In last month’s column, I showed that significantly stronger outdoor signal strength is required to overcome the attenuation by walls, non-line-of-sight paths and reduced height that affect indoor reception. I recommended aiming for calculated outdoor UHF DTV field strengths of at least 68 dBµV/m and ideally 88 dBµV/m. This month, I’ll discuss some ways to achieve higher DTV signal levels. These techniques work without extending the noise-limited contour, thus limiting the interference to other DTV stations.
For background information, read my September 2004 column “DTV Antenna Coverage Illustrated,” which outlines the trade-offs and shows how a station with a well designed antenna can come close to matching the coverage of a higher power station.
COMPARING CONTOURS
Fig. 1 shows the difference in signal level between two antennas, each with an omnidirectional azimuth pattern and an elevation pattern with a relative field of 0.9 at horizontal (zero degrees). The effective radiated power (ERP) for both antennas was set at 200 kW. The contours are very close, with the only variation due to the number of slots used to create the omnidirectional pattern. In an FCC filing, these antennas would be specified as omnidirectional with a relative field of 1.0 at all azimuth angles.
(click thumbnail)Fig. 1: These maps show the difference in signal level between two antennas.The antennas in the model were placed at a height of 2,000 feet on the Richland Tower in Missouri City, southwest of Houston. As you can see from the scale, calculated field strength ranges from 68 dBµV/m (gray) to 128 dBµV/m (red). The Longley-Rice calculation is based on FCC OET-69 parameters, with the exception that error code 3 is ignored and actual antenna azimuth and elevation patterns are used. RadioSoft’s ComStudy Version 2.2.14.6 was used to create the maps. Antenna patterns are from Dielectric’s DASP software.
The left map in Fig. 1 shows predicted coverage from a Dielectric TFU-26GTH-O6 antenna with 0.75 degrees of electrical beam tilt on Channel 48. Allowing for transmission line losses, 13.7 kW of transmitter power was required to achieve 200 kW ERP. The right map, showing a significantly stronger signal near the tower, was based on a Dielectric TFU-16DSC-O4(A) antenna with 1.0 degree of electrical beam tilt. Due to the lower gain of the antenna, 24.8 kW from the transmitter was required to make 200 kW ERP.
This example shows that more power into the antenna, with no change in ERP, will provide stronger signals. The challenge, of course, is making sure that stronger signal is where you want it.
Elliptical or circular polarization (CP) provide another way to put more RF on the ground. Circular polarization requires twice the transmitter power if the vertically polarized azimuth pattern matches the horizontal pattern. Antennas can be designed with a directional vertical azimuth pattern to put the maximum ERP over areas where indoor reception will be more difficult—in the city, for example—to reduce transmitter power requirements. The actual antenna used for DTV Channel 48 in Houston was built by Andrew (now ERI) and incorporates elliptical polarization with different azimuth patterns.
ARGUMENTS AGAINST CP
With the exception of increased transmitter power, many of the arguments against CP no longer apply for DTV. The 1960s view was outdoor antennas are all horizontally polarized, so why bother? Of course, a portable set’s antenna probably won’t be horizontally polarized. Furthermore, inside buildings and in urban canyons, where there is no line of sight to the transmitter, reflections will depolarize the signal.
The other argument against CP was it created ghosts in the pictures. For DTV, this is seldom a problem. Indeed, extra reflections may allow DTV reception in areas where reception would not be possible with a horizontal-only signal. Not only does CP provide up to twice the RF on the ground, it may allow reception in areas that otherwise wouldn’t work.
Improvements to the main transmitter site won’t help if the population you want to reach is shielded by terrain. Thanks to cable and satellite TV, this wasn’t an issue when the only screen broadcasters were concerned with was the one in the living room. However, as station owners start looking to business models based on reaching portable and handheld devices, we’ll be asked to provide a signal to these terrain-shielded areas. Fortunately, single frequency networks, also called distributed transmission systems, can provide a solution.
(click thumbnail)Fig. 2: These maps show coverage from KNBC-DT on Mount Wilson.Fig. 2 shows coverage from KNBC-DT on Mount Wilson. OET-69 parameters were used with the exception that error code 3 was ignored and the actual azimuth and elevation patterns were used. As you can see from the left map, field strength in the important residential area of Santa Clarita, northwest of Los Angeles, isn’t sufficient for indoor reception. Any station transmitting from the main Los Angeles transmitter sites faces the same problem. The map on the right shows the coverage after adding a synchronized transmitter with a panel antenna consisting of a 5-bay TUP-C1 panel antenna oriented at 15 degrees with 1 degree down tilt from a tower site near Santa Clarita. The dark blue areas show where the delay between the two transmitters
is greater than 40 µs and the difference between the two signals is less
than 20 dB. Viewers in these areas may receive interference.
I’ll provide more details on single frequency network design in future columns.
After the shutdown of analog TV in February 2009, if off-air broadcast TV is to remain relevant, reaching wireless devices, in the home, car or office, will be critical. Fortunately, broadcasters, and organizations like the Open Mobile Video Coalition, have recognized this. Our challenge, as broadcast engineers, will be to build DTV facilities that provide more reliable signals to more people in more locations than we’ve had to in the past.
Comments and questions are welcome. E-mail me at dlung@transmitter.com.
