For the last four years, when the subject turns to RF, the chatter is automatically shifted to the latest information on high power UHF high efficiency tubes. Solid state transmitters aren’t in the spotlight because the emphasis of late has been on efficiency and how much money you can save using tubes such as the E2V ESCIOT and the L3 Constant Efficiency Amplifier (CEA).
Fact is, in some parts of the country, efficiency isn’t as important as it is elsewhere. That’s because the kWh utility rates are as variable as the weather.
As Harris’ Dave Glidden is quick to point out, the transmitter site (many of which are leased) is typically small. So the footprint of anything added to that space is a major consideration.
The typical 10-15 kW solid state rigs now take up less space and are extremely reliable. Glidden didn’t have the MTBF (mean time between failure) figures available when we were talking, but that’s probably because (as he admits) solid state transmitter modules take a licking and keep on ticking!
And, privately, manufacturers guess that we still don’t know what to expect in terms of MTBF for the latest and greatest high efficiency tubes. Speculation is that they should be as good or better than anything that came before, but that’s based on educated guessing.
Cooling Is Hot Cooling a transmitter has always been a crucial function of the television transmitter plant.
Choices abound: air, water and oil.
This is where engineering philosophy rears its head!
As Tom Newman of DMT USA (a company dedicated to solid state) puts it, “Water cooling is fine for high voltage, high efficiency tubes, but some engineers balk when the talk turns to water-cooled low voltage solid state transmitters!”
And then there’s the added choice of oil cooling. We hear comments such as, “Water doesn’t mix well with electricity!” “Oil can be a really messy experience!” The fact is, they all work, but RF broadcast engineers have always had their own ideas about what works best, when and where.
However, as Newman points out, when it comes to solid state, “The fact is that you can run more power with a water-cooled module than you can with air cooling. And that allows you to run two devices in one module, increasing the module output to well over 1,000 watts.”
He adds that, according to DMT USA’s calculations, you could run the same power in one water cooled rack that would require two racks of modules with air cooling.
At Axcera, Rich Schwartz acknowledges module power as an area of solid state transmitter developments that has been changing. “Whether or not people in the industry recognize it, we’ve been raising the bar.” But, he goes on to say that Axcera [along with other manufacturers] has been making great strides in the design of heat syncs. That ties directly to LDMOS design and manufacturing improvements.
And he makes an interesting point that a low voltage environment can also have an insurance advantage.
Diminishing Returns Since one of the major features of any solid state transmitter is its high redundancy, if power per module is pushed to the limit, the redundancy comfort factor is negated.
And redundancy goes hand-in-hand with diagnostics and remote control/monitoring.
At Larcan, Jim Adamson insists that Larcan’s solid state transmitter designs have always included diagnostic considerations, starting from the planning stage.
Acknowledging that the transmitter footprint today is important, he muses that if you use one power supply to power up two modules, what happens when that power supply indicates a problem?
You can look at virtually any device in the RF stream, he points out, but what you diagnose, how you back up the system, and how you control the system, comes down to engineering philosophy.
That being said, all solid state transmitter manufacturers agree that the smaller footprint solid state transmitters should be in the spotlight today. The RF site (even if co-located with the studio) is crowded and space is at a premium. At some sites, there’s an NTSC rig and the auxiliary backup, and now a DTV transmitter, so how do you find the space for a DTV backup?
The answers are as diverse as the engineers and directors of engineering who make the decisions for every station. No one formula fits all stations, especially when you add engineering philosophy to the mix.
But one thing is certain: if you haven’t been keeping up on what’s been happening in solid state designs because you’ve been distracted by the glitz and glitter of super high efficiency tubes, you may be missing the future.
Ron Merrell is the executive editor.
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