ProSoft Technology ILX34-AENWG - Manual (Page 77)

Lightning Protection PSW-PWD ♦ ProSoft Software
User Manual ProSoft Wireless Designer
ProSoft Technology, Inc.
Page 77 of 91
July 8, 2009
Nowadays, new controversy has resulted with the use of spline balls, static
dischargers and wicks mounted on antennas and the top of towers. These
devices are said to provide a constant discharge thus decreasing the potential for
a direct strike. Some users claim a diminished amount of direct hits after
installing these devices.
Suffice it to say, if at all possible, do not mount your antenna on the highest
building or tower. Place it a few feet lower and hopefully the fickle lightning bolt, if
it generates a direct hit, will not discharge through the antenna. Furthermore, the
boom or mast should be grounded to the mast or tower. More on this shortly. Do
not forget to ground guy wires that are used on stabilize towers. They are just as
likely to be hit because they extend over a wide area around the tower.
9.5 Input Protection
Input protection is typically provided by a lightning or surge protector at the input
(or antenna side) of a radio. There are three major types of lightning protection
devices for the radio input. They are the spark gap, the gas discharge tube and
the quarter-wavelength (1/4) wavelength shorted stub. Each method has its
pluses and minuses.
The spark gap is the oldest know lightning protection having probably been
invented by Ben Franklin! Basically it consists of two balls or points closely
spaced and directly across the transmission line. When a strike occurs, the high
voltage present will jump across the points and be conducted to ground. RF
transmission devices for 50 Ohm systems such as this have been around since
the 1950’s.
The shortcoming with this older device is that it may not protect against a weak
or lower voltage strike. Adjustment at the factory may produce varying voltage
breakdowns. Furthermore, once a strike is taken, the device may fail or short
circuit so maintenance is required.
A more recently developed input protection device is the gas tube. It works in a
similar manner to the spark gap but can be designed to operate reliably at much
lower voltages down to 100 Volts for low power circuits and 250 Volts or higher
for higher power applications. Another advantage is device is that it can be
designed to operate over a very broadband frequency range with low VSWR up
to about 2 GHz. However, it too must be replaced after a strike although it will not
necessarily warn the user by going to a short circuit so preventative maintenance
is required.
The quarter wavelength shorting stub is becoming a very popular device,
especially above 800 MHz where system bandwidth is generally narrow. It
consists of a tuned quarter wavelength shorted coaxial type transmission line that
is placed directly across the transmission line. Simple types have a narrow
bandwidth, typically 10'-. bandwidth, but are low loss and inexpensive.