A typical need for modulation in communication system pdf VHF radio. The display shows an active frequency of 123.
5 MHz and a standby frequency of 121. The two are exchanged using the button marked with a double-headed arrow. The tuning control on the right only affects the standby frequency. Usually these other frequency bands are only used in oceanic and remote areas, though they work over wider areas or even globally. Military aircraft also use a dedicated UHF-AM band from 225.
95 MHz for air-to-air and air-to-ground, including air traffic control communication. This band has a designated emergency and guard channel of 243. 70 channels from 118 to 132 MHz. Some radios of that time provided receive-only coverage below 118 MHz for a total of 90 channels. On 1 January 1990 the frequencies between 136. 975 MHz were added, resulting in 760 channels.
Increasing air traffic congestion has led to further subdivision into narrow-band 8. Besides being simple, power-efficient and compatible with legacy equipment, AM and SSB permit stronger stations to override weaker or interfering stations. Even if a pilot is transmitting, a control tower can “talk over” that transmission and other aircraft will hear a somewhat garbled mixture of both transmissions, rather than just one or the other. In the newer channel spacing scheme, the largest bandwidth of an airband channel might be limited to 8. However, most airband voice transmissions never actually reach these limits. Different aircraft, control towers and other users transmit with different bandwidths and audio characteristics.