DTV Antenna Coverage Illustrated
At its Aug. 4 meeting, the FCC set deadlines for TV stations to either build out maximized DTV facilities if they elect to stay on their current DTV channel after analog is shut down, or meet minimum coverage requirements on their current channel if they decide to move to another channel after the transition.
Although the commission has put a freeze on DTV applications that would increase a station's combined authorized DTV service area, antenna selection can have a major impact on signal level, even if an omnidirectional azimuth pattern is used and the DTV area defined by the FCC threshold contour remains unchanged.
Directional antennas can reduce transmitter power requirements and increase signal strength in key areas without increasing the service area.
This month, I'll outline some factors to consider when designing a DTV antenna. I'll also illustrate the effect antenna elevation pattern and beam-tilt has on coverage. You may be surprised at the difference in signal levels predicted using the antenna's actual elevation pattern and the FCC OET-69 default elevation pattern.
If equipment and operating cost is not a problem, a maximum power facility using a low-gain omnidirectional antenna in the area's main antenna farm is usually the easiest way to make sure you've given all viewers the best possible signal.
Most stations won't have this option, so before considering antenna and transmitter requirements, it is important to know where the viewers you want to reach are located. Be sure to consider the distance and depression angle as well as the direction (azimuth heading) from the transmitter site.
How many viewers will be using outdoor antennas? Many enthusiasts have gone to the trouble of installing outdoor antennas for off-air HDTV reception.
However, as DTV tuners become common in smaller screen sets, how many viewers will be willing to pay for an outdoor antenna installation or climb on the roof and do it themselves? Second and third sets are not likely to be hooked up to outdoor antennas.
Therefore, supplying sufficient signal strength to work with indoor antennas will be important as the DTV market matures. Putting most of the energy from the antenna on or near the horizon is seldom the best way to accomplish this.
A related question is how many people will want to watch DTV on the move--in the car or on a handheld device such as a cellphone or a mobile device such as in-car TV sets to keep the kids entertained or to obtain traffic/weather/emergency information?
The answers to these questions will determine the transmitting antenna characteristics and consequently the transmitter power requirements.
Many stations have outstanding construction permits for maximum-power omnidirectional facilities, especially if they had a maximum-power omnidirectional VHF analog station.ANTENNA SELECTION
However, for analog UHF stations using directional analog antennas in more densely populated areas, interference is likely to place constraints on the maximum DTV effective radiated power allowed in some directions.
In the first case, the only reason for using a directional antenna would be to reduce power costs or to optimize the signal strength in critical areas without increasing power costs.
In the second case, the use of a combination of electrical and mechanical beam-tilt can increase signal strength in specific areas between the horizon and the transmitter site. This technique is also useful when effective radiated power on the horizon has to be limited in one direction to prevent interference while maintaining a strong signal to a nearby population.
Some stations have made the mistake of installing high-gain UHF DTV antennas operating at maximum power without considering that the much narrower elevation pattern of UHF antennas will cause the signal to "overshoot" many viewers. Think of the typical low-elevation gain VHF antenna pattern as a floodlight, which illuminates a wide area; the higher gain, more focused UHF antenna acts more like a spotlight, which can be aimed at a distant point but won't provide as much light on the ground between the light source and the distant point unless it is tilted down somewhat.
As a result of this difference in antenna patterns, viewers closer to the transmitter site (greater depression angles) may get a better signal from lower-power competitors using low-gain UHF antennas, leaving the high-power station's management wondering why its DTV signal suffers even though the station is operating at full power.REALITY VS. FCC OET-69s
Conventional coverage maps are almost useless when evaluating coverage from high-elevation transmitter sites. The best way I've found to determine the impact of antenna design on coverage is to do a Longley-Rice analysis based on the actual antenna pattern, including electrical and mechanical beam-tilt.
Note that a coverage map based on an FCC Bulletin OET-69 won't provide an accurate indication of signal strength unless the elevation pattern of the antenna used matches the elevation pattern in the Bulletin. Bulletin OET-69 studies use one elevation pattern for all UHF antennas, another one for high-VHF antennas and a third for low-VHF antennas. That's it.
While the pattern is close to what stations on 1,000- to 1,500-foot towers in areas with flat terrain are likely to use, the OET-69 standard elevation pattern for UHF antennas is based on an electrical beam-tilt of 0.75 degree and does not reflect the actual elevation pattern used by many stations.
I chose the Mount Wilson antenna farm northeast of Los Angeles to illustrate the impact of antenna design on field strength. The four coverage maps are based on an omnidirectional azimuth pattern.
I studied two antennas, a Dielectric TFU24DSB-A with A 1.5-degree electrical beam-tilt and a Dielectric TFU-10DSC-O4 with a 1.0-degree electrical beam-tilt. The antenna was located on a hypothetical tower near the Mount Wilson Post Office with the center of radiation at 1,898.4 m above sea level, 152.4 m above ground. I used Channel 38 for the studies.
The maximum effective radiated power was 250 kW for the TFU24DSB-A and FCC antenna pattern and 50 kW for the TFU-10DSC-O4.
Fig. 1 (click here for figures 1-4
) shows the signal level predicted using the elevation pattern from FCC Bulletin OET-69.
The same scale is used for all cases. Note that the signal level is fairly uniform over the Los Angeles area. The signal drops off slightly in a band stretching from West Hollywood through the area south of Los Angeles and over to La Mirada.
Compare Fig. 2, which uses the actual elevation pattern of the antenna supplied by Dielectric, with the signal strength in Fig. 1. The band of weaker signal strength has deepened and moved north, closer to Los Angeles.
The predicted signal in the Pasadena area is much lower than shown when the OET-69 elevation pattern is used. On the other hand, signal levels in areas further from the transmitter site are stronger than predicted using the OET-69 elevation pattern due to the greater electrical beam-tilt of the real antenna pattern.
Fig. 3 shows the impact of adding 1.2 degrees of mechanical downtilt toward 200 degrees true. The signal strength dramatically improves in almost every area. There is some improvement near Pasadena, but the nulls in the antenna's elevation pattern remain visible.
The signal could be improved further by adding additional null fill to the antenna. I didn't spend any time trying to optimize the mechanical tilt angle; there is likely room for improvement there.
In Fig. 4, you can see how a low-gain antenna at lower ERP can equal or even outperform a more powerful station with a higher-gain antenna in some areas. This plot is based on a Dielectric TFU-10DSC-O4 with 1.0-degree electrical beam-tilt operating at only 50 kW effective radiated power, seven dB less than that used in the other figures.
Notice that in areas around downtown and north, the 50 kW low-gain antenna often provides equal or better signal levels than the 250 kW high-gain antenna without mechanical beam-tilt.
The plots were generated using Radiosoft's Comstudy 126.96.36.199. Actual antenna patterns were obtained from Dielectric's Antenna System Planning Program (DASP) Version 6, Build 3 and exported in Comstudy format. DASP has a very limited selection of patterns with 1.5-degree electrical beam-tilt. Greater electrical tilt may improve the signal strength. For the FCC elevation pattern, the default OET-69 pattern generated by Comstudy for FCC azimuth patterns was used. POLARIZATION
In tests I conducted more than 20 years ago, I found a location in San Francisco where the horizontally polarized signals from one transmitter site were almost completely canceled. Stations using horizontal polarization only couldn't be received. However, a circularly polarized station was receivable using a vertically polarized dipole. In another test years later, in a completely different environment, I found pine trees in a forest attenuated the horizontally polarized UHF TV signal much more than the vertically polarized signal.
Remember, the simplest omnidirectional antenna is a vertical, which requires a vertically polarized signal, either direct from the antenna or reflected and rotated from other objects.
If mobile or indoor DTV reception is important, it is worth considering adding some vertical polarization to your DTV antenna after you've maximized the horizontal effective radiated power. Even adding 15 percent vertical polarization could improve reception in some areas.
The benefit of circular polarization for low-VHF channels is controversial, with some stations reporting good results and others, as mentioned in last month's column, experiencing problems. With improved ATSC tuners, the added multipath that can occur when a vertically polarized component is added could turn out to be a benefit. CONCLUSION
The plots clearly show the effect of antenna mechanical tilt-and-elevation pattern on signal strength. If your coverage isn't as good as expected, it may be worth doing a signal-strength plot similar to these, using the antenna's actual elevation pattern and following up with field measurements to verify the locations of the nulls.
Add mechanical beam-tilt if needed to optimize coverage. When replacing an antenna, optimize both electrical and mechanical beam-tilt. If sufficient transmitter power is available, add some vertically polarized signal, especially if using a UHF or high-VHF channel. Your comments and questions on any RF topic are always welcome. Drop me an e-mail at email@example.com.