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Section III TECHNICAL PRINCIPLES OF OPERATION
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TM-11-5820-670-12 Radio Set AN/ARC-131(NSN
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RETRANSMIT OPERATION. - TM-11-5820-670-120020


TM 11-5820-670-12
1-13.
GENERAL. (CONT)
Operation of the radio set is entirely remote controlled. Manual selection of any of the 920 channels is
accomplished by rotation of four frequency selector switches on the control unit. Motors in the
receiver-transmitter automatically tune the radio set to the frequency setting on the control unit.
NOTE
The following frequencies cause transmitter quieting of the receiver and must not be
used during testing, alinement, or operation; 33.90 MHz, 45.20 MHz, 56.50 MHz, and
67.80 MHz.
When configured properly, the radio set is capable of the following operations; transmit, receive,
homing, and retransmit.
1-14.
TRANSMIT OPERATION.
During transmit operations, audio signals generated by the microphone are routed to the receiver-
transmitter through the aircraft interphone amplifier (intercommunications control set) and control
unit. In the receiver-transmitter, the audio signal is converted to a rf signal. The rf signal is then
routed to the communications antenna which radiates the signal.
1-15.
RECEIVE OPERATION.
During receive operations, an fm communications signal is intercepted by the communications or
homing antenna and routed to the receiver-transmitter. In the receiver-transmitter, the fm signal is
amplified and converted to an audio signal. The audio signal is fed through an audio preamplifier and a
power amplifier in the intercommunication control set to the control unit. From the control unit, the
audio signal is fed through the aircraft interphone amplifier to the headset-microphone.
1-16.
HOMING OPERATION.
During homing operations, homing signals, which can be fm or unmodulated, are intercepted by a
controlled-pattern antenna system and coupled to the receiver-transmitter. The homing circuit in the
receiver-transmitter alternately samples the signal from one then the other antenna at a 100 Hz rate,
giving the resultant signal two components. When aircraft leading is alined with the transmitting
station, the signal components have the same signal strength. However, when the aircraft heading is
to either side of the transmitting station, the signal strength of one component is stronger or weaker
than the other. This information is sent to the aircraft homing indicator vertical pointer which provides
a visual indication of aircraft heading, with respect to the transmitting station. Over-the-station
homing is indicated by the homing indicator horizontal pointer which rises as signal strength
increases. Maximum signal strength is indicated when horizontal pointer is center scale (horizontal).
NOTE
A flag alarm system is provided on the homing indicator to indicate that the homing
signal is of sufficient strength. When the flags are not visible, there is sufficient signal
strength; when the flags are visible, the homing signal is too weak to provide proper
homing information.
The radio set is still capable of two-way communications operation while the control unit
mode selector switch is set to HOME, and homing operations are being preformed,
However, homing is temporarily interrupted during a transmit operation.
1-9


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