How the swr has been reduced to less than 21 for most

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how the SWR has been reduced to less than 2:1 for most (if not all) of the HF range. The tuning adjustment of HF antenna coupler is entirely automatic and only requires a brief signal from the transmitter to retune to a new HF frequency. The HF antenna coupler unit incorporates an SWR bridge (see page 35) and a feedback control system (see Figure 5.17) to adjust a roller coater inductor (L1) and high- voltage vacuum variable capacitor (C1) together with a number of switched high-voltage capacitors (C1 to C4). The internal arrangement of a typical HF antenna coupler is shown in Figures 5.18 and 5.19. The connections required between the HF antenna coupler, HF radio and control unit are shown in Figure 5.20. Voltages present in the vicinity of the HF antenna (as well as the field radiated by it) can be extremely dangerous. It is therefore essential to avoid contact with the antenna and to maintain a safe working distance from it (at least 5 metres) whenever the HF radio system is ‘live’. Figure 5.17 Typical feedback control system used in an HF antenna coupler
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84 Aircraft communications and navigation systems 5.7 Multiple choice questions 1. The typical bandwidth of an aircraft HF SSB signal is: (a) 3.4 kHz (b) 7 kHz (c) 25 kHz. 2. The principal advantage of SSB over DSB AM is: (a) reduced bandwidth (b) improved frequency response (c) faster data rates can be supported. 3. HF data link uses typical data rates of: (a) 300 bps and 600 bps (b) 2400 bps and 4800 bps (c) 2400 bps and 31,500 bps. 4. The standard for HF data link is defined in: (a) ARINC 429 (b) ARINC 573 (c) ARINC 635. 5. Which one of the following gives the approximate range of audio frequencies used for SELCAL tones? (a) 256 Hz to 2048 Hz (b) 312 Hz to 1479 Hz (c) 300 Hz to 3400 Hz. 6. How many alphanumeric characters are transmitted in a SELCAL code? (a) 4 (b) 8 (c) 16. 7. How many bits are used in an ICAO aircraft address? (a) 16 (b) 24 (c) 32. 8. The typical RF output power from an aircraft HF transmitter is: (a) 25 W pep (b) 50 W pep (c) 400 W pep. 9. An HF radio is required for use on oceanic routes because: (a) VHF coverage is inadequate (b) higher power levels can be produced (c) HF radio is more reliable. 10. The function of an HF antenna coupler is to: (a) reduce static noise and interference (b) increase the transmitter output power (c) match the antenna to the radio. Figure 5.20 Connections to the HF radio, control unit and antenna coupling unit
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