Expanding DTV coverage Part I
Apr 3, 2009 8:43 AM, By Russell Brown
Three basic systems are commonly used today to expand TV station coverage: translators, boosters and single-frequency networks (SFN). The reason for expanding coverage remains the same (increased viewers), but the areas of today’s expansion have reached into places that traditionally have not been of great concern. Now every neighborhood needs good reception for home viewers as well as for mobile viewers, who could be viewing in just about any place their cell phone works. It’s the engineer’s job to figure out how to do this without breaking the budget.
Coverage issues
Within any TV station’s coverage area, there are many places where the signal from the transmitter is either weak or totally blocked. This is caused by many different factors, but the main one is terrain shielding, which can be a mountains, hills or even buildings. While the station has no control over the terrain, it can take steps to ensure its signal does get through to these places. The “cliff effect” of DTV does not help in this matter, because it does not gradually degrade, but is either there or not there making reception more problematic.
Figure 1: A simple one-channel booster
Click to enlarge
Most stations have switched to the UHF band where these frequencies have an even harder time going through or around obstructions than VHF did. Even the FCC propagation curves F(50/90) are based on only 50 percent of the viewers getting your signal 90 percent of the time, which are not very good odds.
Just as there are several methods to increase coverage, there are sometimes more than one name for each of these systems. (To be sure everyone is talking about the same system, it’s best to actually describe what you want to accomplish.) We will be using the most common names for the various methods and systems.
Boosters
Boosters have been used in TV and FM broadcasting for many years. Basically, they receive an off-air signal, amplify it and feed it to a transmitter antenna. Boosters require shielding and are limited to very low power to reduce the possibility of feedback. The requirements of terrain shielding between the receive and transmit antennas can lead to the use of a separate feed from the studio to the booster to eliminate the feedback problem and provide for higher amplification. If the area to be covered by the booster is not itself completely terrain shielded, the boosted signal will act as a multipath interference signal to the main transmitter’s signal. The advantage of an on-channel booster is the low cost involved because it is merely amplifying the off-air signal. (See Figure 1 above.)
Figure 2: A dual-conversion heterodyne booster
Click to enlarge
Some boosters, known as dual-conversion heterodyne boosters, actually convert the received off-air signal down to IF and then back to the on-channel frequency. The advantage to these types of boosters is the ability to perform some signal processing on the received signal before broadcasting it, including SAW filtering and distortion correction. With this comes the ability to use a higher power output, but terrain shielding is still required. (See Figure 2.)
The most sophisticated booster is a digital signal processing (DSP) booster. These boosters are similar to the dual-conversion heterodyne variety but are capable of more sophisticated RF signal correction.
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