AET Communication Systems 4 — Questions and Answers
Question 1: An HF antenna coupler (ATU) is required because:
- HF frequencies require circular polarization
- Antenna impedance varies widely across HF frequencies and must be matched to the transmitter (Correct answer)
- The HF transceiver output is balanced and the antenna is unbalanced
- HF signals cannot propagate without impedance transformation to 600 ohms
Correct answer: Antenna impedance varies widely across HF frequencies and must be matched to the transmitter
HF antennas are electrically short relative to the wavelength at many HF frequencies, so the ATU performs impedance matching to maximize power transfer.
Question 2: ACARS (Aircraft Communications Addressing and Reporting System) transmits data using:
- Spread-spectrum FSK over HF
- VHF AM with GFSK data subcarrier (VDL Mode 0/A) or VDL Mode 2 (Correct answer)
- Dedicated SATCOM L-band only
- UHF TDMA burst transmissions
Correct answer: VHF AM with GFSK data subcarrier (VDL Mode 0/A) or VDL Mode 2
Classic ACARS uses 2400 bps GFSK data over VHF AM channels, while the newer VDL Mode 2 standard uses GFSK at 31.5 kbps over VHF.
Question 3: Which Federal Aviation Regulation (FAR) part governs the installation and airworthiness of aircraft communication equipment?
- FAR Part 61
- FAR Part 43 and Part 91 (Correct answer)
- FAR Part 121 only
- FAR Part 65
Correct answer: FAR Part 43 and Part 91
FAR Part 43 covers maintenance and alteration of avionics, while Part 91 (and 121/135 as applicable) specifies equipment requirements and operational rules.
Question 4: A VHF communication radio is transmitting on the correct frequency but the transmission sounds distorted at receiving stations. The MOST likely cause is:
- Incorrect squelch threshold
- Audio input overdriving the modulator causing over-modulation (Correct answer)
- Low voltage on the frequency synthesizer
- Incorrect antenna polarization
Correct answer: Audio input overdriving the modulator causing over-modulation
Over-modulation occurs when excessive audio levels drive the modulator beyond 100%, producing harmonic splatter and distorted audio at receivers.
Question 5: Emergency Locator Transmitters (ELTs) required under TSO-C126b operate on which frequency?
- 121.5 MHz only
- 406 MHz (with optional 121.5 MHz homing) (Correct answer)
- 243.0 MHz only
- 406 MHz and 243 MHz simultaneously
Correct answer: 406 MHz (with optional 121.5 MHz homing)
Modern 406 MHz ELTs transmit a digital coded burst processed by COSPAS-SARSAT satellites, and typically also transmit on 121.5 MHz for local homing.
Question 6: In a typical aircraft audio management system, a 'hot mic' mode means:
- The microphone is tested at elevated audio levels
- The crew microphone is continuously open without requiring a PTT press (Correct answer)
- The microphone is switched to high-impedance input
- Emergency channel monitoring is activated on all mics
Correct answer: The crew microphone is continuously open without requiring a PTT press
Hot mic enables voice-activated (VOX) or continuous microphone transmission, typically used in crew oxygen mask situations where PTT operation is impractical.
Question 7: What is the primary advantage of 8.33 kHz VHF channel spacing over legacy 25 kHz spacing in European airspace?
- Increased transmitter output power
- Triples the number of available VHF communication channels (Correct answer)
- Reduces interference from HF signals
- Allows digital voice encoding
Correct answer: Triples the number of available VHF communication channels
8.33 kHz spacing increases the number of usable VHF channels from approximately 760 to over 2280 in the 118–136.975 MHz band, addressing frequency congestion.
AET Communication Systems 4
An HF antenna coupler (ATU) is required because: