Lightning
is a threat to RF communication systems and a potential loss of revenue
to wireless carriers. Reliable and cost effective protection of remote
sites is essential.
An
inherently superior method of protecting RF transmitters and receivers
is a quarter wavelength (l/4)
stub which shunts lightning currents to ground. The quarter wavelength
stub can dissipate several hundred, twenty five thousand (25,000)
ampere lightning strikes and reduces lightning voltages by 70 dB.
Their one million hour mean time between failure make them ideal for
remote sites.
Thel/4
stub suppressor acts as a short circuit for frequencies operating
below the intentional passband and has low RF loss for the transmitted
signal.
The low loss in the passband frequencies occurs because the
intentional signal at the junction of the l/4
stub (l)
is diverted along the l/4
stub for a phase angle rotation of 90°, (2) is reflected 180°
at the short circuit of the l/4
stub, and (3) travels back to the junction of the coaxial cable with
an additional 90° of rotation. The incident and reflected signals
are in phase and the intentional signal does not encounter the short
circuit.
Transients or other signals that have a longer wavelength, like
lightning, encounter the short circuit and are diverted to ground, preventing
it from damaging the transmitter or receiver.
Conventional l/4
stub suppressors have a passband of 5% of the central frequency
with a low VSWR and insertion loss. FCC has developed models that
achieve 20% bandwidth with a VSWR of 1:2:1 and less than 0.15 dB insertion
loss from 400 MHz to 4 GHz. Models with a 30% bandwidth are under
development.
The
limitation of this type of suppressor is bandwidth and that the coaxial
cable cannot simultaneously carry DC current to power an amplifier or
receiver located adjacent to the antenna.
Gas
tubes and solid state suppressors do not have as low a shunting
path impedance as a l/4
stub suppressor. Gas tubes and solid state suppressors have an inherent
delay in sensing the overvoltage causing the impulse breakdown voltage
to be higher than either their DC breakdown value or that of a conductive
path. The gas tube device generates a plasma which gets very hot and
limits its lifetime and peak current capability. The solid state suppressor
diode junction is limited by its heat dissipating capability. The
gas tube and solid state suppressors have a larger bandwidth characteristic
compared to the l/4
stub.
The
l/4 stub conductors are constructed to successfully operate with
no loss of reliability while dissipating several hundred twenty five
thousand (25,000) ampere lightning strikes. The l/4
stub provides about 70 dB of reduction in the lightning current for
transients of 8 µ second rise times and a half pulse with magnitude
ranging from 20 to 1,000 µ seconds. When dissipating 1000 amperes
during a 10/1,000 µ second transient only 5 millijoules is permitted
to be dissipated in a 50 W load. Lightning amplitudes leaking through
the suppressor are reduced to 30 peak volts during the 1,000 ampere
current transient.
A
gas tube or solid state suppressor typically provides 40 to 46 dB
of lightning reduction. A greater amount of lightning reduction
can be achieved the the gas tube or solid state suppressor by adding
a series capacitor. This sacrifices substantial reliability and increases
the possibility of intermodulation distortion. The capacitor has to
carry both the intentional and unintentional signals. The l/4
stub suppressor with a mean time between failure of greater than one
million hours has significantly higher reliability.
The
two tone third order intermodulation intercept point will be in ecess
of the +80 dBm with signal levels of one watt since the l/4
stub is composed of linear coaxial transmission line materials. The
gas tube is similar and the solid state device is of the order of
42 dBm with signal levels of one watt.
Quarter
wavelength suppressors are ideal for Analog Cellular, Advanced Mobile
Phone, Total Access Communication, Nordic Mobile Phone, Digital Cellular
systems including GSM or PCS, Wireless Data and many more RF communication
systems.
