Transmission & Distribution: What's Happening Up There?

Traditionally, we monitor the television signal at every stage of the process from the initial generation point in the cameras until it leaves the transmitter itself.
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Traditionally, we monitor the television signal at every stage of the process from the initial generation point in the cameras until it leaves the transmitter itself. We then trust in the antenna to do its thing, hoping that all will be well and that the RF energy will leave the system somehow, please.

In older systems, the forward and reverse power was sampled, usually with probes in the transmission line in the combiner area. Those samples were detected and applied to meter relays. These were electromechanical devices that could be adjusted to a desired level. If the meter exceeded that level it would cause a contact closure that could be used to shut down the system. This was normally done to protect the transmitter in the case of a VSWR trip or in the event of the VSWR increasing gradually until it reached an unacceptable level, such as would occur if the antenna acquired a significant coat of ice. (As is known by most of the old-timers, some of the old antennas could accumulate enough ice around the feed points to cause the VSWR to increase to an unacceptable level.) Those devices did act to save the system but had no ability to sense minor gradual change or to predict future system faults.

The next level was the era of in-line monitoring equipment such as the Bird Watcher and similar items from other vendors. These systems worked very well within their design limitations. They were truly on/off type devices in that they could be set for a maximum forward power and a maximum reverse power. If either of those were exceeded, a relay closed, giving you either a contact opening or closing that could be used to shut down the transmitter. They functioned a lot like the old meter relays, but they added the service of measuring power continuously. They also added the capability to systems that didn't have the old relay capability, such as the old FM transmitters.

The next level occurred when the transmitter manufacturers started actually including more protection in their equipment. This included VSWR trip circuits and VSWR foldback circuits. The foldback circuits acted to reduce the transmitter power so that the reverse power did not exceed a preset level. Therefore, when the antenna iced up, the transmitter would simply keep backing off on output power to a point that no damage was going to occur to the system. In addition, transmitters came to be equipped with positive automatic power controls aided by microprocessors being utilized in the transmitter control systems.

On to the next level… There is a new Bird power meter, and similar systems are being prepared by other manufacturers. The new Bird system accurately measures average power and provides a digital data stream containing both forward and reverse power levels. This provides the basis for more complicated systems to be used, not only for remote control, but also for trend spotting. Furthermore, these systems provide accurate power measurement without field calibration or the need for a calorimeter other than the rather archaic provisions of the FCC rules. Hopefully, those rules will change someday and allow the industry to take full advantage of the accurate power metering capabilities that are currently available.

The next level leads to devices such as VSWR Vision from Dielectric. Obviously, if the device achieves the level of acceptance that is anticipated, there will be similar systems following from other vendors. This system is online full-time determining the VSWR and recording the VSWR values. Those values can then be downloaded to a central point where trend-spotting programs can be used to identify problems that are starting to occur in the transmission line. This system monitors VSWR incidents and records the time when they occur. The system will then also call designated parties as part of an alarm function to notify them of a VSWR event. With computer analysis of this data, it is possible to determine where the problem occurred and to have repairs effected before the line is totally burned up.

At the top level are complete integrated systems that are being built utilizing a multitude of devices such as the Bird power meter and/or the VSWR Vision. These large systems monitor forward and reverse power on multiple transmission line/multiple antenna/multiple user systems such as are being installed at major antenna sites. In addition to simply monitoring the RF levels on the transmission lines, these systems calculate non-ionizing RF levels in the vicinity of the antennas on the roof of a building or along the length of a tall tower.

The days of a simple prayer that the RF will go away from the antenna are over. We now have more than adequate systems that allow us to monitor the existing conditions, sense unacceptable conditions and provide good prediction capabilities to avoid failures and future downtime. It is much better to schedule the replacement of a failing component than it is to have it occur during a high-income event. No, it never happens when those commercials are scheduled. Downtime will always occur when revenue is at its highest and all the station brass are touring the transmitter plant. That is, if anybody in the station brass actually knows where the transmitter is located.

Don Markley is president of D.L. Markley and Associates, Peoria, IL.

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