Plessey EasyReach, User Manual

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TECHNICAL ASSISTANCE NOTES EasyReach User Manual
32 1H90941
Grounded Antennas
Often the antennas are grounded. A most simple example is the folded
dipole. Only at the intended frequencies are the paths separated by the
characteristic impedance. Away from the radiating elements, the path of
least resistance to good earth, will be the one taken lightning surge current.
Often this path is discovered after the event, where melted-down metal or
burned material is found after a thunderstorm. Deliberate grounding
outside the operating frequency band can also be designed into the circuit.
The simplest method of doing this is the shorted quarter-wave stubs. The
technique is economically suitable when the operating frequency band is
well separated from the effective spectrum of the surge pulses. Grounded
antennas provide good surge protection and are often not sufficient by
itself.
Dedicated Conduction Paths
Away from the radiating elements, surge currents are diverted away from
circuit units to the antenna frame by copper earth straps placed at strategic
locations. This is a low-cost solution and together with grounded antenna
design (as described above) is normally sufficient to protect the antenna
system from lightning. Lightning rods providing protection cones can also
be used to induce surges to take a particular path down to ground via the
antenna frame/tower. The use of heavy-gauge ground wire running up the
mast was found not to be cost effective, as the wire needs to be large and
fitted at least 0.5 m away from the mast, and often the difference in
conductivity between copper and galvanised steel is lost in the difference in
inductances due to smaller surface area of the ground wire. Attention also
has to be provided for the galvanic voltage.
Dedicated Surge Protection Cells
Surge protection cells can be inserted in transmission path between
antenna and ground equipment. Cells are generally of two types. One
type of cell is designed dedicated to a particular frequency band using ‘1/4-
wave’ stub techniques of various sophistication to provide sharpness in the
passband. The other type makes use of the voltage discrimination of some
non-linear diodes or gas tubes. At voltage higher than a predetermined
level, the device shorts the central conductor to its shield, preferably at a
point where the shield itself is well grounded.