Characteristics Of A "Shielded" Loop Antenna - AOR LA400 Instruction Manual

7.

Characteristics of a "shielded" loop antenna

A shielded loop antenna is less susceptible to nearby electrical interference sources, thanks
to the electrostatic shielding of the loop afforded by the grounded metallic conduit enclosing
the wire coils.
By these principles, LA400 responds to the magnetic field rather than the electric field, thus
efficiently isolating the low frequency electrostatic noise from the distant signal to be
received.
Shield circuit of LA400
With this design, all parts of the loop will have the same capacitance to ground. The shield
also protects the loop from the induction field created by nearby disturbances. The induction
field refers to the electric and magnetic fields in the immediate vicinity of an antenna. Those
fields decrease rapidly in strength with distance, and the induction field is usually ignored.
However, wires and other metal objects near the loop can take energy from a passing wave
and produce induction fields that can induce spurious voltages in the loop.
A shield over a loop antenna will not appreciably decrease the amount of magnetic flux that
passes through the loop when a wave goes by - as long as it does not form a complete turn.
A gap is left in the shield so that it does not become a shorted turn. Without the gap, the
shield would reduce the magnetic field linking the loop so that no signal could be received by
the internal wire. With the gap, alternating currents can be induced in the metal shield and
voltages will be induced in the internal wire.
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