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NAB Engineering Handbook: VHF FM Transmitters

One in a series of short Q&As with authors of the 11th edition of the NAB Engineering Handbook

In a continuing series, Radio magazine is interviewing authors of the recently published 11th edition of the NAB Engineering handbook about their particular chapters. Jeff Welton contributed to multiple chapters, but we focus on 7.15: VHF FM radio transmitters.

Radio magazine: Jeff, at the beginning of chapter 7.15 you give a nice overview of vacuum tube transmitter technology. If you were to write an update of this chapter in 10 years, would you keep that overview, or let it pass in to history?

Jeff Welton: In the interest of full disclosure, I have been with Nautel for 27 years, so my exposure to tubes has been limited to what I see in transmitter sites that I visit, or what I have learned from others. With the permission of NAB, the entire tube portion of that chapter was a reprint from the 10th edition, written by Geoff Mendenhall � his description was one of the best I have ever read and I did not see any way that I could improve on it! To your question, especially in FM, there are still a lot of tube transmitters out there. While the grounded grid systems are slowly being replaced due to efficiency and cost of electricity, it�s safe to assume there will still be tube systems in place for a quite a while yet. I think that I would probably keep that overview in a 10 year update� but I would probably have to think much harder about that in twenty years.

Radio: If you were mentoring new broadcast engineers, would you take time to teach them about vacuum tube transmitters?

Welton: Again, tubes will be a part of a lot of facilities for several years to come � I think it would be doing a disservice not to teach the technology to new broadcast engineers, even if it is in passing. The funny thing is that I went to college in the early 1980s. My communications instructor in my final year told us, �tubes exist, be we are not going to spend a lot of time on them, because by the time you get into the industry, they will be almost totally replaced by solid state�. That was over 30 years ago�

Most importantly, new engineers need to be aware of the safety aspects of dealing with the high voltages that come with tube systems � this is something that requires a significantly different mindset than working with a solid state transmitter, where the highest exposed voltage is typically the AC coming into the power supply. Safety is something that is finally being discussed much more frequently, especially as we see younger folks with mostly IT backgrounds being tasked with RF engineering issues and as fewer engineers are doing more work with limited resources.

Radio: The key design element of any transmitter is the componentry used in the amplifiers. Almost everything else in the design accommodates those. Your chapter talks about those components extensively. Do you have any insight in to new components, headed our way in the not-too-distant future?

Welton: That is a good question � and as a manufacturer rep, something that I am a bit reluctant to answer (we try to keep the shiny stuff up our sleeve until it is ready to hit production!). I guess the best way to respond is to look at the past � 25 or so years ago, when VHF FETs became cost effective (relatively speaking) to implement, we saw solid state FM transmitters come into production. With the advent of HD Radio technology, developments in parts tech evolved that allowed us to handle the linear requirements of the digital signal with a reduction in the impact on efficiency that linear amplification tends to cause. Then five or six years ago, LDMOS raised the playing field significantly with respect to efficiency, to the point where we can now build an FM+HD transmitter that has comparable efficiency to a tube rig. A lot of these improvements were less with the amplifier technology, although that has helped for sure, and more to do with the software horsepower available to handle things like dynamic biasing, pre-correction and the like. This is where it is nice to have a relatively young engineering staff at the manufacturing level � the younger folks have exposure to newer parts and technologies, or sometimes the ability to look at something and use it in a completely different manner, to help these improvements occur.

The short answer is that yes, new parts will be coming, of that I am certain. As an example, I was reading an article on nanotubes not too long ago � and woolgathering about a completely software defined transmitter. Is that technology here now? No, at least not in any way that would be economical to produce, but it certainly would not surprise me to see it in my lifetime.

Radio: Aside from upgrades in components that have changed transmitter designs, the need for combined amplifiers for HD radio has changed designs as well. Do you foresee further improvements in efficiency in combined amplifiers in the near future?

Welton: It is kind of funny. In advancing the technology, we have kind of gone backward with combiner theory. In order to provide systems linear enough to handle HD Radio technology and for N+1 backup requirements, we have gone away from tuned systems, such as the 60 degree combiners that we used to use at Nautel, which were quite efficient, to hybrid combiners that are much less so. As computing horsepower improves and we have the ability to dynamically control phase and amplitude of individual amplifiers, I can see this improving, but the challenge of not having a single amplifier failure cascade through the rest of the system would have to be overcome. That is where existing technology shines � in providing isolation of each device from the system as a whole. Again, I hesitate to let any cats out of any bags with respect to what is coming in the near future, but yes, I believe it is safe to say that at some point down the road, there will be improvements to how individual amplifiers (or even entire transmitters) are combined. Physics still sets the boundaries, we just continue to find new ways to work within those limits!