2. The type of bearing indicator used with any particular direction finder depends on the type of
DF system in use, the complexity and physical size permitted, and the accuracy desired for a
particular DF application. Many types of bearing indicators have been designed and used in
DF systems, but those having the greatest application can be grouped into the categories of
AURAL indicators and VISUAL indicators. Visual indicators can be further grouped into three
subelements, left-right indicators, instantaneous indicators, and automatic bearing seeking
indicators. Each type of indicator has certain specific characteristics and applications.
Many of these characteristics are closely related to, and cannot be separated from,
characteristics of the type of DF system with which a particular indicator is used. Therefore,
keep in mind that some of the characteristics credited to a specific indicator are really
characteristics of the DF system with which the indicator is used.
Aural Indicators. An aural indicator is a headset or loudspeaker connected to the DF receiver
audio circuit. It enables the operator to detect the bearing of a signal by changes in audible
receiver output as the antenna is rotated. In DF systems which use aural indicators, the
antenna system must be able to be rotated to the bearing position, and this position must be
characterized by an abrupt change in the antenna response pattern, and, therefore, in the
receiver output.
In most common direction finders which use an aural indication the antenna is a rotatable
loop or Adcock, or a fixed crossed-loop or crossed-Adcock effectively rotated by a
goniometer. All these antennas have, or result in, a figure-eight response pattern with broad
maxima and sharp nulls. Therefore, the nulls of the system are selected as the bearing
points. Because of this, these systems are called aural-null indicators. At higher frequencies
(several hundred megahertz) directional arrays having response patterns with sharp maxima
can be used. Because aural-null systems are more common we will not discuss aural-
maxima systems further.
The most important characteristics of an aural-null indicator are:
(1)
It is the simplest indicator that can be used with a direction finder for the
following reasons;
(a)
it adds nothing to the direction finder because a headset or loudspeaker
is usually included for monitoring purposes.
(b)
it can be used with simple loop or Adcock antenna systems without the
addition of complicated couplings.
(c)
it reduces to a minimum the size, weight, and maintenance factors of the
indicator.
(d)
it is simple to use, even by unskilled operators.
(2)
Because it does not necessitate the use of complicated and elaborate antenna,
coupler, and receiving systems, the sensitivity and instrumental accuracy of the direction
finder is high.
IT0302
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