Don Markley /
12.01.2007
Originally featured on BroadcastEngineering.com
Low-power transmitters
Stations need high-quality, low-power DTV systems.

Up to this point, most of the reporting on DTV transmitters and antenna systems has focused on equipment for the large, full-power DTV systems. That includes transmission lines, multiplexers, filters and the other big pieces of equipment needed for ERP values up to 1MW. Now that the FCC tables of allocations are almost complete, it's time to look at the smaller hardware.

The need for lower power

While there are more than 1700 full-service TV stations in the United States, more than 7000 Class A, LPTV and TV translator stations provide service to both urban and rural communities. The ERP levels on those stations vary from less than 10W for some VHF stations to more than 100kW, depending on class, location and protection requirements. In any case, virtually all of those stations are analog. In addition, several licensees and groups are trying to lift the current freeze on new stations. Many manufacturers are trying to move into that large market. They figure if the number of $1 million deals is shrinking, it's time to start doing a bunch of $50,000 deals.

Add to the LPTV and translator business a growing interest in SFNs, and the need for lower power transmitters becomes even more obvious. The SFN approach holds promise, especially in areas where the terrain is highly irregular. Better service can be offered through a network with lower powered transmitters and shorter towers as opposed to using a giant tower with maximum power that ends up creating holes in the coverage. Each of those networks can have as many as 10 or 12 transmitters in the system.

Therefore, it's obvious why transmitter manufacturers are introducing new features in equipment smaller than the 1kW range.

Transmitters

Larcan recently announced the MXi series, which provides output power of 10W to 350W. Fully solid state, the units are particularly aimed at the translator market, with optional receivers to make a full system for off-the-air reception and conversion to the correct output channel. The systems are all fully frequency flexible and field changeable. The MXD series goes up to 1kW.

Harris' Ranger line goes from 100W to 1kW, and its ATLAS line goes up to 13.5kW. Again, these are both fully solid state. One thing to notice about these systems is that the exciters are comparable with those used for larger DTV transmitters. This is important because the quality of the output signal for any transmitter is fairly fixed by the exciter. Obviously, this is greatly improved by correction based on the output signal quality from the transmitter. However, if the exciter doesn't have high quality, you can't fix it later. It's not like the old days when you could take a cable TV modulator, hang an amplifier on it and shove it out the door. The digital signal is a lot more demanding. Stations should look at the exciter first. If it isn't first rate, the station never will be.

BEXT's Lex100 has a power output of 1W to 1kW, with its next largest model ranging from 250W to 10kW. Many of the larger low-power transmitters grow by adding modules to the basic system. The better equipment allows hot switching or replacement of modules to permit maintenance work without shutting the entire system down.

Axcera offers the solid-state Innovator at power levels up to 350W and the Innovator LX at levels up to 3kW. There isn't a rational argument for using a tube-type device at the low power levels. Such tubes are becoming harder to find and to replace with domestically manufactured parts. An enormous amount of experience exists with solid-state modules at these lower power levels, so station technicians should not encounter any unanticipated problems.

Rohde & Schwarz's NH/NV7000 transmitter features liquid-cooled LDMOS devices. These are available up to 5kW. The company also makes the NH/NV7001, which uses fully air-cooled devices and is available from 75W to 300W.

Both Teko Telecom and Screen Service develop low-power transmitters and translators, although they call them transposers. Screen Service has a series of seven models, starting with a 5W exciter, which becomes the centerpiece for solid-state transmitters up to 8kW.

Lucid provides American Technical Services with low-power transmitters that can be configured as translators. These units are available in models up to 2kW for DTV.

DMT has transmitters at low and medium power designed for either LPTV or translator service. The power outputs range up to 3kW.

Thomson Grass Valley's Elite 100 transmitter goes to 1200W. The Elite 1000 is liquid-cooled with a power output of up to 4kW.

Antennas

As the allocations for the full-service TV stations become finalized, there will be a continuing need for LPTV and translator stations to change channels. One of the nasty things about most TV transmitting antennas is that, other than wideband panel antennas, they don't like being moved to different channels.

Scala's SL-8 omnioid is one rugged and inexpensive option. However, it is not available as a directional model. For the lower power stations, primarily translators, Scala has a range of antennas that are available in various powers and gains. Another antenna for LPTV is derived from the Bogner series of antennas, now available through RFS America.

Low- and medium-power TV antennas are available through Dielectric, ERI (which acquired the old Andrew line) and Jampro. Those systems are variations of the medium-power antennas used by standard broadcast stations. The three companies also manufacture diplexers and multiplexers for combining several LPTV stations into a single antenna. That type of operation can minimize the need for new tower space and new antennas. The multiple station operations most often use panel-type antennas. These systems feature power handling and bandwidth capabilities, but the panels often pose a problem for LPTV stations. The protection requirements for LPTV and translator stations are totally contour-based and are usually different for the various frequencies at a given site.

MCI's panel antennas offer design flexibility. As an example, one of the company's antennas has different beam tilt in varying directions to meet some unusual protection requirements. It also manufactures a manifold-type multiplexer for both low- and high-power applications. That type of multiplexer can be set up to allow stations to be plugged in as they come online rather than requiring everyone to buy in at the same time.

Conclusion

Now that the business is slowing down on the really big hardware, it is obvious that manufacturers are cranking out lower powered systems. The quality of the equipment is just as good as that for the big systems, which is a far cry from the way it was 30 years ago.


Don Markley is the president of D.L. Markley and Associates.

Send questions and comments to: don.markley@penton.com



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