased-array antennas with hundreds of elements to produce a reasonable EIRP of perhaps 10 W at 60 GHz using a 64-element array. While this is more than adequate for commercial communications applications such as IEEE 802.11ad, more power is needed for EW systems. At higher frequencies above 100 GHz, the EIRP would be even less, and in all cases, heat dissipation is a major challenge.

The TWT that can deliver RF power of several hundred watts at frequencies up to 100 GHz and tens of W at 200 GHz, which makes it not only appealing but a necessity if EW systems are to defeat adversaries at these frequencies. Microwave Power Modules (MPM) that are even smaller at these high frequencies than their lower-frequency counterparts that potentially make them usable in airborne radar and imaging applications where SWaP requirements are severe. When fed to a high gain reflector-type antenna an EIRP of more than kW is achievable.

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SB Figure 1 Anatomy of a 100 W cold-cathode TWT developed by L-3 Communications Electron Devices.

The power available from new TWTs may make them more enticing in the future, especially for remote sensing applications at higher frequencies. An example is a MPM designed by L-3 Electron Devices has an instantaneous bandwidth of more than 3 GHz from 231.5 to 235 GHz (G-Band) and produces a peak RF output power of 32 W with 10 mW of drive and efficiency of about 9 perccent (see SB Figure 1).

Cold Cathode Coming

The most noteworthy achievement in TWTs of late is the previously insurmountable challenge of producing a cold-cathode device, development of which has been ongoing since Charles “Capp” Spindt and Kenneth Shoulders of SRI International published a seminal paper in 1966. The benefits are significant, as a cold-cathode tube operates at ambient temperature without the need for a cathode heater, a traditional factor limiting tube life so that theoretically, tube life could be almost infinite. It would also eliminate warm-up time, making operation almost instantaneous. Current density could be much higher as well because emission would no longer be limited by operating temperature, making it easier to focus the electron beams.

The biggest challenge has always been reliability, as the cathode consists of tens of thousands of micrometer-size molybdenum cones deposited on a circular silicon substrate with an area of about one square millimeter. The high fields within the structure and the thin film gate electrode make it possible for an electrical short to occur between the gate and one of these cones. When that happens, the entire array of emitters burns up and the device fails catastrophically. In a traditional thermionic TWT degradation is “graceful,” allowing its end of life to be predicted by the amount of barium remaining.

However, L-3 Electron Devices has developed a way to reduce the damage caused by such a short, and were able to interrupt the breakdown path between the base of the cones and the gate by adding a dielectric layer between them. The company believes that at its current rate of development, the cold-cathode TWT could become a commercial product within about five years.


However, GaN has advantages that cannot be matched by GaAs, silicon or any other current semiconductor technology capable of producing RF power. It can operate at about 5× the voltage of GaAs and has 10× the power density per unit of die area, twice the current handling ability and higher power-added efficiency above about 10 GHz and higher than silicon above about 1 GHz. And while both GaAs and GaN can cover broad bandwidths, a GaN-based amplifier can have an instantaneous bandwidth 4× wider than a GaAs with the same output power.

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Figure 2 GaN on SiC wafers, shown here by the European Space Agency, have reached 6-in. in diameter.

Compared to silicon LDMOS, GaN has a power density advantage and can potentially operate far into the mmWave region. In short, GaN will power most of the radar systems and possibly EW systems in the future and has a long roadmap before it reaches maturity. LDMOS will continue to serve some radar as well as many communications applications for many years, at least for narrowband applications below about 4 GHz. LDMOS is the hands-down winner in terms of ruggedness and cost, with the ability to operate in a load mismatch or more than 65:1 while current commercial GaN on SiC devices (see Figure 2) are rated at 20:1 or less.

LDMOS discrete transistors can achieve high RF power levels, currently up to 1.8 kW with a bias voltage of 65 VDC as well. However, as LDMOS is a more narrowband technology so multiple amplifiers are required to cover a wide frequency range. LDMOS is still used in L-Band radar, IFF and avionics systems where it is competitive with GaN, and this market is highly competitive. GaN is also currently more expensive, but that may change as volumes increase.

Manufacturers of discrete GaN RF power transistors are actively pursuing the opportunities at L-Band with devices at various power levels. Qorvo's QPD1025 GaN on SiC transistor, for example, is currently the highest power, commercially available GaN on SiC device, delivering 1800 W from 1 to 1.1 GHz. Although narrowband, so are the key applications at L-Band that cover only 1030 to 1090 MHz. The device also operates at 65 VDC, directly targeting 65 VDC LDMOS with power-added efficiency up to 77 percent and gain up to 22.5 dB.

MACOM is a notable exception to the GaN on SiC trend as the company has dedicated its development efforts exclusively to GaN on silicon (Si) substrates and has a GaN on Si device that delivers 1 kW of pulsed power at L-Band. Although generally considered GaN's “second-tier,” MACOM believes it can make GaN on Si a formidable competitor for all but bleeding-edge (i.e., defense) applications. The primary advantage of silicon is its low-cost and proven high-volume manufacturability, as it can use standard CMOS fabrication processes, dramatically increasing product capability and reducing cost.

Current GaN devices have a power density of about 11 W/mm2 but several special processes have reported even higher levels. Fujitsu currently has the highest reported power density at 19.9 W/mm, although the DARPA GaN on diamond project reported results similar to this level a few years back. The ability to produce so much power from a tiny semiconductor device is remarkable and sure to increase in coming years. That is particularly appealing in an AESA radar where the ability to produce more power at each antenna element is highly desirable.

Advances in GaN's power density will result from enabling the technology to get as close as possible to its theoretical maximum. One of the ironies of GaN is that its enviable high-power density also results in the creation of heat in a very small area, so future achievements will be directly the result of getting rid of it, quickly as it is thermally limited in many cases.

This process begins at the die level, where silicon carbide (SiC), the material to which it is most often attached as a substrate, has thermal conductivity 6× that of GaAs and 3× that of silicon. Other material combinations such as copper-tungsten, copper molybdenum and copper-molybdenum-copper are used with silicon and GaAs devices, but currently only copper (and diamond in the future) can compete with SiC in thermal conductivity.

Once the heat is moved away from the die, the next stop for the heat is a structure that moves it further away, where it can be dissipated naturally or with liquid cooling. This can be a heat sink or heat spreader. The material that will allow GaN to achieve its potential is diamond as its thermal conductivity is 4× that of SiC or copper, in fact, higher than any other material on Earth.

There are some devices currently using diamond as a substrate material and aluminum-diamond metal-matrix composites (MMC) are used as heat spreaders. As applied to RF applications, chemical vapor deposition (CVD) diamond is the near-exclusive domain of Element Six (a subsidiary diamond goliath De Beers), which has achieved thermal conductivity of an astonishing 2200 W/mK. Their process for GaN on diamond devices was recently sold to RFHIC which plans to commercialize it. Nano Materials International (NMIC), which makes aluminum-diamond metal matrix composites used for heat spreaders, has achieved thermal conductivity of about 500 W/mK, and the composite is becoming more popular as GaN moves toward higher power densities. Akash Systems is using GaN on diamond exclusively to address satellite/space applications.

The DoD also wants GaN to operate at voltages higher than 50 VDC to increase efficiency and a few 65 VDC devices are available, including several from Integra Technologies, one of which operates between 420 and 450 MHz with a 150 VDC supply and delivers more than 1 kW with a 100 μs pulse width at a 10 percent duty cycle and drain efficiency greater than 70 percent. Several device manufacturers are offering these 65 VDC devices, such as Qorvo and Sumitomo, with others following.



Electronic Warfare

Since the end of the Cold War, there has been surprisingly little emphasis by DoD on increasing EW capabilities, especially in the Army, which spent little money in this area other than for IED jammers-until now. For the recent and almost feverish interest in EW, the West mostly has Russia to “thank,” as it demonstrated in Ukraine and Syria how far it has come in EW development and anti-access/area denial capabilities in general. It also got a boost from China's effort to ramp up its anti-access/area denial capabilities in the South China Sea. The result is more attention to EW than at any time since the end of the Cold War. The DoD is working to “reinvent” electronic attack, protection and support across all the services, with a particular focus on the Army.

Providing effective EW has never been easy and today it is more difficult than ever, requiring a modular, scalable, adaptive, more selective and precise approach over a wider range of frequencies into the high millimeter wavelengths with greater resistance to interference, whether from friend or foe. EW systems must be able to capture huge amounts of data, process it and deliver a response in near real-time from more sources than ever as the spectrum is densely packed with legal emitters. As the electromagnetic environment varies from country to country, there is no one-size-fits-all solution.

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Figure 3 DARPA, other agencies and defense contractors are working to first integrate the many elements of EW, for which machine learning and artificial intelligence are the essential ingredients (Source: DARPA).

To accomplish all that DoD wants to achieve will require a more tightly managed approach, making EW systems a more integrated part of the battlespace (see Figure 3). One way to accomplish this is by allowing an AESA system to handle both radar and EW, requiring a level of integration that currently does not exist. It will be difficult to achieve because EW systems must have 12 to 16× the bandwidth of a radar system, which among other things presents significant challenges for antenna designers and at the lowest frequencies make the two applications fundamentally incompatible. However, DoD is considering the approach as it would significantly reduce hardware requirements.

Another trend in EW is toward cognition-machine learning, an artificial intelligence that would allow RF systems to change their characteristics in near real-time to adapt to conditions as they are experienced by the system. In contrast, EW systems currently use lookup tables to sort out what they have ingested and applying a rules-based approach to determining what to do about them. However, the increasing use of digital signal processing and radar systems make it necessary for future EW systems to complement current threat databases with this real-time information that would be gathered during operation. This not only addresses the need for greater situational awareness but allows EW system to adapt to new threat signatures immediately.

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Figure 4 The Navy's Next-Generation Jammer, represents the future of EW as it uses state-of-the-art GaN devices in an electronically-steerable architecture (Source: U.S. Navy).

The most recent program attempting to accomplish this is the Reactive Electronic Attack Measures (REAM) program for which Northrop Grumman was awarded a $7.2 million contract to develop machine algorithms that would ultimately be used on the EA-18G Growler EW aircraft that, coincidentally, will also be the first recipient of the Navy's GaN-powered Next-Generation Jammer (see Figure 4). The REAM program is designed to produce detection and classification techniques for identifying waveform-agile radar threats and responding automatically with electronic attacks.

A Quantum Leap

In the long-term, the greatest threat to EW systems comes from quantum technology and how it is employed in radar systems. The technology has so many benefits that whoever deploys it first will have a huge (if temporary) advantage, making their EW systems as well as stealth technologies potentially useless. China seems determined to deploy this first and has been loudly claiming it has demonstrated the first “single-photon quantum radar system” that skeptics consider dubious. The accomplishment has been attributed to the development of single-photon detectors that very efficiently capture returning photons.

The Chinese say they tested such a system in an outdoor environment and it demonstrated the ability to detect stealth aircraft at 62 miles with accuracy high enough for missile targeting. China's media was quick to point out that this was 5× what a laboratory prototype jointly developed by researchers from Canada, Germany, Britain, the U.S. achieved a year earlier. DARPA is funding research with the University of Waterloo, Lockheed Martin and several other companies to develop quantum radar systems as well.

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Figure 5 Whatever country manages to develop a “quantum radar” will be able to render EW systems and most stealth technologies obsolete (Source: China Electronics Technology Group).

Quantum radar takes its name from the theory of quantum entanglement (see Figure 5) in which two particles can share a relationship (and are thus “entangled”) that allows analysis of one to be used to learn about the other, even though the two are a long distance away from each other. A crystal is used to split a photon into two entangled photons, creating what is called parametric down-conversion, and in a radar, multiple photons will be created in entangled pairs.

The first pair, for example, is sent at microwave frequencies like a conventional radar, and the second set is retained in the transmitting system. By studying the second set of photons, it is possible to learn a lot of about the first. This data includes if the pair struck an object and if so how far away, how fast, how large it is as well as its direction. In addition, as quantum radar uses subatomic particles rather than electromagnetic energy, it is not constrained by materials technologies used to create stealth by reducing radar cross-section. In the same vein, it will ignore jamming and spoofing including chaff.

Although stealth aircraft can be identified over short distances using systems operating at VHF and UHF frequencies, they can only do so over short distances and have difficulty determining range. Consequently, the quantum radar has enormous potential advantages in detecting stealth aircraft and could effectively render current techniques useless.

Summary

A complete discussion of all the technologies and defense programs and platforms of the microwave community would fill a novel-sized book, as there are so many legacy, current and development programs. Consider, for example, that the HF region that gets little attention but is becoming the focus of SIGINT. It is the perfect “place” for both state and non-state actors to send messages over long distances, at little cost and with very low probability of intercept. There is also remote sensing, IFF, avionics, air-traffic control, attempts to create a workable solution for battlefield communications, missile seekers, ballistic missile defense, UAS payloads and whatever is behind the doors of the many black programs. With all this on the table, there should be plenty of opportunities for the microwave industry as far as anyone could safely project.