When approached to write this cover story 50 years on from Tore Anderson’s original article, which appeared in the very first issue of Microwave Journal, I agreed to write it for two particular reasons. First, as a Brit, it is a privilege to have the opportunity to acknowledge how well and professionally Microwave Journal has served the international microwave industry for the last half century.
Secondly, I am privileged to have had the honour of meeting Tore Anderson at European Microwave Week in London, England, in 2001. The event was particularly notable as it occurred only a few weeks after the tragic events of September 11. However, it is perhaps a sign of the man’s commitment, interest and downright defiance that well into his 80s he was prepared to travel to London, when men half his age were not. I cannot claim to have known him well, but he seemed a charismatic, distinguished gentleman, with a passion for his field of work.
He not only impacted on me personally, but also on the Cobham Group of which Credowan is a part. He was instrumental in setting up Hyper Technologies, our sister company, in Les Clayes Sous Bois, France. Anderson set them on their path to becoming France’s leading supplier of waveguide components for the space and defence industries, which they have been since the late 1960s.
This article will hopefully pay respect to and reflect upon his original article, offer opinions on some of the important developments since 1958, and gaze into the microwave crystal ball to see what the future holds. My 30 years of industry experience has been gained on this side of the Atlantic and although I shall acknowledge international developments the perspective will be from a European standpoint.
Ferrite Technology
Anderson’s 1958 article opened with a comment concerning microwave ferrite technology and it is perhaps unremarkable that 50 years later this technology is still used in waveguide and coaxial technologies for one of the most fundamental building blocks of microwave systems, the isolator or circulator. The power levels used in radar systems and satellite communications ease ever upwards, and with it the need to reassess structures and materials. However, due to its fundamental physics, ferrite technology has always been able to offer relatively straightforward tracking of these power levels to satisfy this market need, although I do not believe that there will be a major leap forward in the foreseeable future.
Test Equipment
The biggest single factor aiding the development of the microwave industry over the last five decades, with waveguide assemblies and components being no exception, is for engineers to have the ability to test what it is they are designing and making. Slotted line six-port reflectometers and similar highly complex test set-ups, which existed in the 1960s, eventually gave way to proper S-parameter measurements, using network analysers, originally supplied mainly by the Hewlett Packard Co. I remember the UK launch of the HP8510 network analyser, which, perhaps for the first time, offered a dynamic range, speed and multiplicity of test points that could finally accurately measure passive microwave components reliably and consistently. Until this point our industry had, in many ways, become bogged down with metrology issues, where microwave measurements were made using complex and difficult equipment manufactured to truly outstanding dimensional accuracies.
The reality, however, was that due to variations of temperature, etc., this equipment was not offering the kinds of consistency needed. Systems designers often needed to build in measurement error, as part of their system’s architecture. Temperature-controlled standards laboratories abounded in all major companies. There was a need to constantly verify and calibrate equipment, which was frankly often being used at the absolute limit of its accuracy level. The HP8510 broke this down. Figure 1 shows a 10-year old instrument still in use in the Credowan clean room, doing microwave testing on high dynamic range cavity filters for the Inmarsat satellite programme.
The HP8510 required calibration kits for both coaxial and waveguide use, which had to be manufactured to previously unprecedented high standards of accuracy. Instrumental in their development was a sadly missed character in our industry, Mario Maury, who drove the Maury Microwave Co. to becoming a world player in this field. The Wiltron Co., now part of the Anritsu Group, quickly followed with its 360 equipment, offering many of the features of the HP8510.
Fuelled by this healthy commercial competition, network analyser development has progressed. Some time early in the 1990s, Marconi Instruments in the UK shook up the market with the launch of the 6200 microwave test set. This was in fact a scalar system, but was very easy to use, small and compact, offering lots of functions. For a while it looked like the industry may route in that direction, with microwave instrumentation becoming lower in cost, more functional and easy to use.
It is fair to say that in 2008 test issues are much less of a problem. The days of disputes between design and test teams over the 10th of a dB are long gone. Disputes between suppliers and customers over whether a product may or may not be to specification are now mainly behind us. This in itself has generated huge improvements.
What About the Plumbing?
Tore Anderson, like me, was, I think, a waveguide man at heart. He covered plaster casting techniques in some detail, but I do not believe that this technology has made it to the 21st century. However, lost-wax casting, which was pioneered by MDL in the US in the 1950s and was brought to Europe in the 1960s by Christopher Shaw of MicroMetalsmiths, still forms the basis of a significant percentage of waveguide technology. These simple castings, which sell for as little as $20 to $30 each, are suitable for both flame brazing and dip brazing, two techniques that are used all over the world for the assembly of aluminium waveguide assemblies.
The second technology that could not have been envisaged in 1958 was the degree to which CAD modelling and advances in CNC machining would open up opportunities for producing parts machined from solid metal and then assembled either by the use of screws or dip brazing. The ability to electromagnetically model and programme accurately facilitates machined designs that can be near perfect. This has pushed the barriers, particularly for products such as microwave filters, to performance levels that were previously unobtainable.
Also, various companies around the world have successfully developed zero tuning screw technologies, enabling lower cost production. At the other end of the scale, many world leading filter companies are now producing waveguide filters with microwave performances way above what was possible with the previous generation. This advance in machining technology is illustrated in Figure 2, which shows a Triplexer machined to cavity tolerances of 0.0005 of an inch. This was something that was unachievable only 10 years ago.
This technology has also challenged an associated manufacturing technique, that of plating. Waveguide structures fundamentally improve their loss characteristic by some 30 percent if silver plated, when compared to unfinished aluminium or alochromed aluminium, a technique that has been known for many decades. The application of an effective silver plate, normally laid over a nickel undercoat, has become a great art. But the search for a consistent and electrically high quality silver plate is on and although current technologies offer excellent results I predict that both a cost and performance step can be achieved by the cracking of this particularly challenging nut.
Flexible Waveguide
Many in our industry might not appreciate that Airtron Inc., which originally started in New York State in the 1940s, pretty much invented the concept of flexible waveguide. For those non-waveguide specialists reading this article, flexible waveguide is basically one of two technologies. Flexible twistable waveguide pioneered by Airtron, and still being produced in significant quantities today by companies like Microtech in the US and Mitec in Canada, consists of a helically wound interlocking brass silver-plated strip.
This offers the customer not only the ability to twist the waveguide, but also to bend it in either the E or the H plane (narrow or broad wall). This product must be used with care, however, as over enthusiastic bending or twisting can lead to RF leakage problems. It is not designed for broadly dynamic applications, but more to take up tolerance, vibration and low level movements.
Pioneered to some degree since Anderson wrote his article has been so-called seamless flexible waveguide. This product cannot be significantly twisted, but again can, within reason, be bent within the E and H planes. This is formed from a tube that is then crimped to offer a similar looking profile to twistable waveguide. These products have been developed up to high frequencies. Figure 3 shows the wide variety of waveguide sizes now available in flexible waveguide, ranging all the way from 1.5 GHz, WR650 sizes, right through to 94 GHz, WR10.
Predicting the development of flexible waveguide is problematic, but I believe that the battle to consistently manufacture such waveguide above 18 GHz is not a ‘done deal’ yet. Products beyond 40 GHz are still far from economic and the phase stability of all flexible waveguide also needs improving. If progress is to be made, the next few years need to yield genuine progress in this area of waveguide technology. Integration of subsystems also offers a threat to the long-term use of this product, with integration removing the need for many interconnecting waveguide components.
Anderson also briefly discussed double-ridged waveguide, and it is perhaps worthwhile considering this topic in more detail. Double-ridged waveguide offers much broader frequency coverage than its rectangular waveguide equivalent. 7.5 to 18 GHz is covered by WRD750 and 18 to 40 GHz is covered by WRD180, which are two of the most frequently used sizes. While the microwave performance is not of the same quality as conventional rectangular waveguide, and the manufacturing challenges are considerably more difficult, these waveguide sizes are used for a number of key applications.
The most common application of double-ridged waveguide is probably the electronic warfare market where they are utilised for broadband jammer systems. Most airborne and naval systems have jammers as a fundamental part of their set-up. They are generally substantially full of waveguide components, waveguide assemblies, flexible waveguide, couplers, loads and transitions, all of which are double ridged.
The second application for these parts is frequency agile radars, although they are generally less common now than they were 10 or 15 years ago. The third and final use of double-ridged waveguide is for so-called ‘tri-band’ assemblies. This is where a single antenna can be pointed at three different satellites, working in three different frequency bands. Again, this is less popular now than it was perhaps 10 years ago, but remains a current application.
Circular Waveguide
Towards the end of Anderson’s article he talks about circular waveguide being, in his view, a component of the future. I am personally unaware of it being a significant option for most systems. One of the main reasons being that in many situations it is quite impractical, which is a problem when trying to design components. It also has serious performance problems when having to turn corners.
However, elliptical waveguide certainly still has a major part to play, particularly in the communications sector of the waveguide industry, but in general the integration of microwave radios and ever smaller and more co-located design options is reducing the overall requirement. Although circular waveguide is particularly useful for long straight lengths, in many ways the next technology I will cover has perhaps supplanted it.
Fibre Optics
The purists amongst you will wonder why I am writing about fibre optics in a waveguide article, but the reality is that for transmission of analogue or digital data, fibre optics has taken over a number of functions that previously would have been serviced by long waveguide runs. One element of this business is rotary joints, which is a technology I shall try to explain in more detail. Most radar systems have a 360° scan. In microwave terms, in order to join the parts on the rotating antenna to the parts on the ship or ground system which is not rotating, a rotary joint is required.
Waveguide rotary joints are generally constructed from a simple concept: a transition from rectangular to circular waveguide, a precision device that rotates one part against the other part, which, now being circular, can be done with almost no interruption of signal, and finally a transition back to rectangular. Most waveguide joints, however, are more complicated, in that they also need to transmit coaxial signals through. There are two options: to make the waveguide section more annular, thus surrendering the centre line to the rotating part/another technology (perhaps coaxial); or to position these elements down the centre of the waveguide and to compensate for the potential interruption.
Space: The New Frontier
When reviewing developments in the waveguide world, one cannot escape the fact that the dominant application for conventional waveguide assemblies has become, at least within Europe, the satellite industry. The year 1958 was just before the first unmanned satellite was launched, which I imagine had no waveguide fitted to it. The major breakthrough for the industry occurred in July 1962 when Telstar was launched. This offered extremely limited satellite capability; however, it is the first event I am aware of where a waveguide travelled into space. I believe it was a piece of WR229, but you may know otherwise.
November 1972 saw the launch of the first Anik A1 satellite over northern Canada that supplied telephone communications for the first time to a remote area, which would have previously been uneconomical. This heralded an era during the 1970s and 1980s when satellite technology gained momentum and progressed.
In the 1970s satellites would have offered 12 channel capabilities and incorporated around 50 or 60 waveguides. Modern satellites can have upward of 500 waveguides and can offer 64 high power communication channels with a great degree of interconnection and redundancy built in. The demands of quality and performance in the space industry are an order of magnitude higher than that in the defence industry, which has driven processes and design techniques ever forward.
The space industry, for instance, uses ultra-lightweight waveguide assemblies, with wall thicknesses as little as 0.4 mm, which is a quarter of the wall thickness of waveguide used by other industries. This offers a massive weight reduction, which is especially significant when you realise that each kilogram costs in the region of $50,000 to put into space. There is, however, no compromise on electrical performance, and with power levels in satellites now fast approaching 3 kW CW, waveguides will have an important role to play for a long time to come. Waveguide offers massive power handling and budget savings over the only sensible alternative, coaxial technology, at least at the frequencies of interest.
Computer-Aided Modelling and Design
When considering microwave technology in the last 50 years the impact of software modelling cannot be ignored. Ansoft launched HFSS in 1990, and while there are many competitive software programmes in use, in Europe, it is dominant. When launched, for the first time, it provided design engineers with the ability to quickly and relatively accurately model the structures within a few hours of the performance specification parameter being defined.
This meant that in the components industry, even at quotation level, an exercise lessening risk could be undertaken, which increases the reliability of timescales and costs. This eliminates from the loop many of the traditional risks to both customer and supplier. Figure 4 illustrates an early version of the HFSS.
More than ever software offers the design engineer the opportunity to get it right the first time. This saves machining time, prototyping and proving. In the current era perhaps the most exciting benefit of these programmes is the ability to export them into CNC programming, saving a huge amount of time. In my youth, scruffy bits of paper were generated by the design team, where hoards of draughtsmen (I was one of them) converted these into workable prototypes by pencil.
This task would often have to be performed many times before a unit was complete. These days design offices are under far more pressure to deliver to ever-tighter time scales and this is only possible because of improvements in the tools available to design. The future is attractive, as these software tools become more reliable and easy to use, bringing the concept of direct prototyping tantalisingly close.
High Frequency Applications
In 1958 the waveguide world stopped at around 18 GHz; now it is 100 GHz+. Applications for radio telescopy are at 77 GHz or above and radar systems are routinely working at 95 GHz, offering highly accurate and reliable short-range target identification. Only those of us in the microwave industry appreciate the ‘wow factor’ of buying a German car that has a small 94 GHz radar fitted to its bumper to aid parking. I suspect exploitation of these frequencies will keep us waveguide boys busy for many a year to come.
Final Thoughts
When attempting to look forward in any technology the probability is that you will get some things right and some things wrong. However, much of what Tore Anderson prophesied 50 years ago did come to fruition and continues to do so. The fundamental laws of physics surrounding the microwave industry do not change, but probably what keeps us all working away is the people, who, by and large, are kind, considerate and honest.
It is still possible to be successful by supplying components of good quality, and to offer friendly flexible services to customers. We are not yet totally driven by the pound, Euro or dollar. Those who carry a little influence should still work hard to keep the flame of enthusiasm burning, the same flame that I saw in a man in his 80s in London in 2001.
Nigel Bowes joined RF/microwave connecter manufacturer Sealectro, Portsmouth, UK, in a junior design role in 1977, progressing to a technical support role in 1979. Spells at TCE, Bradley Microwave and EEV led to him joining Credowan in 1990, initially as sales manager, then as sales director from 1995. Bowes was a founder member of the ARMMS RF and Microwave Society, the UK offshoot of ARFTG.
