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!