Nautel XL12 Technical Instructions

Contents

12 000 WATT AM BROADCAST TRANSMITTERXL12Page 2-201 January 20022.1.2.1 Station Reference Ground: The site mustcontain a station reference ground, as defined inNautel's Lightning Protection for RadioTransmitter Stations booklet. This ground mustprovide a continuous, low impedance path to theearth. The transmitter cabinet's designatedreference ground point, the shield of the coaxial feedcable and the ground connection of the powersource's surge protection devices must be connecteddirectly to the station reference ground.2.1.2.2 AC Power Source: All conductors fromthe AC power source should be protected bybi-directional surge protection devices that areconnected between each conductor and the stationreference ground. In addition, the conductors shouldpass, as a group, through a ferrite toroid. Theinductance formed by this toroid will be transparentto the AC voltages but will present an impedance totransients originating in the power source. A surgeprotector panel, that contains suitably rated varistorsis available from Nautel for this purpose. If used, thesurge protector panel should be installed in closeproximity to the station reference ground.NOTEThe AC power source usually presents the lowestimpedance path to ground potential for alightning strike and will normally carry most ofthe lightning induced current away from thetransmitter site.When lightning hits the power source, asignificant amount of induced current may flowtowards the transmitter. In this instance, theobjective is to route the current around thetransmitter, instead of through it, to the bestground available.2.1.2.3 Antenna Feed Cable : The shield of theantenna feed coaxial cable should be connecteddirectly to the station reference ground where itenters the building. In addition, the centre conductorand the shield of the feed cable should pass througha ferrite toroid which is positioned between theshield ground, at the building entrance and the shieldtermination, at the transmitter reference ground.This toroid will be transparent to the RF signal, butwill present an impedance to transients originating inthe antenna.2.1.2.4 Antenna/RF Output Disconnect: Aswitching circuit that disconnects the antenna fromthe transmitter's RF output when the transmitter isturned off should be incorporated into the RF feed/antenna design. This switching circuit will preventlightning induced transients from entering thetransmitter when its solid state devices are mostsusceptible to electrostatic failure.2.1.2.5 Antenna Tower: The antenna tower is themost likely target for lightning strikes. It isimperative that it contain lightning protection devices,such as air-gap spark balls, as the first line ofdefense against lightning strikes.2.1.2.6 External Control/Monitor Wiring: Allexternal/control wiring, that may be subjected tolightning induced transients, should be interfaced tothe station reference ground by surge protectiondevices where they enter the building. all conductorsand their shields should pass through a ferrite toroidwhich is positioned between its surge protectiondevice and the transmitter. This toroid will betransparent to control/monitor signals, but will presentan impedance to lightning induced transients.2.1.3 ELECTRICAL POWER: The transmitteris configured during manufacture to operate fromone of a variety of 50/60Hz AC power sources. Theoption selected is specified by the purchaser. Thepreferred option is a three-phase, four-wire, wyeconnected, AC power source meeting all of thefollowing requirements:2.1.3.1 Nominal Voltage: The primary winding ofthe main AC power transformer contains taps toaccommodate voltages that differ from the idealvoltage of the power source. These taps representfive percent increments and are selected duringinstallation to provide the optimum nominal voltagefor the transmitter.2.1.3.2 Voltage Stability: The AC power source'snominal voltage must be stable to within plus orminus ten percent under all loading conditions. Thetransmitter contains circuitry that maintains the RFoutput at the preset carrier level for voltagevariations within this range.