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tion that would be reliable under
conditions of signal variation resulting
from interference at the
D/F antenna between direct and
reflected rays.
System Description
The system installation plan for
the Radio Direction Finder AN /-
CRD-6 is shown in Fig. 1. The
facility provides bearing information
automatically and essentially
instantaneously on CW, voice or
tone modulated signals on any one
of 10
preset crystal controlled
channels of the 1750 available
channels in the 225 to 400 MC
frequency range. Bearing display
is accomplished by means of a
pointer type indicator.
The system comprises a local
station and one to four remote
stations. The local station consists
of two identical direction finding
antennas mounted on separate
masts at different heights to pro -
vide height diversity with respect
to the received signal, a target
antenna to permit system self -check
and alignment, and a prefabricated
shelter containing the local station
electronic components. The shelter
contains a comparator to determine
and select the antenna in the
stronger r -f field, a multi- channel
crystal controlled radio receiver, a
local bearing indicator, a local control
unit, and a multi -channel crystal
controlled target transmitter.
Each remote station consists of a
remote control unit and an azimuth
indicator identical to the local indicator.
These units are designed for
installation into existing and planned
control tower consoles and also
for standard rack installation. The
master remote station which may
be located up to five miles from the
D/F site, exercises complete control
of the system over a four wire
D/F ANTENNA
\'
ROTAI NG
REFLECTOR
30 "
MODULATED
-"' "'
I
MOTOR
1800
RPM
RF
STATIONARY
DIPOLE
RADIO
RECEIVER
P RASEME TER
30n. QUADRATURE
2 0 REFERENCE REFERENCE VOLTAGES
VOLTAGE
GENERATE
telephone type cable. A total of
three additional remote stations
that also exercise complete control
of the system may be employed in
parallel with the master station.
The most distant parallel remote
station can be located 10,000 ft. or
less from the master remote
station.
When furnished for permanent
fixed station operation, without a
prefabricated shelter and submersion
proof transit cases, identical
equipment is known as Radio
Direction Finder AN /FRD -2.
AMPLIFIER
Fig. 3: Basic direction finding principle
D/F Principle
Fig. 3 illustrates the basic direction
finding principle employed
in the AN /CRD -6. The antenna
consists of a stationary, vertical
dipole about which a parasitic reflector
is rotated at a speed of 1800
RPM. As the reflector rotates, it
amplitude modulates the received
Fig. 4: Height diversity uses cancellation at one point, addition at the other
REFLECTED ENERGY AT
ANTENNA SITE IS SMALL
COMPARED TO ENERGY OF
THE DIRECT SIGNAL AT
THE LOWER ANTENNA
DIRECT AND REFLECTED
SIGNALS CANCEL AT
HIGH ANTENNA
DIRECT AND REFLECTED
SIGNALS ADD AT LOWER
ANTENNA
h
30
signal at the rotation frequency.
The resulting rotating horizontal
field pattern obtained is unidirectional
and essentially cardiod in
shape. The reflector drive motor
also rotates a 2 -phase generator,
whose frequency is equal to the
frequency of modulation applied to
the received signal. The bearing of
the received signal is thus proportional
to the phase difference between
the modulation envelope of
the r -f and the reference voltage.
The modulated r -f output of the
antenna is passed to the input of a
radio receiver. The signal is then
amplified and demodulated and the
resultant output is fed through an
amplifier to the rotor of the phase
meter, the basic element of the
azimuth indicator. The phase meter
consists of two stator windings in
space quadrature and a rotor winding.
Reference voltages from the
antenna, equal in amplitude but
displaced 90° in phase, are applied
directly to the stators. Since the
two stator windings are in space
quadrature and are supplied with
quadrature voltages, a rotating
magnetic field is established in the
phase meter. When a signal is present,
the rotor is energized by a
voltage whose useful component has
a frequency of 30 CPS. The rotor,
therefore, will assume a position
where there is no reaction between
its magnetic field and the stator
magnetic field (at the point where
;Tel, -Tech & ELECTRONIC INDUSTRIES June 1956 91