Sophisticated Monitoring and Control System In St. Louis

Broadcasters know QEI Corporation as a manufacturer of both high and low power solid-state and single tube transmitters. But many are not aware that they were the first manufacturers to offer a fully integrated microprocessor remote control in their transmitters way back in the early 80's. A lot has changed since then. We all have learned to live with computers controlling virtually every part of our lives and certainly our transmission sites have not emerged untouched by this technology.

In recent years QEI produced mobile and transportable RF transmission systems that include as an option an intelligent monitoring and control system. This system was originally developed by QEI for industrial, scientific and medical (ISM) applications of its transmission products. The monitoring and control system is capable of making high-speed calculations on multiple RF output port loads and then addressing these changes by matching the output tuning to the changing condition. It was obvious to the QEI systems engineers that there was a broadcast application for this technology in sophisticated transmission system control and monitoring.

QEI was recently awarded a contract to apply this technology to replace an existing monitor system for an eight-station combiner and antenna in St Louis, MO. With over 2 Million-Watts of instantaneous peak input power into the antenna; speed and accuracy of control are paramount. The system design specification calls for continuous monitoring of each stations forward power and calculated VSWR into the combiner module. Further the system must maintain watch over antenna forward power, antenna VSWR, patch panel interlock status and Nitrogen flow rate. It must accomplish this in real time using a graphical interface to visually depict the status of the event, send appropriate alarms and take the corrective action. Additionally the monitor and control system must log data for each station, combiner and antenna at the occurrence of a critical change in status.

In the event of combiner or antenna component failure, the QEI monitor and control system applies priorities to algorithms for controlling the transmitter modes of operation. For example the following conditions could be monitored and addressed (see diagram for display):

Priority 1:

In the event top or bottom half of the antenna VSWR exceeds 1.3:1 a warning relay is closed to alert the stations. As VSWR reaches the critical set point of 1.5:1 all stations are taken off air for 5 seconds. The monitoring system is then automatically reconfigured to trip on reflected power and the transmitters are brought back on the air at half power. The system then waits 30 seconds for the transmitters to stabilize and sets a new algorithm for reflected power and maximum forward power. In the event they exceed these parameters all stations are shut down.

Priority 2:

If a station combiner module input VSWR exceeds 1.3:1 a warning relay is closed to the station. At 1.5:1 VSWR the system will take station using the module off the air.

Priority 3:

Should the top or bottom half of antenna forward power exceeds 115kW close a warning relay to all stations. At 125 kW forward power put all stations into half power mode. Wait 30 seconds. If forward power still exceeds 115kW take all stations off the air.

Priority 4:

If a module input exceeds 35kW on any station close a warning relay to station exceeding limit. At 40kW put station-using module into half power mode. Wait 30 seconds. If input power still exceeds 35kw take station using module off the air.

Priority 5:

Should the top or bottom half of the antenna loose Nitrogen pressure close warning relay to all stations and take all stations off air.

Priority 6:

When Nitrogen supply needs to be changed, close a warning relay to all stations.

This represents how a typical high power combining system can use this fault detection and corrective action sequence. Since the QEI monitoring and control software is uniquely adapted for each installation many other control and monitoring sequences are possible. Through careful analysis of combiner and antenna failure modes, a monitoring system may be developed to not only provide early warning of component failures but also can limit costly damage and downtime. The QEI monitoring and control system is designed to protect the station investment in a community antenna system.