A new way to monitor DTV transmission line

To a broadcaster, the health of the transmission line, from the transmitter through the antenna, is a prime concern. The line, the antenna, combiners and other associated components carry high RF power and are constantly exposed to the weather and the seasons. The stress of this weathering and aging can eventually cause problems, so it’s important to monitor the line and the other components.

Currently available transmission-line monitoring systems are frequency-domain, VSWR-based systems. When such a system reports an increase in VSWR, say, from 1.10:1 to 1.20:1, the station cannot determine if the problem is in the transmission line, the antenna or the filters in the transmitter building. And detecting and locating problems in the transmission line is especially critical for DTV stations because they use different power safety margins than NTSC stations. The power levels that an NTSC station uses to specify its transmission system include both the peak-to-average ratio as well as the black-picture-to-normal-picture ratio. DTV stations have no such ratios and rely solely on the 10 percent margin of safety specified for all transmission systems.

This article discusses a new transmission-line monitoring method that allows DTV stations to locate a problem on the line to within approximately 48 feet. It uses the multipath-cancelling reference signal transmitted from the DTV station’s antenna, along with the adaptive-equalization filter in a demodulator, to estimate the location of anomalies on the station’s transmission line.

The concept

Figure 1. The basic components of the monitoring system include couplers, an attenuator, a tee, a demodulator and a PC. Click here to see an enlarged diagram.

To counter multipath effects, TV transmitters produce multipath-cancelling reference signals that receivers can use to detect and eliminate ghosts. NTSC transmitters send a ghost-canceling reference (GCR) signal; ATSC transmitters send a similar equalization signal. Inside the receiver, a demodulator with an integral equalization filter detects the reference signals. In ATSC receivers, a digital signal processor dynamically adjusts the taps in the equalization filter to compensate for detected anomalies. The NTSC GCR signal allows receivers to detect ghost signals that arrive any time from 3µsec before the main signal to 45µsec after it. DTV equalization has expanded the range of correction to a minimum range of -30µsec to +45µsec. Some DTV systems may have total forward and backward equalizer ranges greater than 90µsec. But, in all cases, the receiver’s equalizer filter contains taps 93nsec in duration.

Because the equalizer filter corrects for pre and post echoes, its tap values are a measure of the echoes (VSWR discontinuities) in both time directions. By connecting an equalizer-equipped demodulator to a directional coupler on the transmission line in the transmitter building, the tap values in the first 1- to 2µsec on the positive side will respond to reflection coefficients (discontinuities) in the line. Because the signal travels from the transmitter through the transmission line to the antenna at nearly the speed of light, 1000 feet of line represents approximately 2µsec of tap values. The coupler monitors the reflected signal in the line, which traverses a two-way path. Thus, the resolution from the 93nsec tap is approximately 48 feet.

Figure 2. This graph shows the effects of introducing a forward-measured signal into the reflected measurement. Click here to see an enlarged diagram.

Method of operation

Figure 1 shows the basic components of a monitoring system designed around this concept. At the heart of the proposed system are a Z Technology DM1010W professional ATSC/8-VSB demodulator and a proprietary, menu-driven, PC-based Visual Basic program. Two high-directivity directional couplers connect the transmission line and the demodulator. The coupler sampling the reflected RF signal provides the primary RF feed to the demodulator. For normal, faultless transmission-line operation, the reflected signal is the only RF signal required. But, to maintain resolution and avoid losing the reference-signal equalizer lock, a forward coupler sends a second RF source into the reflected sample signal at a reduced level. The system provides a graph of dynamic-equalizer tap energy, which is proportional to TDR-like discontinuities in the entire RF transmission-line system, from the directional-coupler sample point through the antenna. The Visual Basic program interrogates the tap data provided by the demodulator software and compares the measured values to a site-specific set of warning and shutdown alarms. An experiment performed to test this concept set couplers at -50dB with -44dB directivity in a DTV transmission line with 4250W TPO at channel D38. At the levels these couplers provided, the demodulator operated at an AGC level of 40dB in a range of 0dB to 60dB. For DTV TPOs higher than that listed above, attenuators are necessary for optimum performance.

Figure 3. This monitoring system’s Visual Basic display shows the data collected in the experiment used to test the system. Click here to see an enlarged diagram.

Figure 2 contains four sets of data and illustrates several features of the system. Note the large signal at tap 1 that serves as an equalizer-locking signal for the demodulator. The resolution in the horizontal axis of the demodulator system is 48 feet per tap. The distance resolution is related to the tap time constant. Tap 3 represents the tee combiner at the base of the tower. Taps 4 through 33 represent normal transmission line with two or three flanges and six or seven line insulators captured by the 48-foot tap interval. Those quantities of flanges and insulators will vary depending on whether or not the tap lines up with a flange. In this example, taps 34 and 35 represent a shared line-splitting tee that splits channel 13 from channel 38. The measured transmission path is the channel-38 path and, therefore, taps 36 and 37 represent the channel-38 antenna. These transmission-line discontinuities are different at each site. There also may be additional components and their associated discontinuities at other sites.

Figure 4. This system’s software allows the station to set and adjust alarm points and thresholds. Click here to see an enlarged diagram.

The four data sets in Figure 2 also depict the effects of increasing the forward signal content to the demodulator. Each subsequent data set shows the effects of increasing the forward component by 10dB. Since the equalizer locks onto the largest signal, this forward insertion level protects the equalizer lock by desensitizing the downstream signals.

The PC-based Visual Basic program retrieves the data shown in Figure 2, converts all numbers to their absolute values, and scales the graph to make the reference pulse 100 percent, as shown in Figure 3. The program also accepts a series of customizable warning and trip limits for numerous taps that identify special components, and another set of warnings and trips to assign to all remaining components on the transmission line. The program performs proprietary calculations on tap values and analyzes the result in an effort to find less-obvious sources of potential burn-up.

As stations convert to DTV and their income streams shift from NTSC to DTV, their need for pre-emptive maintenance will become more urgent to avoid costly shutdowns. The new high-power, DTV, real-time, TDR-like transmission monitoring system described here can resolve down to the component level. This allows a station to monitor flange joints, elbows, gas barriers and the antenna and to respond pre-emptively to the first signs of a problem, before destructive burn-up has started or progressed to completion. It can allow visual analysis when an operator wants to know how far down he should turn the power. Is 50 percent power going to be sufficient to stop the burning? Will 70 percent power be sufficient to maintain status quo until someone can do a visual inspection of the line? And, when the riggers and the field-service technicians arrive, they won’t have to guess where the problem is.

This monitoring system also can detect previously undetected burn-up situations. In recent years, at least two sites experienced burn-ups in the middle of a vertical run, and no alarms sounded until the burn-ups destroyed a length of line. It’s been theorized that, in these situations, the facilities didn’t observe large frequency-domain VSWR because much of the missing reflected energy was absorbed as heat at the point of failure. The charred remains acted as a load. The proprietary algorithms built into the Visual Basic program in the monitoring system described here can detect these types of failures.

Site-specific configurationFigure 4 shows the alarm levels a station can assign to elbows, transmission-line flanges, channel combiners, channel splitters, elbow complex and antenna. The reason for the differentiation in alarm levels is that a small change of VSWR at the antenna can be a normal change due to temperature or weather conditions at the antenna. As long as the VSWR returns to normal, no alarm will sound. But, even a small change in the enclosed transmission-line components might indicate a problem, so the system monitors them closely. The station assigns a specific tap point of the demodulator to each component at a site for a unique one-tap-per-one-component, site-specific layout. The station can target any site-specific component between the directional-coupler monitor point and the antenna for special attention. Users at the station can program and adjust the settings for alarm warning and trip levels. They can observe site-specific responses and adjust the proper alarm limits to allow for anomalies. And they can connect outputs to transmitter trip functions, remote monitoring or any other intranet-compatible device through a local intranet. When any of the alarms are triggered, the PC program alerts the station, the manufacturer and any site-specific contact personnel through modems or intranet.

The system can e-mail alarm or warning conditions to the appropriate person’s cell phone. Additional I/O for local control is available for transmitter interface, if required. Also, a station can use the monitoring system as an option in conjunction with many existing monitoring systems.

Customizing input and output

The station can set the program to retrieve data as frequently as every three seconds. The computer then will complete its analysis on all subsequent data and generate appropriate responses. For periods of time when the system notes no anomalies, the program can retain limited sets of original data while discarding excessive quantities of repetitive data. (It can generate more complete and continuous data for anomalous events.) The retained data is available for historical records, analysis and reports. In the event of an alarm, the program can generate two types of responses: e-mail and I/O alarms. The program can send e-mail notifications to all addresses listed for notification by modem or local intranet. Two types of e-mail are available: one suitable for cell-phone notification and another for PCs that includes more detailed data, including copies of the plots depicting the alarm condition. Stations that install this monitoring system as a stand-alone system might require automatic trips. For these sites, a USB connection can supply a suitable number of I/O to provide sensors and control. Stations that also have existing conventional monitoring systems with integrated control can use this monitoring system’s PC I/O to interface the two systems seamlessly so that only one of the systems reports alarms to the designated recipients.

Andre Skalina is vice president of research and development, and William A. DeCormier is associate principal electrical engineer, at Dielectric.