In order to demodulate a signal transmitted without a

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does not convey any information. In order to demodulate a signal transmitted without a carrier it is necessary to reinsert the carrier at the receiving end (this is done in the demodulator stage where a beat frequency oscillator or carrier insertion oscillator replaces the missing carrier signal at the final intermediate frequency—see Figure 5.9). The absence of the carrier means that less power is wasted in the transmitter which consequently operates at significantly higher efficiency. 5.2 SSB modulation Figure 5.2 Santa Maria oceanic service (NAT-A) showing operational frequencies and times together with typical variation of MUF for a path from Madrid to New York shows how the service provided by the Santa Maria HF oceanic service makes use of different parts of the HF spectrum at different times of the day and night. Note the correlation between the service availability chart shown in Figure 5.2(a) and the typical variation in maximum usable frequency (MUF) for the radio path between Madrid and New York. The following HF bands are allocated to the aeronautical service: 2850 to 3155 kHz 3400 to 3500 kHz 4650 to 4750 kHz 5480 to 5730 kHz 6525 to 6765 kHz 8815 to 9040 kHz 10,005 to 10,100 kHz 11,175 to 11,400 kHz 13,200 to 13,360 kHz 15,010 to 15,100 kHz 17,900 to 18,030 kHz 21,870 to 22,000 kHz 23,200 to 23,350 kHz.
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HF communications 75 Test your understanding 5.1 1. Explain why HF radio is used on trans-oceanic routes. 2. Explain why different frequencies are used for HF aircraft communications during the day and at night. 3. State TWO advantages of using SSB modulation for aircraft HF communications. Figure 5.3 Frequency spectrum of an RF carrier using DSB and SSB modulation Figure 5.3 shows the frequency spectrum of an RF signal using different types of amplitude modulation, with and without a carrier. In Figure 5.3(a) the mode of transmission is conventional double sideband (DSB) amplitude modulation with full-carrier. This form of modulation is used for VHF aircraft communications and was described earlier in Chapter 4. Figure 5.3(b) shows the effect of suppressing the carrier. This type of modulation is known as double sideband suppressed-carrier (DSB-SC). In practical DSB-SC systems the level of the carrier is typically reduced by 30 dB, or more. The DSB-SC signal has the same overall bandwidth as the DSB full-carrier signal but the reduction in carrier results in improved efficiency as well as reduced susceptibility to heterodyne interference. Figure 5.3(c) shows the effect of removing both the carrier and the upper sideband. The resulting signal is referred to as single sideband (SSB), in this case using only the lower sideband (LSB). Note how the overall bandwidth has been reduced to only around 3.5 kHz, i.e. half that of the comparable DSB AM signal shown in Figure 5.3(a). Finally, Figure 5.3(d) shows the effect of removing the carrier and the lower sideband.
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