VHF COMMUNICATION SYSTEM
The VHF Communication system
provides amplitude modulated
voice communication between aircraft
and ground or
other aircraft. The system
is used mainly
during take off
and approach. Communications take place
in the 118
to 1 5O MHZ
frequency range with
25 KHZ sparing between channels.
The nature of
radio-wave propagation at
these frequencies is
such that communication is Limited to
the line of sigh
distance and that's
why the communication range
increases as the aircraft gains
altitude.
The
advantage of VHF comm.
is that the
signal is not
often distorted or
rendered unintelligible by
statics or other
types of interference.
A typical VHF comm. system consists
of transceiver, antenna, control panel
with associated microphones
and load speakers.
A simplified VHF comm. system block
diagram is shown
in figure
VHF SYSTEM
BLOCK DIAGRAM
1) Receive
mode
The
receiver is
tuned by a
tunning signal from
the control panel according
to the frequency
selected, so the
received signal from ground
station is fed
to receiver through
antenna relay which connects
the antenna to
the receiver circuits
in case of reception
(usually relaxed curing
reception) . The signal is
mixed, detected &
amplified in the
receiver section, delivered to
the aircraft audio
system.
2) Transmit
mode
The mic
audio signal is
fed to the
modulator in which
inoculation & Power
amplification is done
in the transmitter
section which is tuned
to the frequency selected on
the control
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