(3)
The indicator cannot detract from the accuracy inherent in the remainder of the
system.
(4)
The readability of the indicator is not influenced by the type of signal received,
whether it is continuous wave (cw), interrupted continuous wave (icw, or modulated
continuous wave (mcw). Readability shows how the indicator can detect relatively small
changes.
(5)
The readability of the indicator is high on weak signals in the presence of noise
or interference signals because the human ear can distinguish between desired and
undesired signals.
(6)
The readability is poor on fading signals because it cannot discriminate
between a fade and a null indication.
(7)
The readability of the indicator is poor on swinging signals because the antenna
cannot be rotated fast enough to follow the swinging bearing. A swinging signal is one that
because of propagation errors seems to move back and forth through several degrees of arc.
(8)
An extra operation must be performed to determine which of the two directions
indicates the correct azimuth (sensing).
3. Visual indicators. Visual indication has been accomplished through various means. Each has
the common characteristic of operation by comparison of antenna voltages. These voltages
may originate from any of several antennas. Among the types of visual indication are left-
right meter, crossed pointer meter, magic eye, matched line oscilloscopes, and instantaneous
oscilloscope indicators. Because the instantaneous indicators are most common this lesson
will limit itself to them.
An instantaneous indicator, using a cathode ray tube, continuously and automatically
presents on the tube screen a pattern or trace that points toward the azimuth of the arriving
signal as read from an azimuth scale situated around the tube face. The presentation of this
pattern is accomplished without manually rotating the DF antenna to the bearing point.
The antenna system must be one of the following types:
(a)
It may be a fixed oriented, crossed loop or crossed Adcock variety and
effectively rotated through 360 degrees at some continuous and constant rate by a spinning
mechanical goniometer or electric goniometer method.
(b)
The antenna system may be a single-loop or Adcock that is continuously
rotated by mechanical means at a constant rate through 360 degrees.
(c)
The antenna system may be of the fixed oriented, crossed loop or
crossed Adcock variety and the output of each of the antennas must be maintained as
separate signals through individual channels in the indicator.
(d)
The antenna may be of the CDAA type and employ either antenna
switching or goniometers to effectively rotate the antenna through 360 degrees at a constant
rate.
2-17
IT0302
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