Last month I described revisions the FCC was considering to its Longley-Rice model for evaluating coverage and interference. In it I noted that the commission was looking at a way to more accurately account for the lower antenna elevation gain and increased antenna beam-tilt often used by LPTV stations. This month I'll use two recent examples of DTV-to-analog interference to show how I analyzed the interference and determined some possible reasons how it could happen, even though in both cases the DTV station was operating near or below the minimum FCC-allocated UHF DTV power and the current FCC OET-69 model showed little or no interference. As you may suspect, antenna elevation patterns are involved.
One case involved a DTV station in one market interfering with an analog station on the same channel in an adjacent market. Interference between stations in these two coastal markets is not uncommon. In the other case, a viewer sent me an e-mail saying a station's adjacent-channel DTV station was interfering with its analog signal, which I've found to be a much less common condition. I won't publicize the three stations here, but if you have a specific case you are interested in drop me an e-mail.
What caused the interference? The viewer noticing the adjacent channel interference thought it might be caused by out-of-channel energy in the station's DTV transmitter or overload in his TVset. Either could be the cause, but if the station's DTVtransmitter was operating in accordance with FCC rules, any out-of-channel energy should be minimal. Most modern DTV transmitters not only include a filter to reduce spurious emissions, they also have precorrection circuits built into the DTVmodulator to reduce intermodulation products, sometimes automatically. If we assume the transmitter is clean, then the problem has to be with the receiver.
TVsets include a filter designed to let only one channel through the intermediate frequency (IF) amplifier(s) to the video and audio demodulator. Cost and size limit how selective these filters can be and, as a result, some of the adjacent channel signal can make it through the IF to the demodulator. Since the DTVsignal appears as noise to the analog demodulator, the signal appears noisy. In some cases, it is even possible to see the DTVfield sync region in the picture. I've noticed noise from an adjacent DTVchannel on analog signels quite often with my small Casio handheld TV, even when the ratios are well within the OET-69 limits, but haven't noticed it on larger sets. The small handheld TV doesn't have the IF selectivity the larger tuners have.
Another way adjacent channels can cause interference is through reciprocal mixing. In this case, if the oscillator in the tuner used to convert the incoming VHF or UHF station to IF has a lot of noise -- which would appear on a spectrum analyzer as sidebands or a raised noise floor around the oscillator frequency -- these sidebands or noise can mix with the adjacent DTVchannel and put the noise in to the same IF spectrum as the desired analog signal. Brute force overload, where the incoming signals are greater than the RFstage of the tuner can handle without distortion, is also a possibility. Overload interference I've seen was either caused by antenna preamplifiers or excessive gain in master antenna distribution systems. ATTCtesting found that if both signals are very strong, DTV-to-analog ratio does not have to be as large to have interference.
DON'T BLAME ANALOG
Analog TV tuner performance should not affect co-channel interference. In this case, the DTV signal simply adds noise to the analog signal. As a result, an analog signal receiving co-channel DTV interference will look like a weak signal to most viewers. In some cases I have been able to make out a pattern in the interference on some sets that's slightly different than the background noise present on a weak signal.
In both cases, the ratio of the undesired DTV signal to the desired analog signal is what will determine if interference is visible. In the co-channel interference case I referred to earlier, the effective radiated power (ERP) of the DTV station in the adjacent market just over 50 kW and the ERP of the analog station was 2,510 kW. In spite of this power difference, the analog station was experiencing interference well within its B-Grade contour. In the adjacent-channel interference case, the DTV station appears to be operating under an STA at slightly over 35 kW and the analog station was at 5,000 kW! In the case of the co-channel station, the analog (peak) to DTV (average) ERP ratio was 16.7 dB. In the adjacent channel interference case, the ratio was 21.5 dB.
In the early stages of DTV planning, ATTC studied DTV-into-analog interference and as a result of the testing was able to recommend desired-to-undesired ratios above which interference should not result. The FCC adopted specific ratios in OET Bulletin 69. If the adjacent DTV channel is above the analog channel, its signal at the receiver should not be more than 12 dB stronger than the analog signal. If the DTV channel is below the analog signal, the ratio is 17 dB.
As you can see, the DTV station should not have caused interference to the adjacent analog signal, since both antennas are at the same location and path loss should have been similar for both the analog and DTV signals.
I don't have the coordinates for the location of the person reporting the adjacent channel DTV interference, but knowing the market I would guess he was close to mountaintop transmitter sites next to the city. As you may recall from last month's column, the elevation pattern can have a huge impact on ERP (and thus signal level) close to the transmitter site.
In this particular case, looking at the antenna model numbers it appears the station was using an eight -- bay antenna at a low elevation for its low-power DTV and a 28-bay antenna at substantially higher elevation for its high-power analog signal. Using the maximum height above average terrain shown in the FCC database, viewers 5 km or less from the transmitter site will be at a depression angle greater than 5 degrees from the analog antenna. Even with the electrical beam-tilt of 0.7 degree, at this depression angle the relative field of the antenna will be about 0.1, equating to a loss in ERP of 20 dB.
The eight-bay antenna was mounted about 200 meters lower on a short tower at the same site, so that the depression angle to a viewer 5 km away is less than 2 degrees. Looking at the elevation pattern for a typical eight-bay LPTV antenna, at 2 degrees the relative field is more than 0.95, resulting in an ERP loss of less than half a dB! Therefore, at 5 km the ERP of the analog station will be 20 dB less than 5,000 kW, or 50 kW. The ERP of the digital station however will still be close to 35 kW.
This still allows a 13.5 dB margin, which should preclude the possibility of DTV interference, but this margin will decrease as you move closer to the transmitter site. Variations in azimuth patterns and in the actual elevation pattern of the antenna can decrease the margin. Also, as noted earlier, some analog TV tuners are more susceptible to adjacent channel DTV interference than others. On my handheld Casio, upper adjacent DTV interference is slightly visible even when the peak analog signal is 10 dB above the average DTV signal.
The lesson here is that when designing an interim low-power DTV facility, evaluate interference ratios using the actual antenna elevation patterns, not the default FCC elevation pattern. Also, consider that some TV tuners may be more susceptible to adjacent channel interference than others.
FCC OET Bulletin 69 states a co-channel analog signal must be 34 dB stronger than the DTV signal for no interference to the analog signal. In the case of the co-channel stations, the two stations are separated by almost 200 km. If we assumed both stations transmitted a spherical pattern (maximum ERP in all directions), for interference-free reception the path loss from the DTV station has to be 17.3 dB greater than that for the analog station, even given the large difference in ERP.
In this case the DTV station was also using an eight-bay antenna with a broad elevation pattern. The FCC database doesn't contain any data on the analog antenna that would indicate the elevation gain, but at this power level I would guess the station is using at least a 24-bay antenna. Note that in all these cases I'm assuming the stations are using slot antennas and by "bay" I'm referring to the number of stacked slots or the approximate elevation gain of the antenna.
Both antennas are located at sites high above the communities they serve. The analog antenna includes mechanical as well as electrical beam-tilt. Although it is possible one of the sites where interference was a problem is slightly above the main beam of the analog antenna, the interference does not exist all the time, so it is not the primary cause.
In this case, the answer, as you might expect, is coastal ducting in an inversion layer between the DTV site and the sites where interference occurred. This has been a common problem in the area. As I explained in my October and November RF Technology columns last year, ducting can propagate signals with less path loss than free space! During the times interference was noted, an inversion was present over at least part of the path. Add in some additional loss due to terrain interference with Fresnel zone clearance and a slight loss of analog ERP due to the amount of mechanical and electrical beam-tilt and it is possible the DTV station could pick up some of that 17.3 dB advantage in path loss, even though at one of the interference spots the distance to the DTV station was 60 km more than the distance to the analog station.
CHECK THE ANTENNA
How can this interference be eliminated? In this case the answer isn't clear. ERP reduction on the DTV station isn't practical, since it is already near the FCC minimum. Although a broader elevation pattern or less beam-tilt may help on the analog side, it is not likely to make a significant difference. However, it may be worth checking the antenna to make sure the mechanical and electrical tilt is as specified (modifying coastal weather is difficult). In this case, my recommendation was to increase the elevation gain of the DTV antenna and incorporate more beam-tilt, electrical and mechanical. The broad elevation pattern of the eight-bay antenna will likely couple more energy into the duct formed by the inversion layer than a higher gain antenna with a more directional elevation pattern. However, other stations with high-gain antennas and significant beam-tilt at the same location still propagate into the adjacent market through ducting, so it isn't certain this would reduce the DTV signal level 140 kms away enough to eliminate the co-channel interference.
If there is a resolution to either of these cases, I'll let you know in a future column. Meanwhile, your comments are always welcome. Drop me a note at email@example.com
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