Since the invention of the transistor in 1947, solid state electronics have gradually displaced vacuum tube designs in most electronic applications. However, one area where vacuum technology remains unsurpassed is in the generation of microwaves - from microwave ovens to high power radars.
High power, high frequency radar transmitters use travelling wave tubes (TWTs) to amplify the radio frequency signals in today's high performance radar systems and satellite communications, where they are unequalled for reliability and efficiency.
Invented in the same decade as the transistor, TWTs have steadily become increasingly sophisticated. They can now achieve efficiencies of over 40% in S band radars, compared with less than 20% for solid state transmitters. In X band, the difference is even more marked, with TWTs being nearly three times as efficient as solid state systems!
The performance of modern TWTs, in terms of pulse fidelity, reliability and durability, has led them to populate the world's satellite communications networks, and they are at the heart of practically every state-of-the-art military radar, electronic warfare and complex weapon system in service today.
According to Howard Smith, engineering director at UK microwave technology specialists TMD Technologies: "At any serious power and/or frequency, TWT systems can always outperform solid state systems."
The key weakness with solid state transistors in high power microwave generation is the poor efficiency, which causes high thermal dissipation, and thus high junction temperature. The high junction temperature degrades the reliability, and variation in junction temperature degrades the RF performance.
However, these limitations with solid state radar transmitters are not always recognised. According to TMD Technologies chairman Peter Butcher: "We are not opposed to solid state in microwave; in fact we have designed a number of solid state amplifiers which are operating satisfactorily around the world. But for high power, and frequencies above 3 GHz, the benefits of TWT systems in terms of performance and reliability far outweigh solid state alternatives."
"Our task now is to demonstrate this to the marketplace, which over the years has been misled into believing the opposite is true."
The high efficiency of TWT systems is not just an advantage in its own right, but also leads to higher reliability and longevity. The energy saving also has the increasingly important advantage of having a lower carbon footprint, where the difference in efficiency is as significant as between a conventional incandescent lamp and a modern low energy light bulb.
Date: 7 July 2008
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TMD Technologies Limited, Unit 3, Swallowfield Way, Hayes, Middlesex, UB3 1DQ, UK
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