Russell Brown /
02.01.2010 12:00 PM
Dielectric's Symphony
In multistation broadcast tower facilities, controlling and managing the complex RF system is critical.

Dielectric's Symphony RF control system allows multistation broadcast towers to control and monitor the transmitter outputs of every station, thus providing a comprehensive system for tower management. This system ensures the safety of tower workers and quickly switches to auxiliary antennas in case of damage to the main one.

Symphony replaces many custom, in-house built control or lock-out systems that used key locked switches and neon lamps to indicate the position of multiple broadcast transmitters' RF switches. These types of controls would prevent a transmitter from being switched back to an antenna that was being serviced, but they could not force stations to switch to the auxiliary antenna if the need arose in the middle of the night.

Why it is needed

The RF power meters and monitoring circuits of a transmitter are used to monitor and protect the transmitter and its immediate transmission system. In a single station site where the output of the transmitter directly connects to the transmission line on the tower and the antenna, this works fine. If a fault occurs somewhere within the line or antenna, the transmitter's virtual standing wave ratio (VSWR) circuits detect the increase in reflected power and cut the RF output of the transmitter. This works because of the direct connection between the transmitter's output and the final destination, which is the antenna.

When community antennas are used, this changes. In order to share a common antenna, the station's output must pass through multiple switches, channel filters and combiners to reach the community antenna. Now the power carried by the transmission line to the community antenna has the combined power outputs of all the transmitters feeding it, which results in much higher power levels than any single station would produce. The power levels and VSWR at these points become critical to all the stations connected to it. It becomes impossible for any station to monitor these parameters due to the isolation caused by the channel filters and combiners. Plus they would never see the combined signal of all the stations in the RF system.

A few years back, one tower with a community antenna system did not have any monitoring for the combiners and transmission line to the antenna. It wound up burning many hundreds of feet of transmission line due to an undetected fault. A system like Symphony would have detected the problem and shut down that transmission line, and the transmitter's outputs, before the loss of all that line.

How Symphony works

In order for the Symphony system to control the RF parameters of all the stations, it utilizes programmable logic controllers (PLC) technology, which is widely used in industrial control systems and is recognized as being highly flexible, easy to use and most importantly, reliable. PLCs have inputs and outputs that connect to various equipment. These are controlled both by the human operator and a defined set of rules, which will take action if faults are detected within the RF system. (See Figure 1.)

The PLCs are connected to the RF switches to control and monitor their position. RF power sensors are attached to probes on the transmission lines that monitor both forward and reflected power levels. Temperature is also monitored.

Symphony provides a graphical user interface for monitoring and controlling every part of the system. Touch screens are used in both the individual transmitter rooms as well as central control points, such as the tower manager's office or a common lobby of the tower's building where everyone can see the system as a whole. (See Figure 2.)

In the transmitter room

Each station gets a PLC box and a rack-mounted touch-screen controller. The LCD screen shows all RF switches used by that station and provides control of them. Direct control is accomplished by touching the image of the switch and then choosing its position. Or the system can be programmed to allow for simple on-screen push buttons that set all switches at once, i.e. TX to main antenna, to aux antenna or dummy load. In order to move the switches, there is an interface between the PLC in the room and the transmitter's interlock circuits to turn off the RF. The RF power monitors will not let any switch move unless that station's power has fallen below 500W. The PLC also has inputs for remote control of the switches and status outputs, as shown in Figure 3.

Some transmitters are now much more integrated with their RF switching and would basically fight any external system moving its switches. To solve this, Symphony can be programmed to send commands to the transmitter, letting it move the RF switches, and then the system monitors the switches to be sure they are in the correct position.

Due to its programmability, the system is flexible and can be adapted to almost any situation; the challenge is to plan ahead and know what you want it to do.

Heart of the system

The main controller is usually situated near the RF switches because this is where the bulk of the interface wiring is located. There are several PLCs, each monitoring and controlling RF switches, accepting input from power sensors that supply a 0V-5V output, as well as pressure sensors that monitor the transmission line pressure.

This main control station has a large touch-screen LCD monitor to allow for systemwide overview and control. This screen can be duplicated in several other locations around the tower building, providing complete control of the entire RF system for the tower.

The entire system is connected via Ethernet. If communications are lost for any reason, the system won't let the RF switches move for the affected section and will alert the operators to the problem. The interlocks to the transmitter will only open if there is a fault detected with the associated RF switch and not just on a loss of communications. This Ethernet/IP-based control allows the system to be remotely monitored via the Internet.

Symphony is a sophisticated single-to-multistation RF management system, providing centralized monitoring and control for the RF system from single TV or FM transmitters to towers that use community antennas.

Russell Brown is chief engineer at KMTP-TV in San Francisco and writer of Broadcast Engineering's “Transition to Digital” e-newsletter.

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