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Transmission & Distribution: More thoughts on DTV transmitters

By now, every transmitter technician and chief engineer who passes the basic broadcasters' physical exam (can hear thunder and eat mush) has gazed at the current crop of DTV transmitters. This has been done with emotions that varied from fear to outright lust, depending upon the degree to which a new challenge appeals to each individual. Because that first exposure has past, it may be time to sit back and think about just how to make that decision as to which transmitter to buy.

First, the power needed has probably been determined based on a combination of the allocated power, the facilities sought through maximization, and the antenna system initially selected through discussions with the station's consulting engineer. The proposed power level will have a great deal to do with the transmitter choice, especially when choosing between solid-state and vacuum technology. If the station will be running a megawatt from an omni-directional antenna, the cost of solid-state transmitters will probably be prohibitive. On the other end of the scale, solid-state is an excellent choice for transmitters at or below 5kW average power.

Let's look at what is offered. You've got your air-cooled solid-state boxes, your liquid-cooled solid-state boxes, and you've got your big IOTs cooled by liquid, vapor or air. You've got your short tubes that are liquid cooled and your more conventional looking tubes that are cooled by air. You also have some variations of klystrons combined with tubelike parts. The sum of it is that they all work - they make RF to be delivered to the antenna. To the author's knowledge, there really aren't any transmitters from the better-known manufacturers that could be considered to be really bad. That being the case, there are still a number of things to be considered that should influence your purchasing decision.

The obvious first consideration is performance. While all the accepted transmitters will make RF and will look good into a dummy load, just what has been done to make them perform at their absolute best? For example, much has been said in DTV discussions about the group delay inherent in the antenna, transmission line and, in some cases, combiners. It isn't enough to simply correct for distortions in the driver and amplifier stages. A complete system should also provide correction for distortions to the signal that are introduced by the radiating system. If this is not done, the perceived bit-error rate at the receiver is increased, which lowers the margin before the signal is lost and, in reality, reduces the effective service area. Some exciters provide such correction capability. Unfortunately, they are not the least expensive on the market but, the extra cost may well be acceptable if the overall service is improved.

Almost everyone now has full-blown computers buried inside the transmitter to control everything from system monitoring to signal correction circuits. What happens when one of those computers develops a little glitch and needs to be rebooted? Everyone who has spent any time at all with a computer knows that they all hiccup on occasion whether caused by voltage transients, improperly applied fingers on the keyboard or an incorrectly read file from the hard drive. In such instances - either the old CTRL-ALT-DEL combination is applied or a full push the power button shutdown is needed to restore sanity to the computer. When such reboots are needed for the computer in the transmitter, will the system stay on the air? Will the correction settings be lost and, if so, how quickly will the system return to a properly corrected state? The transmitter itself should stay on the air and provide reasonable quality RF during such control problems or even during failure of the computer. The number of little systems in the transmitter that can take it off the air should be as low as possible for maximum on-air time.

One really big area to investigate is just how well the transmitter will be supported by its manufacturer. First, does the manufacturer provide a school for the model of transmitter you will be using? Is that school actually taught by knowledgeable engineers or by some technician that isn't needed elsewhere? Do the training materials adequately cover the circuits in the transmitter down to the component level? After all, it should be possible for the transmitter staff to repair most of the circuit boards, not simply swap them out for new ones. All of the transmitter crew should attend the school. That will probably require attendance at more than one session which, in turn, means that the school needs to be offered more often than once per year. Last but not least, the cost of the school should be included in the transmitter price.

The next item of customer support is actual technical assistance and parts availability. No manufacturer is worth a tinker's dam without 24-hour-a-day, 365-day-a-year technical assistance. If you can't call at 3 a.m. from a mountaintop in West Elbow, MT, and get someone on the line in a reasonable time that can provide you with real help, get a different manufacturer. It's as simple as that.

That leads to the next phase, which involves repair parts. Everything considered a reasonably necessary repair part in that transmitter should be available for immediate shipment to get you back on the air as quickly as possible. If that is not the case, look for a different manufacturer.

In addition to the obvious consideration of the initial cost of a transmitter, what will be the operating costs for the system? That involves the expected life of the amplifier devices with their cost, the expenses for other consumables like coolant and the amount of power that the system will consume from the line. It should be possible to come up with a fairly accurate budget to determine just how much the system will cost the station on an annual basis and to use that information with the original price to determine the actual cost of generating RF.

Finally, the manufacturer should willingly provide you with a list of users of the model of transmitter you are considering. Call a few stations and talk to the chief engineer about his experiences. Was the transmitter reasonable to install, how reliable has it been, what have been his experiences when dealing with the factory, what problems has he encountered and would he buy the same box again. There is an old saying about asking someone who owns one. It certainly applies to the biggest single piece of equipment at the station.