Fixed antennas use some form of goniometer. The term goniometer is applied to a device
used to couple two or more input circuits (usually connected to antennas) to an output circuit
(usually connected to the receiver). This is done in such a manner that the degree of coupling
varies with the rotation of a shaft. The coupling between one input circuit and the output
circuit decreases while the coupling with the other input circuit increases. When properly
connected, a well constructed goniometer provides an output at each position of its shaft
identical to that which would be produced by a single figure-eight pattern antenna oriented to
the corresponding position. Thus, the goniometer provides an equivalent for the rotation of
an antenna, and makes it possible to use large fixed antenna systems (either loop, Adcock,
CDAA, or others) which would be too bulky for an operator to rotate.
Goniometers must meet these basic requirements:
(1) The fixed elements must be electrically
identical.
(2) There must be a complete absence of
coupling between the fixed elements.
(3)
Accurate positioning of the fixed
elements at the same angle as the antennas (usually 90 degrees) is necessary.
(4) Coupling between the rotating element
and the fixed elements must vary with shaft revolution in the same manner as the variation of
antenna response with azimuth angle. These requirements are met in practical goniometers
to the extent that the maximum error is less than plus or minus one degree.
In applying to the goniometer to a practical direction finder, it is often desirable to locate the
goniometer some distance away from the antenna. To accomplish this, it is necessary to
provide transmission lines between the antenna and its goniometer. These lines must be well
shielded and are usually balanced to ground to avoid stray pick- up. Additionally, the
transmission lines connecting the several antennas to the goniometer must be electrically
identical, particularly in time delay, over the entire frequency range of the equipment in order
to preserve accuracy and provide deep nulls.
The principal types of goniometers are:
(1) Inductive, usually consisting of two fixed
windings at right angles to each other and enclosing a third winding which rotates on a shaft.
(2)
Capacitive, with two fixed sets of plates
enclosing a rotatable set of plates, and
(3) Electronic, which utilizes circuitry instead
of physical rotation, and, therefore, allows higher speeds than can be obtained by mechanical
rotation.
The use of a goniometer introduces an additional possibility for octantal error above that
caused by antenna spacing. This is caused by nonuniformity within the electric fields of the
stationary windings of the goniometer. In early days this nonuniformity was considerable, and
octanta! error was large. In more modern goniometers, the windings are distributed in such a
manner that this cause of octantal error is practically eliminated. The octantal error is
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