STRING, TACKS AND SEALING WAX: AM TRANSMITTERS OF THE FUTURE
There I was at the latest NAB exhibition, looking at the new offerings in AM transmitters from the various manufacturers, and thinking about how, in the last few decades, the Tx makers have taken all sorts of liberties with the way RF stuff is made, and how by and large they seem to have gotten away with it.
When I went to school (admittedly that was more that a little while ago), there was a great deal of stress placed on using non-ferrous materials around RF. Most everything was silver plated. There were absolutely no sharp edges anywhere. And it was all made to be 50-ohm, whatever that meant. The big transmitter makers of the day, for instance RCA and Continental, pretty much stuck to that, and they made a series of transmitters that worked the way we expected, and perhaps more importantly, they looked like we expected them to look. After a while, you grew accustomed to big silver plated coils and hardware, and neat silver-plated tubing carefully bent in smooth right angles. Everything built very big and very imposing-looking, and always with an eye to mechanical strength. It seemed to add a level of comfort to the inner Teuton in the average broadcast engineer. Certainly the right angle part did.
Well, I like to blame the next chapter of our story, if blame is the right word, on Nautel. It was Nautel that came along in the early 80's, and replaced RF connectors with barrier strips and crimp terminals. Nautel taught us that a couple of strands of hookup wire, twisted together inside of a piece of copper tubing, could serve as a very nice transmission line. Certainly, their AMPFET 10 transmitter, with its plexiglas front and it's relative dearth of meters, didn't even look like it was a transmitter. And so began what I secretly think of as the Home Depot era of AM transmitter design. Obviously some new minds, unencumbered by our old hoary broadcast engineering methods, were at work in the factories.
It's been a slippery slope since, as other manufacturers discovered that they could save a buck or two, or streamline production, or just mess with our minds by using "unconventional" techniques. The new Broadcast Electronics 50 kW AM is a sight: there's no big iron (it's all switching power supplies), and the control system is chock full of RJ45's and DB25 connectors to make the IT folks feel right at home. The real shocker, though, is the output matching network: multiple strands of smallish Litz wire, tywrapped together on a plastic frame to make a high-power coil. In lieu of a traditional rugged porcelain insulator with nonferrous hardware, a little strip of PVC plastic with a tywrap on top!
Not to be outdone, the folks at Nautel, in their new 50 kW rig, have replaced the homely coil with a rectangular design: one side of the rectangle is a printed circuit board, and the other three are formed by a bunch of parallel copper U-straps, spaced apart with Teflon tape. You tap the coil by pushing a wire with an automotive-style lug on to a mating contact on the PCB. Ahem, it brings a whole new meaning to the term "quadrature coil." (Sorry, very bad pun.)
I remember some bad jokes in the past about AM frequencies being so low that they're almost DC, compared to other bands in use today. Some had even hinted that, owing to their low frequencies, AM transmitters shouldn't be considered to be "true RF." Now the transmitter makers are systematically proving that, in many respects, that joke was always on us!