How are radio waves generated 2

The technical basics of radar


Abbreviation: RADAR = ““ ““ ““ ““ “'' Radio Aircraft Detecting And Ranging '', i.e. the detection and location of aircraft using radio waves.

The principle of the radar

The principle, after which the location with radio waves works is the use of the Echo effect of electromagnetic waves. To do this, the speed of propagation of the waves must be known and the time from sending the waves to receiving them again must be measured. The distance to the next obstacle then results from a simple formula.

Distance to target = speed of light * transit time / 2;

c * t
s = --------

Radio waves behave similarly to light waves. They spread out in a straight line and, under certain circumstances, are reflected, refracted and bent.

reflection : Occurs on almost all objects. The incident wave is reflected back at an angle that is equal to the angle of incidence.

refraction: Occurs when the wave passes from one medium to another. Depending on the nature of the media, the direction of the wave is changed.

diffraction: Diffraction occurs when the diameter of the "" hit "" object is in the same size ratio as the wavelength.

Electromagnetic waves are also reflected from the ionosphere and the earth's surface.

The development of shortwave technology made it possible for G. Breit and M.A.Tuve to determine the height of the ionosphere by means of high-frequency pulses in 1925.

Today radar for the location of ships, vehicles and missiles mostly operate at a wavelength of a few meters and centimeters.

Radar frequencies

Source: IEEE Standard 521-1976, Nov 30, 1976

Band frequency range usage

HF 3-30MHz OTH (Over the horizon) monitoring
VHF 30-300MHz very long distance monitoring
UHF 300-1000MHz very long distance monitoring
L 1-2GHz long distance monitoring
En route traffic control
S 2-4GHz medium-range monitoring
Terminal traffic control
Long haul weather exploration
C 4-8GHz long distance tracking
Airborne weather exploration
X 9-12GHz short haul tracking
Missile guidance
Mapping, marine radar
Missile interception
Ku 12-18GHz high-resolution map creation
K 18-27GHz Little used because of water vapor
Ka 27-40GHz Very high-resolution map creation
Airport surveillance
Milli- 40-100 + GHz experimental application

Structure of a radar device

The transmitter

The Channel consists of oscillator + amplifier + modulator.

The oscillator generates the carrier frequency of the signal.
The modulator superimposes a signal on the carrier frequency (mostly in the form of a pulse).
The amplifier after all, it pumps enough energy for the signal so that it can be sent far enough.
(Earlier transmitters did not have amplifiers, the oscillator had to be strong enough.)

The oldest oscillator is the magnetron. Magnetic induction creates a strong, oscillating magnetic field in a cavity in the middle of the device. The magnetic field is then conducted to the antenna by a waveguide.

Semiconductor components (Gunn elements, avalanche diodes) are increasingly used as oscillators, since the oscillator no longer needs to generate the transmission power. Compared to electromagnetic devices, the semiconductor components are smaller and more reliable and can also reach higher frequencies.

A vacuum tube is still mostly used as an amplifier today. Depending on their execution, they become Klystron, TWT
(Traveling-Wave Tubes) or something like that. In principle, they always work in the same way: a controlled flow of electrons creates a strong electromagnetic field that changes with the current.
The latest development for amplifiers is the so-called gyrotron.

The pulse length of the modulators should be sufficiently small, the reliability and the efficiency as high as possible. The costs as low as possible. Both modulators made from tubes and from semiconductors (shunt diodes) are used today.

The antenna

The decisive factor for the antenna is that the send and receive directions are as narrow as possible. Parabolic antennas or antenna fields are therefore used, with several antenna masts standing parallel to each other.

Radar types

There are two main types of radars: Continuous wave radars,
that transmit constantly, the frequency changing sinusoidally and recognize the received signals by their frequency shift.

The second type is the more common Pulse radar.
This has directional antennas that can systematically search areas or track objects.

There is also talk of primary and secondary radar. In the case of the primary radar, the same antenna is used to transmit and receive. The secondary radar, on the other hand, uses active signal transmission. A transmitter is installed, for example, in an airplane, the main transmitter is stationed on the ground.

Problems and Limitations of Radar

Noise is one of the biggest problems with radar. There are actually two types of noise.
One is the noise of the radar itself. All electronic components (except for the conductors) generate noise. On the other hand, better components and more effective circuitry can be used. If necessary, the components can be frozen, which also reduces the noise.
The second variety is the noise created by the environment. Most of the noise can be suppressed by filters.

Experienced radar operators can distinguish noise from the actual signal. However, false signals from radar are unavoidable.

problem ground: Very turbulent air conditions, reflection from trees etc. Thanks to the reflection of the radar waves by the earth's surface, the radar can actually have a greater range near the earth's surface than high up in the air. The radar wave creeps on the ground, so to speak. However, this phenomenon is difficult to use. First, trees, buildings, hills and other objects reflect the radar signal and hide the actual target. Second, the air close to the ground is also somewhat more turbulent; the radar wave can be broken at the limit of different air density and temperature. The radar wave is also often reflected multiple times from the ground, which makes it even more difficult to interpret the signals received.