Другое: RADIO SYSTEM BASICS (Основы системы радиосвязи)

RADIO SYSTEM BASICS
* A radar system is basically an evolution of a radio system, and it is useful to define the basic elements of a radio system first.
A radio system consists of a transmitter that produces radio waves and one or more receivers that pick them up, with both transmitter and receiver(s) fitted with antennas. The very earliest wireless telegraphy radio systems used a transmitter that simply generated a burst of radio energy by opening an electric circuit with a telegraph key and causing a spark. The radio waves propagated through space and set up an electric current in a receiving antenna, which in turn closed a relay switch. Messages were sent using Morse code.
The problem with this simple scheme is that the spark generated waves over a wide and indiscriminate range of frequencies, with a single receiver picking up and mixing up transmissions from every transmitter in the line of sight. The way to get around this problem is to fit each transmitter with a variable oscillator, an electronic circuit that generates electrical signals at different frequencies, as set by a knob turned by the transmitter operator. A receiver picks up this signal with its antenna, with the signal run. Run through a variable filter, an electric circuit consisting of an inductor coil and a variable capacitor that can be set by a knob turned by the receiver operator to block out all frequencies except one. This scheme allows multiple transmitters to operate in a given area without interference. The transmitter operator sets the transmitter oscillator to a given frequency, and then uses a telegraph key to gate the oscillator output on and off into an amplifier circuit, which drives it out the antenna. The receiver operator sets the receiver filter to the same frequency. The receiver picks up radio waves on all frequencies and amplifies them. The amplified received signal is run through the variable filter, and then into a detector circuit to convert high-frequency signals into a direct-current signal to activate the relay switch.
The detector includes a rectifier, a one-way valve for electricity, that eliminates half the waveform, with this rectified signal then passed through a low-pass filter, consisting in the simplest case of a resistor and a capacitor, that eliminates the high frequency carrier and leaves behind the low-frequency audio signal. The block diagram below gives a simple representation of such a system. For simplicity, the transmitter and receiver can only set to one of seven specific frequencies, which would be unrealistically restrictive in the real world.