standards and guidelines for communication sites - Radio And ...

STANDARDS AND GUIDELINES FOR COMMUNICATION SITES TOWER LIGHTING PROTECTION
Damage to the GPS receiver can be induced directly from an antenna head strike, or through induced
coupling from other transmission lines on the same antenna structure. There is little that can be done to
effectively protect the GPS antenna unit itself from direct lightning strikes, other than to assure that it is
never mounted at the highest point on the tower. The GPS system coaxial transmission line shall be
installed and grounded (earthed) in accordance with the practices described in Chapter 4, “External
Grounding (Earthing).” A special GPS coaxial RF SPD shall be installed within 610 mm (2 ft.) of the
cable entry port and bonded to the MGB as described in Chapter 5, “Internal Grounding (Earthing).”
Also see “RF Components Protection” on page 7-36 and Figure 7-22 on page 7-42.
The GPS compatible RF coaxial lightning protector is different from normal coaxial protectors in that it
is designed to pass the DC bias through the coaxial line which is required to power the antenna-mounted
amplifier. Before selecting a coaxial SPD, it should be verified that the GPS receiver can operate with
an SPD in the coaxial line. In most installations the GPS receiver will require optimization to
compensate for the delay caused by the SPD and the variations in the length of the transmission line
(which is site specific). The GPS antenna transmission line shall be cut to the proper length as excess
transmission line shall not be coiled either inside or outside the shelter, room or equipment area.
All coaxial RF surge suppression devices shall exhibit an open DC circuit through the device except
those specifically designed to pass DC for CCTV, tower top amplifiers and GPS active antenna
installations. These devices shall incorporate a circuit to pass the DC power while providing suitable
surge protection for the DC circuit path. The input and output ports shall not be directly connected and
may have a capacitive or reactive network installed within the device to permit the RF and DC energy to
pass.
A DC injector/DC path coaxial hybrid RF SPD will typically have a turn-on voltage nominally 20%
above the maximum DC operating voltage, with a turn-on time of 4 nsec for 2 kV/nsec. This protector
will also offer a VSWR of less than or equal to 1.2:1 (return loss of -20.8dB) over the frequency range,
and an insertion loss of less than or equal to 0.3 dB, handling a 20 kA, ANSI C62.1 8/20 waveform of
200 Minimum (+ and -) Alternating at 200 A per 62.41-2002.
The proper surge suppression device shall be selected for the specific application. Different active GPS
receiver antennas will have different DC power supply voltages on the transmission line. The device
selected should have a turn-on voltage that is nominally 20% higher than the maximum DC operating
voltage; however, the turn-on voltage shall be low enough to provide suitable protection.
7.8 TOWER LIGHTING PROTECTION
All tower lighting system power, control and alarm conductors shall have SPDs installed at the point of
entry into the building, shelter, room or equipment area. If the equipment room or area is located within
a larger building and several ft away from the point where the conductors enter the building, the SPDs
shall be installed within 610 mm (2 ft.) of the equipment room or area entry point or wall penetration.
AC power conductors shall have normal mode (L-N or L-L) surge suppression devices installed.
Common mode (L-G) devices shall not be used on AC power circuits, although common mode devices
will provide the most effective suppression on alarm and modem circuits and may be used on these
circuits.
68P81089E50-B 9/1/05 7-45