Avionics News January 2023 - 55

CONTROLS:
ANTENNA
FREQUENCY SELECT
VOLUME
SQUELCH
ON/OFF
BUSS VOLTAGE
TRANSCEIVER
MICROPHONE AUDIO
MICROPHONE KEY
AUDIO OUTPUT
FIGURE 1:
The configuration of a VHF
communication system will
vary but will contain these
two components. A control
head is shown in blue, and a
transceiver is shown in green.
Some systems combine both of
these blocks into a single unit.
Others include the controls in
the display system.
transmitted at the same time, the air traffic controller
would either hear one or neither. If two AM transmitters
operate simultaneously, it's possible the air traffic
controller could hear both, although poorly, and the
controller may be able to sort things out.
The antenna used for VHF communication is of the
Marconi style and may be mounted on the top or the
bottom of the aircraft. On high-speed aircraft, these
antennas may look like a blade or fin. Some VHF comm
antennas may be combined with GPS or SXM antennas,
all in one package.
Over the years, the VHF communication system
configuration has come full circle. The most popular
early configuration was to have the transceiver remotely
mounted in the aircraft. In other words, the transceiver
would be in an equipment bay, away from the instrument
panel. On the instrument panel would be a control
head, which would include a display for the pilot to see
what frequency was selected, and controls for volume,
frequency control, and squelch. As electronic technology
improved, it became possible to include the control head
within the transceiver and have the entire radio small
enough to mount in the instrument panel. The panelmount
configuration became popular. The most popular
form of this system was to combine the communication
transceiver with a navigation receiver, all in one panelmounted
package. Large aircraft maintained the original
remote configuration, but virtually all single-engine
and many twin-engine aircraft had panel-mounted
communication transceivers. The most-modern avionics
packages include display systems that take up the entire
instrument panel with information screens. This has
forced the transceiver to be remote mounted again,
taking us full circle.
The VHF communication system works well for
aircraft flying over or near land; however, aircraft flying
over oceans or over remote lands, such as the Canadian
Bush, need a different type of communication system.
Long-distance communication can be achieved using
high frequencies, which are the frequencies from 3
MHz to 30 MHz, taking advantage of sky wave radio
propagation. Aircraft used in remote areas or over the
Continued on following page
FIGURE 2:
The configuration
of an HF
communication
system will be
similar to that of a
VHF system with the
addition of a coupler
for the antenna.
ANTENNA COUPLER
AND/OR
POWER AMPLIFIER
TRANSCEIVER
OR
RECEIVER EXCITER
ANTENNA
CONTROLS
BUSS VOLTAGE
MICROPHONE
AUDIO
avionics news * january 2023 55
Avionics News January 2023

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