1. Very High Frequency
(VHF) Communication
AVIONICS
TECHNOLOGY
VHF communication systems are employed largely for controlling air
traffic.
These systems are installed in all types of aircraft so the pilot may be
given information and directions and may request information from air
traffic control centers, control towers, and flight service stations.
On the approach to any airport with two-way radio facilities, the pilot
of an aircraft calls the tower and requests information and landing
instructions.
In airline operations and all instrument flights, the flight of an aircraft
is continuously monitored by air traffic control (ATC), and the aircraft's
crew is given instructions as necessary to maintain conditions of safe
flight.
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VHF communication systems operate in the frequency range of 118 to
136.975 MHz. For international operations the frequencies may extend
to 151.975 MHz.
The nature of radio-wave propagation at these frequencies is such
that communication is limited to line-of-sight distances.
The advantage of VHF communication, however, is that the signals are
not often distorted or rendered unintelligible by static and other types
of interference.
VHF communication radios are currently available with 720, 760, or
360 channels.
The 720- or 760-channel radio is preferred by most pilots owing to
its versatility in frequency selection.
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In 1976 the FAA changed the minimum frequency spacing for VHF
systems from 50 to 25 kHz between 118 and 135.975 MHz.
This change made the 720- channel radio possible.
Recently the FAA and FCC authorized the general use of frequencies
up to 136.975 MHz.
This change added 40 channels to increase the selection to 760
channels.
Some older 360-channel radios are still in use; however, they are
quickly becoming obsolete.
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VHF System Description
Aircraft VHF communication systems consists of a VHF transceiver, control
head, antenna, and an interface to the aircraft audio system for access to the
microphone and cockpit speaker.
In light aircraft, the transceiver is mounted in the instrument panel and
contains all the necessary controls and displays.
In larger aircraft, the control head, which is used for selecting the receiver and
transmitter frequencies, is usually located in the center console between the
pilot and copilot, and the transceiver is remotely located in the radio rack aft or
below the crew station.
VHF communication transmitters provide AM voice communication transmission
between aircraft and ground stations or between aircraft.
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VHF System Description (cont’d)
Because of the nature of VHF radio signals, the average communicating distance from aircraft to ground is:
Approximately 30 mi [48 km] when the airplane is flying at 1000 ft [305 m]
Approximately 135 mi [217 km] when the airplane is at 10,000 ft [3048 m].
Transmitting frequency is determined by the position of the selector switches
on the VHF control panel.
The transmitter is tuned at the same time and to the same frequency as the
receiver.
The most modern VHF communication radios incorporate the latest digital
design features.
In general, the use of microprocessors and digital circuits has allowed for a 50
percent reduction in parts count and an 80 percent reduction in internal shop
adjustments as compared with the use of analog circuits.
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VHF System Description (cont’d)
A modular design of a modern digital system reduces maintenance time by
providing easy access to all circuit boards and components.
VHF communication equipment for light aircraft is typically combined with a
VHF navigation (NAV) radio system.
The VHF transceiver is a solid-state or digital system that can receive or
transmit on any one of the 720 channels in the COMM range of frequencies.
The frequencies are spaced at 25-kHz intervals throughout the range.
Frequencies are selected simultaneously for both the receiver and the
transmitter by rotating the frequency selector knobs.
The large outer knob is used to change the megahertz portion of the frequency
display, and the smaller concentric knob changes the kilohertz portion.
The small knob will change the frequency in 50-kHz increments when it is
pushed in and in 25-kHz increments when it is pulled out.
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VHF Transceiver Frequency Selection
Some control heads employ a universally accepted 2-out-of-5 frequency
selection scheme.
Other control heads use a digital serial data bus, such as ARINC 429, to select
the desired frequency of a remotely-mounted transceiver.
With 2-out-of-5 tuning, any two out of a maximum of five frequency selection
inputs to the transceiver will be grounded by the selector switch in the control
unit to correspond with the desired frequency selection.
For example, if the control head displays the frequency 21.5, the "A" and "C"
10-MHz, "A" and "B" 1-MHz, and "C" and "D" 0.1-MHz frequency selection
inputs to the transceiver will be grounded by the discrete signals from the
control head.
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VHF Transceiver Frequency Selection (cont’d)
Table: Two-out-of-five frequency selection
2 of 5 Code Table
No.
A
B
1
X
X
2
X
C
D
X
3
X
4
X
X
X
5
X
6
X
7
X
X
X
8
X
9
E
X
X
X
X
X = Ground
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VHF Transceiver Frequency Selection (cont’d)
The ARINC 429 serial data bus provides a balanced differential signal using
nominally zero to 5-volt switching levels from the control head.
ARINC 429 is also used to send digital data from the avionics equipment to the
cockpit displays.
ARINC 429 messages are comprised of 32-bit data words.
Each bit in the data word is set at either V if no voltage is present, or "1” if +5
volts DC is present.
This serial data stream runs across the two-wire bus at speeds of up to 100
kilobits per second using a command-response protocol.
The message format requires that a record, consisting of up to 126 data words,
begin with an initial word that notifies the receiving unit that a message is
being sent, and ends with a final word that is used to test for errors in the
record.
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VHF Transceiver Frequency Selection (cont’d)
Initial words and final words do not contain data in bits 11 through 29.
The first 8 bits in the initial word may contain one of the following messages:
Request to Send, Clear to Send, Data Follows, Data Received OK, Data
Received Not OK, or Sync Lost.
The first 8 bits in the final word is the file label, and bits 9 through 29 is the
error control checksum, which is the addition of bits 9 through 29 in all the
intermediate words in the record.
ARINC 429 does not provide for error correction, but only error detection within
the serial data stream.
Typically, manufacturers of avionics systems will provide not only ARINC 429,
but a variation of this format, such as the Collins Commercial Standard
Digital Bus (CSDB), as their own unique data bus to be used for sending and
receiving data between only their brands of equipment.
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VHF Control Panel
To tune the transceiver to the
desired operating frequency, it is
necessary to first enter the selected
frequency into the STANDBY
display.
The frequency is then activated by
pushing the transfer button, and
the word USE will be displayed.
Another frequency may then be
entered into the STANDBY mode.
The STANDBY mode will store the
selected frequency to allow for a
"quick switch" of the frequency
being used by the receiver.
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This becomes very helpful while
operating an aircraft in crowded
airspace in which several
communication frequencies are
used for air traffic control.
Control panels for VHF
communication systems vary in
design, depending on the
manufacturer of the equipment and
the requirements of the aircraft
manufacturer.
Typically, the control panel located
in the flight deck contains the
frequency selectors and the digital
displays for the main and standby
frequencies.
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VHF Control Panel (cont’d)
Most VHF systems for corporate
and transport-category aircraft use
a separate radio control panel, and
the receiver transmitter (r-t) is
located in the electric equipment
center.
A VHF instrument panel
Also on these aircraft, the VHF
communication radio system is
often independent of the VHF
navigation system.
On light aircraft the r-t and control
panel are often one unit mounted in
the instrument panel.
Interior of a VHF communication radio
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VHF Antenna
Antennas for VHF systems are lowdrag stub units extending from the
top and bottom centerline of the
airplane.
These antennas are matched to
their respective transmission lines
by means of carefully measured
lengths of tuning line.
The antennas are used for both
transmitting and receiving.
VHF antenna configuration
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VHF System Operation
The receiver portion of a VHF
communication system is typically
the superheterodyne type.
The IF is a lower frequency and is
easier to control through the
receiver.
The antenna receives an induced
signal from the electromagnetic
fields passing the antenna.
The IF is amplified to produce a
stronger signal, which is sent to the
second-stage mixer where again a
lower frequency is produced.
This signal is sent through a bandpass filter to an RF amplifier.
Once amplified, the signal passes
through a low-pass filter and into
the first-stage mixer.
The mixer converts the RF into an
intermediate frequency (IF).
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This signal is amplified and sent to
the detector, where the audio wave
is separated from the carrier wave.
The audio signal is then amplified
by the buffer and broadcast into the
aircraft by the speaker.
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VHF System Operation (cont’d)
The buffer amplifier receives inputs
from the AGC (automatic gain
control) circuit, which ensures
correct signal amplification at varied
input signal strengths.
The transmitter receives an input
signal from the microphone or data
inputs.
This signal is amplified by the audio
buffer and sent to the modulator
(synthesizer).
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The modulator produces an AM
signal, which is filtered, amplified,
and sent to an ALC (automatic level
control) circuit.
Similar to the AGC in the receiver,
the ALC ensures that a consistent
output signal is sent to the antenna,
even at varying input signal
strengths.
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VHF System Operation (cont’d)
Block diagram of a typical VHF
communication system for a large aircraft
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Rockwell Collins VHF-20A Transceiver
The Collins VHF-20A, a typical
remotely-mounted VHF transceiver,
provides AM voice communication in
the frequency range from 117.00
MHz through 135.975 MHz, in 25kHz increments.
The VHF-20A consists of a power
supply, frequency synthesizer,
receiver, modulator, and
transmitter.
The VHF-21/22 is an advanced
microprocessor-based version of the
VHF-20A that employs the ARINC
429 and Collins CSDB.
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Collins VHF-20/21/22 transceiver
Collins VHF-20A block diagram
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Rockwell Collins VHF-20A Transceiver (cont’d)
The VHF frequency synthesizer,
having only one crystal controlled
oscillator, derives accurate RF
output frequencies through the use
of a phase-lock-loop and solid-state
switching circuits.
The synthesizer interprets 2-outof-5 frequency information from the
VHF control head and provides all
internal RF signals required by the
VHF receiver and transmitter.
Collins VHF-20/21/22 transceiver
Collins VHF-20A block diagram
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Rockwell Collins VHF-20A Transceiver (cont’d)
In the receive mode, the synthesizer outputs a DC tuning voltage to the
variable-voltage capacitors in the preselector to eliminate mechanical tuning.
The synthesizer also applies an injection frequency to the mixer to output a 20MHz IF.
The 20-MHz IF amplifier, which is AGC controlled, provides the required
selectivity and signal amplification.
The detected audio is amplitude and bandpass limited and applied to the audio
output amplifier.
Squelch circuits disable the output amplifier if proper signal-to-noise ratio or
carrier level is not present.
When the push-to-talk switch on the microphone is applied, the synthesizer
removes the receiver injection and provides transmitter excitation at the
selected frequency.
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Rockwell Collins VHF-20A Transceiver (cont’d)
Power is applied to the transmitter by a +16-volt DC transmit series regulator,
and the broadband RF amplifiers raise the synthesizer excitation to 20 wafts
minimum output.
The RF output is low-pass filtered and applied through the transmit/receive
switch to the antenna.
The AM modulator is a variable voltage power supply that varies the transmitter
drive voltage consistent with the microphone inputs.
Carrier modulation is detected by a sidetone detector and applied through the
receiver audio amplifier so the pilot can monitor his or her voice transmissions
through the aircraft's audio system.
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Bendix/King KX-170A/KX-175 VHF Transceiver
The Bendix/King KX-170A/KX-175 is
a combination panel-mounted VHF
communications transceiver and
navigation receiver that operates on
either 28-volt or 14-volt DC power.
Characteristics of Bendix/King
KX-170A/KX-175 are:
The communications section is:
Dual-conversion
Superheterodyne receiver with a
9.0-MHz IF
861.25-kHz second IF frequency
360 channels are synthesized at
the first mixer
Low-side injection is used for
channels 127.00 MHz to 135.95
MHz
High-side injection for 118.00
MHz to 126.95 MHz
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The received antenna signal is
coupled to the preselector
through a diode transmit/receive
(T/R) switch.
A two-pole, varactor-tuned RF
filter couples the antenna to the
RF stage.
A second varactor-tuned filter
couples the amplified RF signal to
the first mixer and supplies
additional image and ½ IF
spurious rejection.
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25. AVIONICS
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Bendix/King KX-170A/KX-175 VHF Transceiver (cont’d)
Characteristics of Bendix/King
KX-170A/KX-175 are (cont’d):
The second IF contains two
integrated circuit (I.C.) amplifiers
with three double-tuned
interstage networks for additional
receiver selectivity.
An active detector/noise limiter
provides audio gain, rate noise
limiting, and 90% AM clipping of
noise spikes.
A two-stage AGC amplifier is
used to control the gain of the RF
stage and the first IC amplifier in
the second IF strips.
The amplifier RF signal is mixed
with the synthesized injection
frequency in a balanced mixer.
A two-pole crystal filter couples
the difference frequency to the
second mixer and provides image
and ½ IF selectivity.
The 8.13875-MHz crystal
controlled second local oscillator
develops injection for the second
mixer.
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Bendix/King KX-170A/KX-175 VHF Transceiver (cont’d)
Characteristics of Bendix/King
KX-170A/KX-175 are (cont’d):
The receiver outputs 6 dB into
the AGC with no input signal.
This eliminates conventional gain
threshold effects and establishes
a constant "signal plus noise" at
the detector output.
The filtered noise is amplified
and used to operate a squelch
gate.
The transmitter is a solid-state,
four-stage, broadband, 30-dB
gain, RF power amplifier.
Modulation is applied to the
driver and final stages.
The detector noise bandwidth is
approximately 15 kHz.
The low-pass filter provides
harmonic spurious rejection.
A noise filter passes "white noise"
containing frequency components
above 7 kHz.
A series regulator supplies 8.5
volts to RF and audio circuitry.
A zener regulator maintains 5.0
volts to digital circuitry used in
the frequency synthesizers.
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AN/ARC-197 VHF Transceiver
The AN/ARC-197 VHF communication system provides an aircraft with two-way,
plain voice radio communications in the very high-frequency range.
Its actual frequency range is from 116.000 to 151.975 MHz.
In the P-3C aircraft (maritime patrol aircraft), this system interfaces with four of
the intercommunication stations in the transmit/receive functions.
The pilot, copilot, TACCO, and NAV/COMM stations can transmit and receive
over this radio.
The other stations in the aircraft have received function only.
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AN/ARC-197 VHF Transceiver (cont’d)
Major Components
There are three components to the AN/ARC-197 system:
The RT-1397/ARC-197 transceiver
The C-11067/ARC-197 VHF-AM control panel
The 949880 VHF antenna
The RT-1397/ARC-197 transceiver is a solid-state unit,
consisting of:
a power supply
frequency synthesizer
receiver modulator
transmitter
one indicator
one push button,
one microphone jack
one headphone jack on the unit
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RT-1397/ARC-197 transceiver
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AN/ARC-197 VHF Transceiver (cont’d)
The RT-1397/ARC-197 transceiver
(cont’d):
The indicator is labeled TRANSMIT
POWER, which illuminates when
output power is greater than 10
watts.
The push button is labeled SQUELCH
DISABLE, which will disable the
squelch for low signal levels.
The microphone and headphone
jacks are used for maintenance and
emergency VHF communication in
case of ICS failure in-flight.
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RT-1397/ARC-197 transceiver
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AN/ARC-197 VHF Transceiver (cont’d)
C-11067/ARC-197 VHF-AM Control
Box
The control box controls the
operation of the system.
There are two dual function knobs
and a display window on the control
panel.
The display window shows the
selected frequency of the system.
The outer ring of the dual function
knob on the left applies system
power and selects the test function.
The inner knob changes the
frequency of operation in 1-MHz
steps over the range of control.
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VHF-AM control box
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AN/ARC-197 VHF Transceiver (cont’d)
C-11067/ARC-197 VHF-AM Control
Box (cont’d):
The outer ring of the dual function
switch on the right is labeled VOL,
and it is not used in the P-3 aircraft.
Volume is controlled by the ICS
system.
The inner knob of this control is used
to change the frequency of operation
in 25-kHz steps over the range of
control.
VHF-AM control box
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AN/ARC-197 VHF Transceiver (cont’d)
949880 VHF Antenna:
The 949880 antenna is located in the
tailcap on top of the vertical stabilizer
of the P-3 aircraft.
This antenna radiates and receives
the VHF radio frequency signals.
Signals routed to and from the
antenna go through a VHF bandpass
filter, which reduces the crosstalk
between the VHF and UHF systems.
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AN/ARC-197 VHF Transceiver (cont’d)
Functional Description
There are two modes of operation with
the AN/ARC- 197 radio.
The receive mode
The transmit mode
Receive Mode:
The received RF signals from the
antenna are routed through the
filter, and applied to the receiver
circuits in the transceiver.
The frequency selected on the
control box is applied to the
frequency synthesizer.
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The synthesizer uses a single
phase-locked loop to generate RF
injection frequencies, in 25-kHz
steps, from 116.000 to 155.975
MHz.
The RF injection frequencies, along
with DC tuning voltages,
electronically tune the receiver to
the selected frequency.
The AM detected audio is applied to
the audio amplifier circuit.
Squelch circuits disable the output
amplifier if the required signal-tonoise ratio or carrier level is not
present.
The output audio is then applied to
the ICS interconnection box for
distribution to the various stations.
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AN/ARC-197 VHF Transceiver (cont’d)
Functional Description (cont’d)
Transmit Mode:
The VHF XMTR control signal from
any one of the four ICS master
control panels applies a ground to
the transceiver as the VHF key
signal.
This VHF key signal provides the
push-to-talk command to the
transceiver to switch it from the
receive to the transmit mode of
operation.
The synthesizer generates
transmitter drive frequencies from
116.000 to 155.975 MHz in 25 kHz
steps.
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Audio from any of the four ICS
master control boxes are applied to
the modulator circuit.
The modulator circuit provides 90percent amplitude modulation.
The transmitter uses five stages of
amplifiers to raise the output to 20
watts.
The RF output is routed to the
antenna, through the filter, for
radiation.
The transceiver also produces a
sidetone output, which is provided
to the ICS system in the same
manner as the receiver audio.
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