Section IV. DETAILED FUNCTIONAL DESCRIPTION, ATU

TM 11-5825-271-34

Section IV. DETAILED FUNCTIONAL DESCRIPTION, ATU

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

sensitivity

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.

2-11