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Solid State

The hot topic among UHF broadcasters has been centered on the latest and greatest super efficiency tubes. Leading the pack have been E2V and L3 communications, with the ESCIOT and the CEA (constant efficiency amplifier).

Despite the attention given to the race for higher and higher operating efficiencies, some manufacturers are bullish about where solid state transmitters fit into the future of over-the-air broadcasting.

At DMT USA, a long time European transmitter manufacturer now marketing in the U.S., Executive VP Tom Newman says, the reasons are time-proven. “One of the most compelling arguments for solid state is the redundancy built into these transmitters.

“If your remote transmitter site isn’t easily accessible, solid state becomes very important, because any device, or module, can fail but you won’t go off the air. And when a module goes down, the replacement costs won’t be anything like replacing a high efficiency tube!”

Understanding that no engineer wants to make constant trips up a mountain to replace even cost-effective modules, Newman is quick to point out another highlight of the LDMOS solid state devices.

“It comes down to what we call mean time between failure, or MTBF. The MTBF records on LDMOS devices are really almost unbelievable. Compare that to the last high efficiency tubes, where we’re still waiting to see how long they last.”
But even a staunch solid state supporter like Newman will admit that when efficiency is the driving force, solid state transmitters are operating closer to the old Klystron, or about 20%. But Newman and others insist that there are compelling tradeoffs, and they begin at the point where it makes sense to slide over from solid state to the new tubes, and that’s at between 15 and 20 kW.

Rich Schwartz, the Director of Marketing and Product Development at Axcera, understands both side of the choice, since Axcera is delivering both technologies.

“Solid state efficiencies right now are affected by all the combing that goes on between modules. It takes a lot of modules to get up to the higher powers, so efficiencies will be affected as the power goes up. Also, the multi-module solid state transmitters also use more power supplies.” You get ultimate redundancy, but you pay the price.

“One thing that seems to be important today, once you approach the 15 kW boundary, is that some stations do not have engineers who are comfortable with high voltages. Anyone can replace a module.

“And, if and when you lose one module, I doubt you’d see much difference in your Grade B signal. With a tube transmitter, it’s a whole different situation.”

And you can also take another look at operating efficiencies when you calculate the differences between cooling methods. Air cooling versus water versus oil. It does make a difference. As one director of engineering recently commented, “When we went to the higher efficiency devices, we noticed a big difference in our air conditioning bills”. With air cooling, the cost would have been even less.”

A few years ago, the industry was treated to a “pink elephant.” That is, a surprising technological development in solid state that would change the industry. But it just wasn’t quite ready for prime time.

That was the highlight of a special press conference to
announce the development of a Silicon Carbide device that could operate at higher powers and generate lots less heat. Successfully produced, it would have translated to higher power solid state transmitters running on fewer modules. Of course, that would have a direct effect on operating efficiencies. But, alas, the pink elephant is still caged, looking for a way to consistently produce high quality Silicon Carbide devices. Anyway, as the module numbers would have decreased, so would redundancy.

Meanwhile, transmitter manufacturers are looking at the newer LDMOS devices that are capable of higher powers than we’ve seen in the past. They aren’t exactly the promised land, but they are inching toward more economical transmitters with excellent MTBF numbers.

Ron Merrell is the executive editor.