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EasyReach User Manual TECHNICAL ASSISTANCE NOTES
1H90941 31
TECHNICAL ASSISTANCE NOTES
Surge Protection for Radio Equipment
General
Devices exist to assist in the protection of radio equipment from surges caused
by lightning strikes and nuclear blasts. The level of protection based on the fact
that damage thresholds for microwave diodes are in the order of 1.E-8 joules.
This note does not address surges in power lines.
Surge Characteristics
Surges in equipment conductors are induced by broad band, high-energy
electromagnetic pulses. These pulses are caused by lightning (LEMP) and/or
nuclear blasts (NEMP). Most of the energy in lightning pulses is confined within
DC to 1 MHz with rise time in the nanoseconds range. Electric field associated
with LEMP are in order of 500 kV/m while those from NEMP are about one order
of magnitude less. In the normal configurations with an antenna mounted on a
mast and connected to the transmitter/receiver via coaxial cable/waveguide,
induced voltages (currents) are in
common mode at the entrance point (around
the top of the mast). Potential difference will develop between different
conducting paths as the surge is transmitted towards ground equipment due to
different (transient) propagation characteristic of the cable/waveguide. For
coaxial cables for example, higher inductance of the inner conductor delays and
flattens (longer rise time) the pulse relative to the one received from the shield.
LNAs located outside right at the antenna are often not affected by large
common mode surges. Apart from high potential difference, the resultant high
surge currents can also excessively heat, to melting, the various conducting
paths on their way escaping to ground.
Methods of Surge Protection
The causes for surges thus can not be controlled or eliminated. The effects of
surges, however, are minimised by good (low inductance) earth paths where
surge currents are passed harmlessly down to good earth. The paths may be
one or all combinations of the following:
1. the earth paths that are connected to signal line but are open circuit
at the operating frequency; or,
2. the improved existing or dedicated conduction earth path; or,
3. the paths that are short-circuited to good ground at the surge voltage.
Each of these three methods is briefly explained below.