Vor/Loc Receiver - Honeywell Bendix/King KX 165A Maintenance Manual

BENDIX/KING
4.2.3

VOR/LOC RECEIVER

4.2.3.1 Principles of VOR system
4.2.3.1.1 General
The basic function of VOR is to provide a means to determine an aircraft's position with reference
to a VOR ground station and also to follow a certain path toward or away from the station. This is
accomplished by indicating whether the aircraft is on a selected VOR station radial or by deter-
mining which radial the aircraft is on. A means to differentiate between radials and identify them
is therefore necessary. For this purpose, advantage is taken of the fact that the phase difference
between two signals can be accurately determined. The phase difference between two signals
which are generated by the VOR station is varied as the direction relative to the station changes
so that a particular radial is represented by a particular phase difference. Refer to
non-directional reference signal is generated with a phase that at any instant is the same in all
directions. A second signal is generated with a phase that at any instant is different in different
directions. The phase of the variable phase signal is the same as the phase of the reference sig-
nal only at the 0° radial (North). As the angle measured from the 0° radial increases, the phase
of the variable phase signal lags the phase of the reference signal by the number of degrees of
the angle from 0°. The reference and variable phase signals, which are 30Hz voltages, are carried
by RF to make radio transmission and reception possible. The VOR receiving equipment must
separate the 30Hz reference and variable phase signals from the RF carrier and compare the
phase of the two signals. The phase difference is indicated on a course indicator or RMI.
4.2.3.1.2 VOR generation
The VOR electromagnetic field is composed of the radiation from two ground based antennas ra-
diating at the same carrier frequency. The first is a non-directional antenna radiating an amplitude
modulated carrier. The frequency of the modulating signal varies from 9480Hz to 10440Hz back
to 9480Hz 30 times per second. That is, a 9960Hz subcarrier amplitude modulates the RF carrier
and is frequency modulated by 30Hz.
The second antenna appears electrically as a horizontal dipole which rotates at the rate of 30 rev-
olutions per second. The dipole produces a figure 8 field pattern. The RF voltages within the two
lobes are 180° out of phase. The RF within one of the lobes is exactly in phase with the RF radi-
ated from the non-directional field. The rotating figure 8 pattern reinforces the non-directional pat-
tern on the side. See
of 30 revolutions per second; the rate at which the dipole antenna rotates.
The signal received by aircraft within radio range of the VOR station is an RF carrier with ampli-
tude varying at the rate of 30Hz because of the rotation of the cardioid pattern. The carrier is also
amplitude modulated at the station by the 9960Hz signal which is, in turn, frequency modulated
on a sub-carrier so that it may be separated from the 30Hz variable phase signal.
4.2.3.2 Principles of localizer system
The localizer facility provides a visual display of the aircraft's position relative to a straight line of
the runway. The ground based localizer antenna system generates two patterns. Refer to
4-5. One pattern is directed toward the right side of the runway, the second to the left. The two
patterns have the same carrier frequency but different audio modulating signals. The pattern to
the left of the runway (in normal approach) is 90Hz, amplitude modulated, while the pattern to the
right is 150Hz, amplitude modulated.
The ratio of the levels of the 90Hz to 150Hz audio, after demodulation, is dependent only upon the
position of the aircraft within the patterns. The patterns are adjusted so they are of equal strength
on a vertical plane extending out from the runway centerline. When the aircraft is on this plane,
the 90Hz and 150Hz voltages are equal.
Rev 0, Jun/2000
Figure 4-4. This results in a cardioid field pattern which rotates at the rate
15610M00.RCD
KX 165A
Figure 4-4. One
Figure
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