1. High Frequency (HF)
Communication
AVIONICS
TECHNOLOGY
HF communication systems are long-distance communication systems and
are not employed on all aircraft.


Airlines may or may not utilize these systems, depending on their particular
requirements.
HF systems are not usually found in light aircraft.
Many airlines that employ HF communication systems do so because these
systems provide for an extended range of communications between
aircraft and from aircraft to ground stations.
HF communication systems are used primarily on larger aircraft that
require extended communication range.
Typical aircraft HF communication systems consists of:

a control head located in the cockpit
an HF transceiver and power amplifier located in the radio rack

an antenna coupler located in close proximity of the antenna.

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Aircraft HF antennas used on earlier aircraft consisted simply of an
insulated copper wire that extended from the aft of the fuselage by means
of an electric motor that adjusted the antenna's resonant length to
correspond with the transmitted or received frequency.
Modern aircraft use:

a fixed length antenna of either the long wire variety that extends from
vertical stabilizer to a point on the top of the fuselage, or

an antenna that is molded into a composite panel that forms the leading
edge of the vertical stabilizer or some other surface on the aircraft.
The antenna coupler matches the fixed length of the long-wire or
embedded antenna to the proper resonant frequency by loading the
antenna through a series of LC tuned circuits.
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HF System Description
The high-frequency (HF) communication systems operate in the frequency
range of 2.0 to 30 MHz.
The HF range is actually a middle-frequency range, in as much as it starts just
above the standard broadcast band, which ends at approximately 1700 kHz.
This frequency group consists of ground waves; therefore,

HF communication systems are used for long-distance radio transmissions.
The HF system on an airplane is used to provide two-way voice communication
with or digitally coded signals for ground stations or other aircraft.
The HF radio control panel is located where it is easily accessible to the pilot or
copilot.
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HF System Description (cont’d)
A typical panel includes a frequency selector, a squelch or RF gain control, and
a mode selection switch.
On most aircraft the antenna for an HF radio is covered by plastic-type shields.
The cover may be of fiberglass or a similar material that will allow
electromagnetic waves to reach the antenna.
A typical HF radio control panel
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HF System Description (cont’d)
On modern aircraft a flush mounted
antenna is used that does not
increase induced drag.
The probe antenna is used for both
receiving and transmitting and is
matched to the transmission line at
any frequency by means of an
antenna coupler.
An antenna coupler system may
consist of a remote coupler unit and a
coupler-unit control.
The antenna coupler system is
necessary to maintain an efficient
match between the antenna and the
transmitter at a wide range of
frequencies.
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A typical antenna coupler system
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HF System Description (cont’d)
The complete HF transceiver is installed in an electronic equipment rack and is
remotely controlled from the control unit on the flight deck.
The HF system consists of:

The HF receiver-transmitter

The HF control unit

The antenna coupler system

The antenna
The receiver-transmitter operates at high frequencies between 2.0 and 29.999
MHz in one of several different modes.
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HF System Description (cont’d)
The different modes are:






single sideband (SSB)
upper sideband (USB),
lower sideband (LSB)
full carrier amplitude modulation (AM)
continuous wave (CW)
data modes
The different modes determine the waveform characteristics of the signals
transmitted and received by the radio.

All the data mode are used for voice communications.
The data mode is used for digital-type information that is linked to equipment
external to the HF radio system.
The data communication system is known as air-ground data link, or data link.
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HF System Operation
Rockwell Collins HF-220 Communication System
The Rockwell Collins HF-220
Communication System provides longrange voice communications in the
frequency range of 2.0 MHz to
29.9999 MHz.
The HF-220 system is controlled by
the Collins CTL-220 control head.
The mode control on the left side of
the CTL-220 provides the selection of
either:
 AM
 USB
 split-channel, reduced-carrier
(A3A)
 split-channel, suppressed-carrier
(A3J) transmissions
Rockwell Collins CTL-200 HF control head
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Rockwell Collins HF-220 Communication System (cont’d)
Signals are received and transmitted by the TCR-220 transceiver and amplified by
the PWR-200 power amplifier before being sent to the AAC-220 automatic antenna
coupler.
The HF-220 system is automatically tuned by the antenna coupler.
The power amplifier provides 100 watts peak envelope power (PEP) in SSB mode
and 25 wafts RF output in the AM mode.
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Rockwell Collins HF-220 signal flow diagram
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Rockwell Collins HF-220 Communication System (cont’d)
Referring to the signal flow diagram of the HF-220, the operation of the system is
initiated when the on/off mode selector on the CTL-220 control is rotated clockwise
out of detent.
A relay in the PWR-200 is then actuated and switches +28 volts from the aircraft
power bus to the PWR-200, TCR-220, and AAA-200.
Control logic in the TCR-220 grounds the key-line whenever the microphone PTT
switch is pressed.
A relay in the TCR-220, actuated by the transmitter key-line, supplies power to the
transmit circuits and a relay in the PWR-200, actuated by the same key-line, applies
power to the power amplifier circuits.
Frequency selection involves the use of the frequency and channel controls on the
CTL220, and the program card, synthesizer, and reference divider/multiplier in the
TCR-220 transceiver.
Direct tuning of the CTL220 control allows selection of any frequency in the 2.0-MHz
to 29.9999-MHz frequency range with 100-Hz frequency spacing.
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Rockwell Collins HF-220 Communication System (cont’d)
Channel selection on the CTL220 allows selection of any one of 20 specified
frequencies programmed on the program card.
CTL220 clock and data lines send the selected frequency or channel information in
serial form to the program card.
The system provides either duplex operation (the transmitted frequency is different
from the received frequency) or simplex operation (the transmitted and received
frequencies are the same).
The HF220 system receive and transmit frequencies are established by a double loop
circuit in the synthesizer of the TCR220.
The synthesizer is frequency and phase locked to an oven stabilized, crystal
controlled frequency reference oscillator.
The CTL220 converts the frequency or channel selected into 22-bit serial control
information.
The program card converts the serial control information to 22 parallel control lines
that are inputs to the synthesizer.
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Rockwell Collins HF-220 Communication System (cont’d)
If the frequency synthesizer becomes unlocked from the crystal controlled frequency
standard, an interrupted tone is applied to the audio output indicating that the
transmitter is disabled.
In receive mode, either the USB or AM circuits in the TCR-220 are selected by the
mode control switch on the CTL-220.
The received signal from the antenna is applied through half-octave low-pass filters
in the PWR-200 that provide protection from high-frequency, out-of-band signals.
An additional bandpass filter and over-voltage diodes in the TCR-220 provide further
protection for the receive circuits.
The receive signal is then amplified, mixed to 69.8 MHz to remove spurious
responses, filtered through a crystal filter, and mixed down to 500 kHz, the IF
frequency.
A low-noise IF preamplifier, with either the 500-kHz SSB or AM filter, is selected by
control logic in the TCR-220.
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Rockwell Collins HF-220 Communication System (cont’d)
Control logic in the TCR-220 actuates a product detector for SSB signals or a diode
detector for AM signals.
The resulting audio signal is remotely controlled by use of an electronic attenuator
with the DC control located in the CTL-220.
Two systems of AGC are used. The normal IF AGC operates up to an RF input level
of approximately 100 microvolts.
When the 100 microvolt limit is reached, the RF AGC circuits are actuated and
attenuate the RF amplifier and the IF preamplifiers up to an input level of 10
millivolts.
The squelch control on the CTL-220 adjusts the IF gain, resulting in smooth quieting
of the receiver as the control is rotated clockwise.
An audio switching circuit mutes the audio whenever the AGC drops below the
threshold determined by the position of the squelch control.
A receiver test function is provided by a switch on the CTL-220 which inhibits the
audio muting.
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Rockwell Collins HF-220 Communication System (cont’d)
The CTL-220 also provides a "clarifier' control to improve the reception of offchannel SSB station.
This control allows the BFO, and thus the received SSB signal, to be varied plus or
minus 100 Hz from the center frequency.
The transmit circuits can be actuated in two different ways:

by grounding the key-line (microphone PTT button)

by grounding the tune-in progress (TIP) line
The TIP ground is automatically applied whenever the AAC200 is being tuned.
The PWR-200 amplifies the RF signal from the TCR-220 to the 100-watt PEP level.
This is accomplished with two stages of push-pull amplification.
The RF output from the PWR-200 is applied to the AAC-200.
The AAC-200 automatically matches the 50-ohm output of the PWR-200 to the 10to 30-foot grounded wire antenna.
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AN/ARC-161 HF Communication System
The AN/ARC-161 HF radio system provides two-way communications between the
aircraft and any similarly equipped platform.


It provides transmission, reception, and processing of intelligence and tactical data.
The cipher voice communication is provided by the TSEC/KY-75 high-frequency
security unit.
The HF communication system consists of two radio sets and interfacing equipment.

These sets are called HF-1 and HF-2.
The two radio sets provide two-way, single-sideband (SSB) and plain voice (AME)
communication in the frequency range of 2.0000 to 29.9999 MHz, and they
automatically tune 280,000 channels at 100-HZ intervals.
The HF-1 antenna has four degraded frequency ranges at which coupling might not
be possible: 4.8000 to 6.0000 MHz, 12.0000 to 13.2000 MHz, 19.1000 to 20.3000
MHz, and 26.5000 to 27.7000 MHz.
The HF-2 antenna has two degraded frequency ranges where coupling might not be
possible: 7.9000 to 9.1000 MHz and 18.9000 to 20.1000 MHz.
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AN/ARC-161 HF Communication System (cont’d)
HF Major Components
Each AN/ARC-161 radio set in the HF communications system consists of the following
components:

a RT-1000/ARC-161 receiver transmitter

an AM-6561/ARC-161 RF amplifier

a C-9245/ARC-161 control box

a CU-2070/ARC antenna coupler (which includes a lightning arrester)

a long-wire antenna
Both radio sets share the use of a TSEC/KY-75 security unit and TSEC/KY-75 remote
control unit.
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AN/ARC-161 HF Communication System (cont’d)
HF Major Components (cont’d)
RT-1000/ARC-161 Receiver Transmitter

The RT-1000/ARC-161 receiver transmitter controls the radio tuning, coupler tuning,
and mode selection.

It also contains all circuitry required for the generating of transmitter signals and the
processing of received RF signals into audio/data signals.
AM-6561/ARC-161 Radio-Frequency Amplifier

This component amplifies the output of the RT prior to transmission.
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AN/ARC-161 HF Communication System (cont’d)
C-9245/ARC-161 Control Box

The C-9245/ARC-161 control box
controls the operation of the HF
radio set.

There are two of these in the
aircraft, one for each radio set.
They are identical and
interchangeable.
C-9245/ARC-161 control box
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AN/ARC-161 HF Communication System (cont’d)
C-9245/ARC-161 Control Box
through 6: MHz select switches –
Selects the desired frequency in the
operating range of 2.0000 to 29.9999
MHz.
SQ control knob – Controls the amount
of squelch threshold voltage in VO and
AME operations.
SQ OFF switch – When this switch is
pressed, the squelch circuits are disabled.
CPLR READY indicator – This indicator
will illuminate when the associated
coupler is properly tuned.
C-9245/ARC-161 control box
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AN/ARC-161 HF Communication System (cont’d)
CPLR FAULT indicator – This indicator
will illuminate to indicate improper
tuning of the associated coupler.
SYS READY indicator – This indicator
will illuminate when the radio set has
passed the self-test routine.
SYS FAULT indicator – This indicator
will illuminate anytime the radio set fails
self-test.
RESET switch – This switch resets the
SYS FAULT indicator to a GO condition
after a NO-GO condition was detected
C-9245/ARC-161 control box
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AN/ARC-161 HF Communication System (cont’d)
XMTR OFF indicator – When this
indicator is illuminated the radio set
cannot be used to transmit.
This indicator will come on during the 3minute warm-up period (the first 3
minutes of initial power up), and
anytime the operating frequency is
changed by more than 1 MHz.
Other than that, anytime there is a fault
in the system that affects the
transmitter, this indicator will illuminate.
VOL control – This control knob is not
used in the P-3 aircraft configuration.
BLANKER ON switch – This switch is
not used in the P-3 aircraft
configuration.
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23. AVIONICS
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AN/ARC-161 HF Communication System (cont’d)
COND switch – This switch selects the
radio set’s condition of operation.
 OFF–This position removes primary
power from the radio set.
 STBY–Applies power to the radio
set’s RF Amp final stage filament,
frequency standard and other
critical circuits.
 HI–Energizes the radio set and sets
the transmitter for high-power
output.
 LO–Energizes the radio set and sets
the transmitter for low-power
output.
 TEST–Initializes the radio set’s selftest functions.
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C-9245/ARC-161 control box
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24. AVIONICS
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AN/ARC-161 HF Communication System (cont’d)
MODE switch – This switch selects the
radio set’s mode of operation.

VO–This position permits forced
mode selection by the COMM
system selector panel.

AME–This position permits AME
transmission and reception.

CW–With the switch in this position,
the radio set will transmit and
receive continuous-wave signals.

DATA, DIV, LSB–In this position,
permits forced mode selection by
the COMM system selector panel.
C-9245/ARC-161 control box
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AN/ARC-161 HF Communication System (cont’d)
CU-2070/ARC Coupler
Long-wire Antenna
The lightning arrester provides a
spark gap to protect the radio if
lightning strikes the long-wire
antenna.
There are two antennas used in the
P-3 aircraft for the HF system.

Both are located external of the
aircraft, and stretch from the top of
the vertical stabilizer to the
fuselage.

The one on the left is used by
HF-1, and the one on the right is
for the HF-2 radio set.
Both of the top mounts for the
antennas are the breakaway type.


The CU-2070/ARC coupler provides
impedance matching between the
receiver transmitter and the
antenna.



If either antenna breaks in flight,
the wire will depart the aircraft.
CU-2070/ARC coupler
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AN/ARC-161 HF Communication System (cont’d)
TSEC/KY-75 Security Unit

The TSEC/KY-75 provides the
encoding and decoding of the
signals when the radio set is in the
cipher mode.
TSEC/KY-75 Remote Control Unit

The TSEC/KY-75 remote control
unit (RCU) enables the operator to
operate the security unit without
moving from the NAV/COMM
station in the aircraft.
TSEC/KY-75 remote control unit
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AN/ARC-161 HF Communication System (cont’d)
TSEC/KY-75 Remote Control Unit
ALARM indicator – This indicator will
illuminate whenever there is an alarm
condition in the security unit.
TSEC/KY-75 RCU switch – This
switch is not used.
TSEC/KY-75 remote control unit
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AN/ARC-161 HF Communication System (cont’d)
PWR/FILL switch – This is a fourposition switch used to select the key
and storage locations for the various
codes used.
a. OFF/ZEROIZE–This position will
zeroize all variables stored in the
TSEC/KY-75 security unit. To select
this position, the operator must pull
the knob out 1/4 inch and turn it
counterclockwise. This prevents
accidental zeroizing during flight.
b. 1, 2, 3–These positions select a
particular key for the cipher
transmission or reception. They also
select the storage area when the key
is loaded into the TSEC/KY-75.
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TSEC/KY-75 remote control unit
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AN/ARC-161 HF Communication System (cont’d)
HF system block diagram
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AN/ARC-161 HF Communication System (cont’d)
HF Functional Description
Receive Function
 The RF signal is received by the antenna, coupled through the coupler and the deenergized transmit-receive relay in the RF amplifier, and finally to the receiver
transmitter receiver RF input.
 RF-to-audio conversion is accomplished in the receiver section using inputs from the
frequency synthesizer.
 The recovered audio component is amplified to a power level sufficient to be used as
an audio input to the peripheral equipment.
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AN/ARC-161 HF Communication System (cont’d)
HF Functional Description (cont’d)
Transmit Function

During the transmit function, voice or data signals from the peripheral equipment are
switched through the COMM switching matrix to the USB and LSB transmit audio
inputs to the receiver transmitter.

A keying signal is also applied from the peripheral equipment.

For emergency or maintenance operations, voice and keying signals can originate at
the mic/headset assembly connected directly to the receiver transmitter.

Audio-to-RF conversion is accomplished when the carrier frequency is modulated by
the audio input.

The modulated RF is amplified by the RF amp to the selected power level of 400 or
1000 watts, depending on the mode selected on the control box, and then coupled to
the antenna for transmission.
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HF Automatic Link Establishment (ALE)
One of the problems often encountered with the use of HF radio:

HF depends on energy being deflected off of the ionosphere layer to
achieve long-range communications.
This layer changes in density and altitude depending on the time of day.
As the ionosphere layer changes, the frequency at which skip waves are most
effective also changes.
As a result, a pilot who was experiencing good communications with a ground
station may find that same frequency used an hour ago is now unusable.
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HF Automatic Link Establishment (cont’d)
The Rockwell Collins HF-9000 with Automatic Link Establishment solves the
problem above.
The ALE processor automatically scans all the pre-programmed frequencies and
selects the one that is most optimum.
The HF-9000 then transmits a carrier or CW signal to a ground station
equipped with ALE which locks in on the selected frequency.
With 26 pre-selected frequencies stored in memory

the linking process takes approximately nine seconds before notifying the pilot

with the presence of white noise in the audio, that he or she can commence
communications
ALE and SELCAL are two features that make modern HF radios very userfriendly from a pilot's standpoint.
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