20 Questions
What is the purpose of the SELCAL system?
For signalling purpose
What happens when an aircraft receives both direct and reflected waves?
Fading or even short-term loss of voice communication
What is the maximum range of a VHF signal transmitted from an aircraft flying at 12000 ft received by another aircraft flying at 9000 ft?
255 nm
What is the primary use of HF communication?
For air traffic control
What is the term for the distance between any two successive ground reflections?
Skip zone
What type of wave is used for short-range communication in HF?
Ground wave
What is the purpose of the SELCAL system?
To selectively activate the receiver for a specific aircraft
What is the duration of each pulse in the SELCAL system?
1±0.25 seconds
What is the function of the decoder in the SELCAL system?
To decode the audio frequency pulses
Which of the following is NOT a component of the SELCAL system?
VHF Transceiver
What is the purpose of the 'RED x' code in the SELCAL system?
To identify a specific aircraft
What is the primary purpose of the coupler in a HF communication system?
To provide a matching network for the antenna
What is the frequency of the 'RED A' tone in the SELCAL system?
312.6 Hz
What is the reason for loss of voice communication in certain circumstances?
Changes in the ionosphere conditions
How does the SELCAL system reduce unnecessary audio signals?
By using a tone selector to mute the receiver
What type of radio system is most suitable for high frequency communication?
AM radio system
What is the purpose of the Tone Selector in the SELCAL system?
To select the correct tone for a specific aircraft
What is the purpose of the frequency and power control panel in a HF communication system?
To select the frequency and set the transmission power
What is the role of the flight interphone system in a HF communication system?
To provide microphone audio input for voice transmission
What is the purpose of the SELCAL system in a HF communication system?
To signal for voice communication
Study Notes
Radio Frequency (RF) Communication
- RF signals travel between transceiver and antenna on a RF-Coax line
- De-modulated audio is guided to the flight interphone system for voice communications and to SELCAL for signalling purpose
- Flight recorder system receives analog pulse to signify that the transmission has been acknowledged
- In certain circumstances, the aircraft may receive both direct and reflected waves, causing fading or even short-term loss of voice communication
VHF Communication
- Maximum range of a VHF signal transmitted from an aircraft flying at 12000 ft can be received by another aircraft flying at 9000 ft, calculated as: Range = 1.25 × (9000 + 12000) = 255 nm
High Frequency (HF) Communication
- HF communication is used for long-range air-to-air and air-to-ground voice communications
- Ionospheric reflection is used for long-range HF transmission
- Factors affecting the range of HF transmission include:
- Transmission power at aircraft and ground station
- Time of day
- Season of the year
- Ionospheric disturbances
- Geographical location
- Frequency in use
- A HF communications system consists of:
- Frequency and Power control panel
- Transceiver
- Coupler (matching network for antenna)
- SELCAL system allows pilots to mute the receiver until ATC transmits a group of two pulses
SELCAL System
- SELCAL system allows pilots to receive selective calling signals
- Each aircraft is allocated a specific code, designated as ‘RED x’, where x is a letter corresponding to the audio frequency of the pulses transmitted
- Table 1-1 shows the SELCAL tone allocation, with frequencies ranging from 312.6 Hz to 1479.1 Hz
- SELCAL communications system consists of:
- Decoder
- Tone Selector
- Selcal Lamps/Switches
- Decoder receives audio from VHF/HF transceiver and decodes the signal to alert the pilot
This quiz covers the basics of radio frequency signals and communication systems in aircraft, including transmission and reception of signals, and the role of various components like transceivers and antennas.
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