Nautel XL12, Technical Instructions

Page: 26 / 238

12 000 WATT AM BROADCAST TRANSMITTER
XL12
Page 2-2
01 January 2002
2.1.2.1 Station Reference Ground:
The site must
contain a station reference ground, as defined in
Nautel's
Lightning Protection for Radio
Transmitter Stations
booklet. This ground must
provide a continuous, low impedance path to the
earth. The transmitter cabinet's designated
reference ground point, the shield of the coaxial feed
cable and the ground connection of the power
source's surge protection devices must be connected
directly to the station reference ground.
2.1.2.2 AC Power Source:
All conductors from
the AC power source should be protected by
bi-directional surge protection devices that are
connected between each conductor and the station
reference ground. In addition, the conductors should
pass, as a group, through a ferrite toroid. The
inductance formed by this toroid will be transparent
to the AC voltages but will present an impedance to
transients originating in the power source. A surge
protector panel, that contains suitably rated varistors
is available from Nautel for this purpose. If used, the
surge protector panel should be installed in close
proximity to the station reference ground.
NOTE
The AC power source usually presents the lowest
impedance path to ground potential for a
lightning strike and will normally carry most of
the lightning induced current away from the
transmitter site.
When lightning hits the power source, a
significant amount of induced current may flow
towards the transmitter. In this instance, the
objective is to route the current around the
transmitter, instead of through it, to the best
ground available.
2.1.2.3 Antenna Feed Cable :
The shield of the
antenna feed coaxial cable should be connected
directly to the station reference ground where it
enters the building. In addition, the centre conductor
and the shield of the feed cable should pass through
a ferrite toroid which is positioned between the
shield ground, at the building entrance and the shield
termination, at the transmitter reference ground.
This toroid will be transparent to the RF signal, but
will present an impedance to transients originating in
the antenna.
2.1.2.4 Antenna/RF Output Disconnect:
A
switching circuit that disconnects the antenna from
the transmitter's RF output when the transmitter is
turned off should be incorporated into the RF feed/
antenna design. This switching circuit will prevent
lightning induced transients from entering the
transmitter when its solid state devices are most
susceptible to electrostatic failure.
2.1.2.5 Antenna Tower:
The antenna tower is the
most likely target for lightning strikes. It is
imperative that it contain lightning protection devices,
such as air-gap spark balls, as the first line of
defense against lightning strikes.
2.1.2.6 External Control/Monitor Wiring:
All
external/control wiring, that may be subjected to
lightning induced transients, should be interfaced to
the station reference ground by surge protection
devices where they enter the building. all conductors
and their shields should pass through a ferrite toroid
which is positioned between its surge protection
device and the transmitter. This toroid will be
transparent to control/monitor signals, but will present
an impedance to lightning induced transients.
2.1.3 ELECTRICAL POWER:
The transmitter
is configured during manufacture to operate from
one of a variety of 50/60Hz AC power sources. The
option selected is specified by the purchaser. The
preferred option is a three-phase, four-wire, wye
connected, AC power source meeting all of the
following requirements:
2.1.3.1 Nominal Voltage:
The primary winding of
the main AC power transformer contains taps to
accommodate voltages that differ from the ideal
voltage of the power source. These taps represent
five percent increments and are selected during
installation to provide the optimum nominal voltage
for the transmitter.
2.1.3.2 Voltage Stability:
The AC power source's
nominal voltage must be stable to within plus or
minus ten percent under all loading conditions. The
transmitter contains circuitry that maintains the RF
output at the preset carrier level for voltage
variations within this range.