Stabiliser and they were sufficiently long to permit

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stabiliser and they were sufficiently long to permit resonant operation on one or more of the aeronautical HF bands. Unfortunately this type of antenna is unreliable and generally unsuitable for use with a modern high-speed passenger aircraft. The use of a large probe antenna is unattractive due to its susceptibility to static discharge and lightning strike. Hence an alternative solution in which the HF antenna is protected within the airframe is highly desirable. Early experiments (see Figure 5.13) showed that the vertical stabiliser (tail fin) would be a suitable location
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82 Aircraft communications and navigation systems Figure 5.13 Original sketches for a tail- mounted antenna from work carried out by E. H. Tooley in 1944 and is now invariably used to house the HF antenna and its associated coupling unit on most large transport aircraft—see Figures 5.11 and 5.12. Due to the restriction in available space (which mitigates against the use of a resonant antenna such as a quarter-wave Marconi antenna—see page 24) the HF antenna is based on a notch which uses part of the airframe in order to radiate effectively. The notch itself has a very high- Q factor and its resistance and reactance varies very widely over the operating frequency range (i.e. 3 MHz to 24 MHz). The typical variation of standing wave ratio (SWR—see page 33) against frequency for an HF notch antenna is shown in Figure 5.14. For comparison, the variation of SWR with frequency for a typical quarter-wave VHF blade antenna is shown in Figure 5.15. From Figures 5.14 and 5.15 it should be obvious that the HF antenna, whilst well matched at 21 MHz, would be severely mismatched to a Figure 5.14 Variation of SWR with frequency for an HF notch antenna (note the logarithmic scale used for SWR) Figure 5.15 Variation of SWR with frequency for a VHF quarter-wave blade antenna (note the linear scale used for SWR) Figure 5.16 Variation of SWR with frequency for an HF notch antenna fitted with an antenna coupling/tuning unit conventional 50 Ω feeder/transmitter at most other HF frequencies. Because of this, and because the notch antenna is usually voltage fed, it is necessary to use an HF coupling/tuning unit
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83 HF communications Test your understanding 5.3 Explain the function of an HF antenna coupler. What safety precautions need to be observed when accessing this unit? Figure 5.18 Interior view of an HF antenna coupler showing the roller coaster inductor (top) and vacuum variable capacitor (bottom). The high-voltage antenna connector is shown in the extreme right Figure 5.19 SWR bridge circuit incorporated in the HF antenna coupler. The output from the SWR bridge provides the error signal input to the automatic feedback control system between the HF radio feeder and the notch antenna. This unit is mounted in close proximity to the antenna, usually close to the top of the vertical stabiliser (see Figure 5.12). Figure 5.16 shows the effect of using a coupling/tuning unit on the SWR-frequency characteristic of the same notch antenna that was used in Figure 5.14. Note
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