Transmission & Distribution: Now less is more popular

As no real surprise to anyone who has not been asleep for the past five years, broadcasters have been having trouble meeting the FCC deadlines for DTV operation.
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Now less is more popular

By Don Markley

As no real surprise to anyone who has not been asleep for the past five years, broadcasters have been having trouble meeting the FCC deadlines for DTV operation. For reasons widely reported in the press, the Commission has permitted many stations to start operation with facilities significantly less than those in their allocation — either with an appropriate construction permit or, in many cases, under the terms of a special temporary authorization (STA) — while work continues to construct higher power facilities.

Today’s transmitters tend to offer more features than past versions. The time delay reflectometer feature on Axcera’s DT2B-based exciter, part of their DTVision signal analysis system, is a good example.

Stations are experiencing significant difficulties in financing their DTV operations and/or getting the new facilities built. In many cases, this still involves obtaining the necessary zoning for new or modified towers and antennas. As has been discussed earlier in these pages, communities are becoming more sensitive regarding any new construction. Increased protests by environmentalists and concerns about such problems as bird kills are making zoning problems seemingly insurmountable.

The temporary use of reduced power has been granted to many stations upon proper application to the Commission with suitable reasons for the need for such operation. Technically, the biggest requirement is showing that the temporary facility will provide adequate service to the city of license. Usually, that requires a simple showing that can be prepared by the station's consulting engineer. The signal strength is not simply that for noise-limited service, but has been established by the Commission to ensure that the urban areas are properly served. The service area for the station, as established by their allocations, will continue to be protected from interference temporarily while the station operates with the reduced facilities. However, that will not continue indefinitely.

The reduced facilities permit the station to get on the air less expensively and with equipment that is easier to install. First, the transmitter is typically one box containing essentially everything needed at the transmitter site. As one example, Harris introduced their Ranger series of UHF transmitters with DTV power output of 460 W or 900 W. While the output power is lower, the features are essentially the same as on their larger kin. For example, the new, smaller units use the same exciter as the larger transmitters. They also have complete circuitry for automatic correction of distortions including the mask filter. The mask filter is built in, which makes the whole installation simple. For remote monitoring, they have the same system of software as in the large transmitters. Finally, these systems can be integrated into a larger solid-state transmitter when higher power operation is desired.

Another example would be the DTValue system from Axcera. That system incorporates a Dolby 5.1 encoder, an SDTV encoder and a complete monitoring package into one cabinet along with all amplifiers and the mask filter. That system is available from 50 W to 500 W. Again, it can be expanded by adding cabinets up to 5 kW.

It would appear that simplicity and reliability have become the two most important characteristics of today’s run of transmitters.

Those two systems are included here as examples of what is available at the lower power levels. Similar systems are available from other manufacturers at competitive prices. Solid-state UHF transmitters are becoming popular for DTV use since the required power output is so much lower than for most of the NTSC operations. Of course, solid-state transmitters have been the system du jour for VHF operations for some time and continue their popularity into the DTV operations.

Low power isn't the only new area in transmitters. More features and greater ease of operation are becoming the rule. Again as an example, Axcera now includes a time delay reflectometer feature in their DT2B-based exciter system. That feature is part of their DTVision signal analysis system. The reflected signals are processed in the modulator, and a graph showing the location of the reflection source is displayed on the front panel without other external equipment. While not expected to be a rigorous transmission line and antenna evaluation system, it certainly adds to the information furnished to the operator above having only a return power meter.

For ease of operation, Harris has introduced a new exciter. The Apex is the latest in their series of exciters and is for any VHF or UHF channel. It is fully frequency agile without any channel setup. The most unique feature is that it has no user adjustments at all. Everything is software controlled, and the software is field upgradeable to comply with any necessary changes in the ATSC standards. Think of it — no user adjustments in a digital television exciter with full digital processing and corrective circuitry. We have come a long way from the time it took a technician a couple of hours to get the colors matched in a camera with lots of user adjustable controls. In a completely solid-state transmitter system, maintenance has come down to dusting, changing the filters and occasionally sacrificing a small animal or two.

With regard to higher power, which is still around, the tetrodes still are popular in the medium power ranges — although a lot of stations are going to a single IOT transmitter. While one may be tempted to cringe a bit at having only one high power device upon which the whole station depends, their reliability has proven to be extremely good with minimal downtime. In the larger markets where zero downtime is necessary, a completely redundant backup system is a reasonable solution to the problem, permitting 24-hour operation while allowing maintenance to be done in a non-panic mode.

The diacrode is still available and has found renewed popularity for medium power. In the air-cooled version, this device has been popular for 5 kW DTV and 10 kW analog transmitters.

Finally, for high-powered UHF transmitters, multiple IOTs still seem to be the preferred solution. Those devices continue to be upgraded and improved both in their linearity and dependability. They have become to UHF what triodes used to be to the AM industry. That is, simple and economical for reliable operation. It would appear that simplicity and reliability have become the two most important characteristics of today's run of transmitters. This may be because the technical staffs of broadcast stations have become smaller. Less time is available for work on the transmitter plant and, sadly, fewer people are around with the technical expertise to work on today's sophisticated equipment.

Today's technician must have far more training than in the days of simple vacuum tube transmitters. The systems simply are far too sophisticated to be worked on by marginally trained personnel. Troubleshooting a system with multiple microprocessors isn't quite the same as watching for a color change in a mercury vapor rectifier. The older readers should explain that one to the younger staff members, who will have no idea what it means.

Don Markley is president of D.L. Markley and Associates, Peoria, IL.

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