Physical basis of radio communication. A simple radio receiver. Radiolocation
Transmitting information with electromagnetic waves is called radio communication; the Russian engineer A. S. Popov demonstrated the reception of radio signals on 7 May 1895, and the Italian G. Marconi worked on it independently in the same years. Sound frequencies are low and cannot be sent directly, because wave energy depends strongly on frequency (I ∝ ν⁴) and different stations would mix; so the microphone signal is added to the high-frequency oscillations of a generator: this is modulation. The modulated wave is radiated by an antenna. In the receiver the antenna picks up the signal, the L–C input circuit (with a variable capacitor) is tuned to resonate with the wanted station, then the detector (a semiconductor diode) separates the low-frequency sound signal from the high-frequency one: this is detection; after amplification a loudspeaker or earphone turns it into sound. In radiolocation a short pulse is sent and the wave reflected from the target is detected; if the time Δt passes between sending and return, 2l = c·Δt, i.e. l = c·Δt/2.
Draw the scheme of a «crystal receiver» (antenna, L–C circuit, diode, earphone) in your notebook with the teacher's help and write the job of each block. Never run outdoor antenna wires near high-voltage power lines and never use one during a thunderstorm.