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Considerations for Very-High-Frequency Connectors in Defense Systems

Parent Category: 2020 HFE

By Peter McNeil

Of the hundreds of different environments in which microwave coaxial cables and connectors are used, defense systems are probably the most demanding. They’re handled by people who don’t realize they’re not just “wires”, but technically sophisticated, precision components. After all, they’re warfighters, not microwave engineers. So, they’re run over by heavy vehicles, used as a handy way to pull equipment carts, and exposed to chemicals, fuels, saltwater, and many other hazards.

Aging and various environmental factors are major contributors to the failure of cables and connectors, but it is arguable that most of the damage is done by the people who use them. That’s why, according to one assessment, about 75% of microwave cable assemblies are replaced frequently, about 35% are replaced once a year, and 20% are replaced at least twice a year.

In the consumer world, cables and connectors are commodities, inexpensive, and expendable. But that’s not the case in defense, aerospace, and other mission-critical applications, where replacement is not only costly and enormously expensive but often a matter of life and death. So, for decades, the cable and connector industry has been publishing documents, articles, videos, and other informational material in the hope of educating customers about the “care and feeding” of their products.

Cable and connector manufacturers have made huge strides in ruggedizing these components over the years, from layering cables with jackets impervious to hazardous substances to making connectors as robust as possible to withstand repeated connection and reconnection. All are remarkable feats of electrical, electromagnetic, and mechanical engineering.

They must also have the lowest possible insertion loss and VSWR, high phase and amplitude stability, the ability to maintain their performance over wide temperature ranges with high resistance to ingress of external signals, and other capabilities. But their latest challenge may one of the difficult, ever: extending this ruggedness to cable and connectors designed to operate well into the millimeter-wave region and employ them in defense systems in the field.

It’s not the first time the industry has been faced with this, as operational frequencies have been increasing over the years, and with the arrival of 5G even commercial wireless systems are likely to operate at frequencies up to 60 GHz or even higher. Flexible cables and connectors designed for use at 60 GHz have been available for years but at frequencies of 100 GHz or higher, not only is it difficult and expensive to manufacture them, ensuring they can maintain optimum performance in defense environments presents entirely new challenges.

From the Bench to the Battlefield

For manufacturers of RF and microwave test equipment, creating a product roadmap consists of one part market analysis and one part clairvoyance. The former is only somewhat useful because it is extremely difficult to ascertain with certainty precisely what will be needed a half-decade or so in advance. That’s more or less the time required for manufacturers to develop instruments and have them ready when customer’s equipment has been released to production.

In addition to the instruments themselves that must be available are calibration kits, and precision passive components including couplers, connectors, adapters, and cables. At the highest measurement frequencies these are not inexpensive nor are they inherently “robust”, so they present issues when used in portable and flightline measurement scenarios. If SMA, TNC, and other comparatively large connectors that must be handled with care, 1-mm and even smaller connectors represent an entirely new paradigm. For reference, the center conductor of a Type-N connector is 3.04 mm; in an 8-mm connector its 0.347 mm. Connectors proposed for use at higher frequencies, such as 0.6 mm and 0.4 mm would have center conductor diameters of 0.26 mm and 0.174 mm, respectively (Figure 1).

2008 connectors fig01

Figure 1 • Bead sizes in connectors, from the K connector at right to connectors at left designed to operate at hundreds of gigahertz. The bead is the dielectric that supports the center conductor. Source: Anritsu.

So, it’s not surprising that connector development has long proceeded in lockstep with the anticipated need by new instruments to cover higher frequencies, driven in large measure by the instrument companies themselves. From Keysight Technologies (formerly Agilent Technologies and before that Hewlett Packard), Anritsu (and Wiltron before it), as well as Rohde & Schwarz have been some of the most active companies in connector development, and have been responsible for many of the advances in their technology.

Up in the Air

The airborne environment is possibly the most demanding for cable and connectors manufacturers because they must satisfy a long list of requirements specific to this environment as they are essential to ensure the safety of the pilot and the aircraft. It’s more difficult to protect cables in military and aerospace environments, starting from the basic coax structure and adding layers to maintain the impedance of the cable. Temperature stability over a very broad range of temperatures is another key requirement. Other protective measures include vapor barriers as well as different types of jacketing and abrasion resistance.

Another problem is that the instruments required to test the end-to-end continuity and other parameters of cables and connectors aren’t always available where needed. Even if they are, these systems can be very complicated, requiring someone with technical knowledge to operate them. In addition, larger flightline test systems are expensive so they cannot be deployed everywhere the Armed Forces needs them, and then there’s the time required to teach people how to use them. To simply the situation, test equipment can be reduced to creating “red light/green light” results, which identifies a problem and even where it is located, and possibly some insight into what’s causing it. As an example, consider the hypothetical scenario below.

As noted earlier, aircraft are complex, demanding platforms in which huge amounts of equipment is located throughout the airframe (Figure 2). So, let’s says that an EW, ECM, or communications system in a fighter aircraft experiences degraded performance. One of the approaches to this problem is to swap out LRUs and replace them (if spares are available). If this doesn’t work, and there is no damage to the antennas, what’s left are the huge lengths of cables and their accompanying connectors.

2008 connectors fig02

Figure 2 •  This is definitely not an environment conducive to finding and repairing or replacing a failing connector or cable.

The next step is to find the discontinuity through distance-to-fault detection, which is only possible with a portable (or benchtop) vector network analyzer (VNA) or a large, extremely comprehensive system like the Textron Joint Service Electronic Combat Systems Tester (JSECT). Measurements are made to determine where the discontinuity is located, which is often at a cable/connector interface, so the solution may be to restore the interface by removing a short length of cable and installing a new connector at its end.

Within the confines of an aircraft, this can be an excruciating experience because the problem area is buried within the airframe, making it inaccessible without removing panels to reach it. This and the repair itself require the skills of a spelunker, significant technical expertise, and surgical precision. With luck, it’s possible that the cable assembly is terminated using a field-replaceable connector, but that’s not always the case.

This would at least make the repair possible, as microwave cable assemblies are manufactured as a piece to ensure optimum performance at their cable/connector interface. There are also multiple protective layers surrounding the center conductor and possibly a vapor seal on a flight-grade assembly and establishing a connection with performance equal to the original is not trivial.

This is just one hypothetical scenario in which a cable or connector or both are the sources of a problem, but it illustrates the time and cost required to solve cable-related issues as well as why it’s essential to recognize that microwave interconnects aren’t simply commodities but precision components that can either make or break system performance. One particularly depressing scenario occurs when cables have been installed in an aircraft, tested before they go in, and after they’re installed reveal that the original or another problem remains.

Higher Frequencies, Bigger Problems

This situation isn’t likely to improve in the future, as systems move into the millimeter-wave region where components get smaller, more fragile, and more expensive. A traditional connector like the venerable TNC or Type-N is massive compared to a connector designed to operate at millimeter-wave frequencies where connectors can have a diameter of 1-mm, and recently even 0.8 mm. At these frequencies, even a tiny nick in a connector’s center conductor, barely visible to the naked eye, can degrade performance.

Millimeter-wave connectors exceeded the 100-GHz benchmark in 1989 with the 1-mm connector credited to HP. It is mode-free to 110 GHz and mechanically innovative, pushing the tolerance limits of fabrication technology because of its tiny dimensions (Figure 3). Another major development, credited to Omni-Spectra, is the blind-mate connector that allows an entire, multi-connector subsystem to be connected to another without the need for intervening cable, significantly alleviating interface issues.

2008 connectors fig03

Figure 3 • Pasternack’s Model 45403 1.mm removable end-launch connector operates to 110 GHz with a VSWR of 1.28:1 or less and insertion loss of 0.1 dB.

Having reached 110 GHz with a truly tiny connector body, the question for test equipment manufacturers was how they would meet the connector challenges posed by VNAs. Remarkably, that question has been answered by the 0.8-mm connector developed by Anritsu that is mode-free to 145 GHz and was created for use with the company’s latest VNA. The 0.8-mm connector is so small it’s almost impossible to see without a microscope but still big enough to be manipulated. The connector has an air dielectric front-side interface like K and V connectors with the center conductor supported by a proprietary, low-loss support bead on one end and a PTFE bead on the other. As the support bead is made of high-temperature material, it can survive exposure to 200o C for short periods.

The design challenge required the development of a proprietary bead to achieve good impedance matching, which with the 0.8-mm connector was necessary to achieve impedance control within 3%. What resulted is a connector whose mode-free performance is actually 170 GHz, although traceability can only be attained to 145 GHz because it’s designed to be used with coaxial cable. The task also required the in-house development of cables and adapters for calibration.

How Small Can They Get?

The late Bill Oldfield, one of the most innovative forces behind high-frequency connector development who worked for years at Wiltron and later Anritsu, stated in a conference paper that it should be possible to create even smaller connectors than 0.8 mm, and proposed 0.6-mm and even 0.4-mm connector interfaces.

Although these smaller connectors could theoretically be built, it doesn’t seem realistic because dirt or even dust too small to see would affect repeatability, and mating and unmating them would be a nightmare, under conditions similar to those in an operating room. And if concentricity cannot be achieved, neither can repeatability.

Such connectors would presumably be usable to hundreds of gigahertz, and while DoD has its sights on terahertz imaging and other applications, it’s inconceivable that connectors like this could be fabricated at a cost DoD or others could afford. Fortunately, at such high frequencies, where electromagnetic energy approaches the lightwave region, interfaces will likely disappear as highly-integrated SoCs including analog and digital functions from baseband to the output frequency will be used instead.

This is already occurring at much lower frequencies, and with the application of different semiconductor materials, coax and connectors will no longer be needed in this scenario. There are indeed waveguide designations into the terahertz range (WR-051), but even a WR-3 waveguide that covers 220 to 330 GHz has internal dimensions of 0.8 x 0.4 mm.

Conclusion

There is lots of spectrum between 145 GHz and 300 GHz, and DoD has its sights on it. This obviously requires instruments that can make measurements at these frequencies and although this is already possible using frequency extenders, all the test fixturing is in waveguide and is apparently likely to remain that way.

From a practical perspective, it appears that high-frequency connectors have been reduced so far in size that further reductions seem unlikely. Nevertheless, when DoD, the wireless industry, or some other formidable group requires higher performance, it creates the incentive for renovation.

About the Author

Peter McNeil serves as Marketing Manager for Pasternack.

November 2020

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