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TM 11-5825-271-34
or CR13, switches on Q5 or Q4. This causes motor B1
2-21. Power
to turn in the correct direction to eliminate the phase
(fig. FO-11)
difference between the voltage and current waveforms.
The 24 Vdc input at J2-A and J2-B (operating power for
For example, when a positive voltage is applied to the
the ATU) is received from the NDB transmitter in single
base of Q2, Q2 conducts and motor current flows from
installations or from the change-over unit in the case of
B+ to Q5, then through the tuning motor M1 to S5 and
dual installations.  Switch S1 is the POWER ON/OFF
S6, and finally through CR12/Q2 to ground. When Q3 is
switch and DS1 is the POWER ON indicator. Switch S3
switched on by a positive voltage, the current flows in the
controls the on/off operation of the TEST switch S2,
opposite direction from B+ through Q4, S5, S6, B1,
which selects forward power, reflected power, or the
CR13 and Q3 to ground Components R1, R6 and CR9
current reading on TEST meter M1. Switch S3 is located
bias the input to the rf switches to increase system
on the side of the ATU cabinet, allowing meter Ml to be
turned on and off without opening the ATU front cover.
c.  Manual override switch S4 applies ground
potential directly to the cathode of either CR12 or CR13,
2-22. Power Probe AlA1
thus switching the motor on to slew the tuning drive
(fig. FO-11)
manually. Transistor Q1 is biased on by the override
The power probe AIA1 provides forward power, reflected
switch S4 to inhibit the servo action while manual tuning
power and rf current signals to the TEST meter M1, and
is performed.
permits the operator to monitor the performance of the
NDB transmitter The power probe AlA1 used in the ATU
2-24. Matching Transformer A1A3
is identical to the power probe employed in the NDB
(fig. FO-11)
transmitter;  refer  to  paragraph  2-19  for  detailed
The matching transformer A1A3 has seven taps to
functional description. Note, however, that m the case of
match the net series-resonated antenna resistance to the
power probe A1A1 in the ATU, the current limit output at
50 ohm NDB transmitter output. The appropriate tap is
P1-2 and the rf level output at P1-12 are not used
selected on TB 1.
2-23. Servo Probe A1A2
2-25. Loading Coils
(fig FO-11)
(fig. FO-11)
a. The servo probe A1A2 continuously monitors the
a. The ATU contains two loading coils, L1 and L2.
phase relationship between the current voltage of the rf
In addition to the upper and lower terminals, each coil
output and, in the event of a phase difference, operates
has an intermediate tap, permitting selection of a partial
motor B1 which fine tunes the loading coils L1/L2 to null
value of the coil inductance. Coils L1 and L2 can be
the phase error. This process eliminates the effect of
connected in series or in parallel between the matching
antenna reactance changes on the VSWR
transformer and the antenna terminal.  Therefore, a
b. The servo probe AlA2 consists of current
number of alternate connections are possible to tune the
transformer T1 with secondary windings across which
reactive component of the antenna impedance for the
identical voltages are obtained from the induced current.
frequency in use.
Two diode bridges, CR1 to CR4 and CR5 to CR8, act as
b. Each loading coil (L1 and L2) has a ferrite tuning
rf switches which, since they are connected m opposite
slug, driven by motor B1, and controlled by the servo
phases, are alternately switched on by the positive and
probe A1A2 to provide fine tuning of the antenna. The
negative half-cycles of the current waveform.  The rf
upper and lower limit switches S5/S6 open when the
signal voltage is fed to both of these rf switches, shifted
tuning slugs reach the appropriate travel limit. Diodes
90 degrees by C1 and R1. If the antenna voltage and
CR1 and CR2 provide a current path to allow retuning of
current are in phase, the net dc potential at the output of
tuning slugs in a direction away from the actuated limit
both switches is zero. If a phase difference between the
switch (S5 or S6).
antenna voltage and current exists, the output voltage at
c.  In all, there are three pairs of loading coil
one switch is positive, and output from the other switch is
assemblies available, each designed to cover a specified
negative. This causes a positive voltage to appear at the
portion of the operating frequency.  The loading coil
base of either Q2 or Q3. Thus, Q2 or Q3 switches on
assemblies are supplied with each ATU to suit the
and, by providing ground at the cathode of either CR12
operating frequency.

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