Radio Theory
This domain tests the physical ideas behind the radio you operate: how a signal is generated, modulated and received, how much spectrum it occupies, and what makes a receiver usable in a crowded band.
Carrier and modulation
- A transmitter produces an RF carrier at the assigned frequency. Aeronautical VHF uses amplitude modulation (A3E): speech varies the envelope, creating an upper and a lower sideband.
- AM bandwidth is roughly twice the highest modulating frequency. With 3 kHz speech the emission is about 6 kHz wide, so it fits inside a 25 kHz or 8.33 kHz channel.
- Aeronautical HF voice uses single sideband with suppressed carrier (J3E). The assigned frequency is the suppressed-carrier reference; the receiver must reinsert the carrier or the audio is unintelligible.
Receiver qualities
- Sensitivity is the ability to work a weak signal; selectivity is the ability to reject the adjacent channel. Both matter more as channel spacing tightens.
- Power ratios use decibels: 3 dB is a doubling, 10 dB is ten times. Impedance match (typically 50 ohms) and low VSWR keep power in the antenna rather than reflecting it back down the feeder.
Worked example. Find the wavelength and a quarter-wave radiator for 121.5 MHz. Wavelength = 300 divided by frequency in MHz = 300 / 121.5 = about 2.47 m. A quarter wave is 2.47 / 4 = about 0.62 m.
Traps. Confusing channel spacing with emission bandwidth; thinking 8.33 kHz spacing means 8.33 kHz of audio; treating the decibel as a linear unit; forgetting that an AM signal is only fully recovered when the carrier is present.