nav topic 4 hf communication system

AV2220 - Aircraft Communication Systems Chapter 2 1

AVIONICS AVIONICS TECHNOLOGYTECHNOLOGY

High Frequency (HF) High Frequency (HF) CommunicationCommunication

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.



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.



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.

HF System Description



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.

HF System Description (cont’d)

A typical HF radio control panel



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.


A typical antenna coupler system



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.




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.




The Rockwell Collins HF-220 Communication System provides long-range 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.

HF System Operation

Rockwell Collins HF-220 Communication System

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



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.

Rockwell Collins HF-220 Communication System (cont’d)



Rockwell Collins HF-220 signal flow diagram



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.




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.




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.




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.




The CTL-220 also provides a "clarifier' control to improve the reception of off-channel 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 10- to 30-foot grounded wire antenna.




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.

AN/ARC-161 HF Communication System



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.

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.




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



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.





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





XMTR OFF indicator – When this indicator is illuminated the radio set cannot be used to transmit.

This indicator will come on during the 3-minute 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.





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 self-test functions.





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.





CU-2070/ARC Coupler The CU-2070/ARC coupler provides

impedance matching between the receiver transmitter and the antenna.

The lightning arrester provides a spark gap to protect the radio if lightning strikes the long-wire antenna.

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.

If either antenna breaks in flight, the wire will depart the aircraft.


CU-2070/ARC coupler



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



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.





PWR/FILL switch – This is a four-position 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.






HF system block diagram



HF Functional Description

Receive Function The RF signal is received by the antenna, coupled through the coupler and the de-

energized 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.




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.




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.

HF Automatic Link Establishment (ALE)



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 user-friendly from a pilot's standpoint.

HF Automatic Link Establishment (cont’d)

nav topic 4 hf communication system

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