In my last column I looked at DTV reception technology at NAB2002. This month I'll look at the other end of the path -- DTV transmission, specifically low-power DTV transmission.
At this year's NAB, many manufacturers were offering products that let broadcasters put a DTV signal on the air at much lower cost than a full-power installation. Broadcasters are also looking at using low-power DTV transmitters as translators or on-channel boosters for extending DTV signals into areas blocked by terrain or outside their primary station's coverage area.
DTV TRANSLATOR TRICKS
In many parts of the country, people rely on TV translators for free over-the-air reception. In Utah the FCC database shows more than 500 licensed translators. Kent Parsons, who has been a translator engineer at KUED, KULC and KUER in Utah for more than 40 years, is experimenting with different techniques for getting full-bandwidth HDTV signals from the Salt Lake City stations to the people served by these translators.
Kent described his experiments at the NAB Broadcast Engineering Conference. One of the things he found was that very low-power levels are adequate for reliable DTV transmission between mountaintop sites. This makes it practical to link low-power translators over great distances. In one test, Kent used three hops to get to a translator in Monroe, Utah. The first hop on that path was 83 miles. On multiple hops, a DTV channel processor providing error correction is essential. Remodulating equipment from Zenith was used for the tests. KTech offers a similar product.
Trading off power for bandwidth, Kent found he could combine two adjacent channel DTV signals and run them through one amplifier in a transmitter. This multiplexing can be applied to microwave links as well. One of the more interesting experiments involved using one microwave transmitter to carry three DTV signals. Kent took DTV signals on Channels 34, 35 and 36 and re-modulated them down to VHF Channels 3, 4 and 5, corresponding to a frequency range of 60 to 82 MHz. Most microwaves use an IF of 70 MHz with a bandwidth of 25 MHz. As you can see, this VHF channel combination nicely fits in the IF passband and yes, it does work!
Kent's main test equipment for this work was a simple spectrum analyzer. Looking at the spectrum display of a DTV channel, it is easy to see when amplifiers are being over driven -- most TV engineers are familiar with the out-of-channel shoulder re-growth that occurs in a non-linear system.
Kent's message to other broadcasters was that translators do work for DTV. The technology is available. When I wrote this, the FCC had yet to establish regulations in the low-power and translator sections of FCC Rules Part 74 for DTV operation.
In another NAB session, Sam Zborowski described a DTV booster system using two off-the-shelf Yagi antennas, TV channel filters from Microwave Filter Co. and a TV amplifier from Radio Shack. Sam pointed out that this simple configuration will not meet the FCC DTV channel mast requirements and, in his opinion, should not be required to. There is a need for inexpensive gap fillers and the FCC should provide a simple procedure for licensing them.
When designing low-power DTV translators or on-channel boosters, the types of antennas viewers are using and the degree to which they are willing to fiddle with them to get DTV reception has to be considered. The design can be simple if the targeted viewer is assumed to have an outdoor antenna and rotor. If the viewer is relying on an indoor antenna, higher signal levels will be needed. These considerations, of course, will apply to interim low-power full-service DTV as well.
DESIGNING INTERIM LOW-POWER DTV FACILITIES
If a low power DTV facility is to be more than an FCC channel holder, it is important to consider how much signal level your DTV audience will require. Although some stations are fortunate enough to have cable carriage for their DTV channel, many will not. Early adopters may go through the effort of putting up an outdoor antenna. FCC rules give viewers that right, overriding local restrictions on outdoor antennas. However, will many viewers fight their local homeowners' association or municipality for an outdoor antenna? If the viewer did not need an outdoor antenna for decent local channel reception, perhaps because they are receiving it by cable or local-into-local satellite, it is probably best to assume they will try an indoor antenna for local DTV reception.
ATSC planning factors dictate minimum field strength of 41 dBu for UHF DTV reception. The FCC increased this to 48 dBu for "city-grade" UHF DTV coverage. However, as more studies are done on indoor reception, it is becoming clear much higher signal levels are needed for reliable indoor reception. Increasing the field strength by 20 or 30 dB greatly increases the chance a viewer will have adequate margin on an indoor antenna system.
This year's NAB provided evidence many broadcasters are planning to build out low-power DTV systems until the number of receivers increases enough to justify the cost of a high-power DTV transmitter and, for UHF, the power to run it. Looking at the offerings from transmitter manufacturers, typical power levels for low-power DTV transmitters range from 100 W to 1,000 W. Antenna manufacturers were showing low-cost, low-power antennas for DTV. At least three manufacturers offer versions of the veteran Scala SL-8. The Scala SL-8 "Paraslot" antenna has a maximum gain of 11.4 dBd. Andrew has offered an AL-8 for many years and Dielectric just introduced the DL-8. (I like that model number!)
Combining the antenna gain and the transmitter output power and allowing for the loss in Heliax transmission line, the DTV ERP from a low-power configuration using an SL-8 type antenna will range from under 1,000 W to over 10,000 W. Is this enough?
To answer that, it is important to know where the viewers are located and what type of antenna they are likely to use. In urban and suburban communities, the signal should be sufficient to work with indoor antennas or small outdoor antennas. In rural areas, where viewers are likely to have outdoor antennas, field strengths in the 41 to 50 dBu range for UHF should be adequate. Therefore, it makes sense to look at several different field-strength contours. I suggest 41 dBu, 61 dBu and 71 dBu for rural, suburban and urban area reception, with antennas ranging from large outdoor antennas with preamplifiers to indoor bow-tie antennas. There is nothing magic about the 61 and 71 dBu levels -- I simply added 20 and 30 dB to the noise-limited threshold level to compensate for poor antenna performance and losses getting a signal inside a house. Some readers will argue these levels are too high and others will say even more field strength is needed.
Now that we've defined the desired signal levels needed at the receiving antenna and the effective radiated power from the transmitting antenna, the only thing left to consider is the location of the transmitting antenna.
I'll use a hypothetical low-power DTV station in San Antonio operating with an omnidirectional antenna and a gain of 10 dBd to compare various antenna locations at the three DTV field-strength levels listed. If indoor reception is an issue, locating the DTV transmitter closer to the urban center, or better yet in the northwest corner of the city, may provide better coverage!
|Site ||Coordinates ||Height AGL ||Line Loss ||ERP|
|Antenna Farm ||29-17-39N 98-15-30W ||450m ||460m-3" rigid 3.9 dB 40.7% ||4.07 kW|
|Northwest ||29-31-10N 98-34-15W ||80m ||90m - HJ12-50 1.31 dB 73.9% ||7.40 kW|
|Studio ||29-29-37N 98-29-53W ||45m ||55m - HJ12-50 0.8 dB 83.2% ||8.32 kW|
|Table 1 - DTV Antenna Sites|
We will compare a hypothetical low-power Channel 36 DTV facility with the center of radiation at 450 meters above ground (AGL) in the antenna farm at Elmendorf, a similar facility on a shorter tower at 80 meters AGL in the northwest part of San Antonio, and the same antenna at 45 meters on a STL tower at a studio location near downtown San Antonio. See Table 1 for details on the sites and ERP.
Coverage from the three sites was calculated using the 10 dBd omnidirectional antennas with a 1 kW DTV transmitter, with the ERP adjusted to transmission line losses. Add 10 meters to the height AGL for the transmission line length. HJ12-50 2.25-inch air dielectric Heliax was used for the two shorter towers. For the large tower analysis, 3-1/8-inch rigid line was used.
|Site ||41 dBu ||61 dBu ||71 dBu|
|Antenna Farm ||1,674 ||1,183 ||387|
|Northwest ||1,596 ||1,373 ||1,110|
|Studio ||1,468 ||1,134 ||557|
|Table 2 - Coverage inside FCC F(50,90) Contour Population from 2000 Census (thousands)|
Table 2 shows the population inside the 41, 61 and 71 dBu contours from each site. As you can see, the northwest site provides better strong signal coverage with some loss in rural coverage. The studio location, while not as good as the northwest location, still provides better 71 dBu contour coverage than the antenna farm location. Fig. 1 shows the population density in San Antonio and contours from the three facilities. The studio and STL tower contours are shown in red, the northwest site contours in brown and the antenna farm contours are in blue, with the concentric contours corresponding to the three field strengths.
I have to stress this is a hypothetical analysis! I chose gains and powers that make it easy to calculate the ERP. In designing a low-power facility, optimizing the antenna pattern and picking up additional gain will allow you to either increase coverage or reduce transmitter costs.
It is also worthwhile to take a look at shadowing. A suitable downtown site may not be available and, at lower height, shadowing from buildings and terrain will be an issue. When comparing facilities, remember that field studies have shown Longley-Rice when used with the OET-69 defaults can significantly overstate field strengths.
If you are planning a temporary low-power DTV facility, I urge you to look at all the location options available and consider the viewers you are targeting. You may find that a low-cost antenna on a 150-foot STL tower at your studio could outperform a similar antenna and more costly transmission line run at your main tower. Higher gain antennas will change the equation.
If you expect viewers to use outdoor antennas, unless you can find a nearby site that is in the same direction as most of the towers, an antenna farm tower will be the obvious choice, even if the field strength at the viewer's location isn't as strong. In some markets, where the population is spread over a wide area, or in two separate areas like Dallas-Fort Worth, a low-power DTV facility may not be able to provide adequate high field-strength coverage of both population centers regardless of where it is located.
Next month I'll look at what transmitters are available for low-power DTV operation and new options for high-power DTV.