Retro-Technics • author: a3310i • last modified: 2021.12.18 •

The advent of semiconductor microwave elements such as Gunn generation diodes, LSAs, avalanche diodes and Schottky diodes for mixers and detectors in the 1960s made it possible to build small, reliable and inexpensive radar devices. These devices also, due to their low power consumption and ease of installation, find numerous applications. The article presents examples of such applications.
Using microwaves instead of light, infrared radiation or ultrasound to detect objects has various advantages. Microwaves are less subject to interference than ultrasonic waves. Fog, rain, smoke or dust and insulating materials affect their propagation much less than light or infrared radiation. Since microwaves propagate less directionally, aiming them at an object is much easier than with photoelectric devices. Common radiolocation devices, such as ship and aircraft radars, work in pulses. Based on the time elapsed between the transmission and the return of the pulse reflected from the object, the distance between the radar unit and the object can be determined. For most applications of miniature radar devices, distance information is not needed. These devices can be much simpler and are based on the use of the Doppler phenomenon.

The Doppler phenomenon is well known for sounds. It also occurs for electromagnetic waves. If a transmitter operating at frequency f₀ approaches or moves away from the receiver with velocity v, then a change in frequency is observed:

where:
c - speed of light
v - radial speed of the object relative to the radar

In a beacon, if the object is moving, the incoming frequency differs from the transmitted frequency. Furthermore, since the reflecting object is for its part a moving transmitter, the resultant change in frequency is twice as great as that resulting from the previous formula and is equal to:

For radar operating at 10 GHz (X-band), a frequency change of 92.5 Hz is obtained when reflected from a person walking at 5 km/h. For Doppler radars, operating frequencies in the X band (6.2-10.9 GHz) have been allocated in many countries. At object speeds in the range of 1 to 1000km/h, output signals are obtained which can be processed by equipment used for acoustic frequencies.
Doppler radar can consist of a separate transmitter and receiver. However, they are usually combined together and a common antenna is allocated, which reduces the size and cost of the device. A block diagram of such a device is presented below.

[001] Doppler radar block diagram.

Using semiconductor microwave elements and low-frequency integrated circuits, Doppler radars weighing up to 500g are constructed, detecting e.g. a moving man at distances up to 10 m (when the output power of the Gunn oscillator is 10mW) and up to 50 m (when the output power is 15mW).

[002] The microwave barrier.

Development of an in-car collision avoidance radar is underway. This radar should help the driver to maintain a safe distance from the vehicle in front of him by continuously indicating the distance and speed of the approach. It can also automatically apply the brakes or, for fully automated driving, control the flow of fuel. The matter is complicated by the fact that this radar must be resistant to numerous interferences caused by reflections from the road, trees, road signs, overpasses. If we add to this a large number of other cars with the same type of radar installed and driving on the road in both directions, many problems arise related to mutual interference causing blinding, obscuring and penetration.

[003] Speed measurement by radar.

Mini radars only react to objects moving in the space covered by the radiation emitted by the antenna. The use of radar as a burglary prevention device in offices, shops, warehouses is becoming more and more common. Without a horn antenna, coverage of a large surveillance area around specific objects can be achieved. To reduce the possibility of accidental activation (false alarm), this application uses such a narrowed amplifier bandwidth that the radar detects only slow motion. If necessary, the radar can be hidden. Microwaves pass through insulating materials, so the radar can be masked with a wooden or plastic wall. Radar with separate transmitter and receiver serves as a microwave barrier. Individual devices can be used to assemble entire building surveillance networks from the outside and inside.

[004] Passive infrared detector combined with photoresistive detector in commercial motion sensing device.

Doppler radar detecting vehicle traffic is used in traffic regulation to protect dangerous intersections such as railway crossings, raised bridges etc. The traffic lights only give passage on the less congested carriageway when a vehicle appears on it.
Mini radars are used to control the movement of fans, windows, metal doors, machine parts. They measure the rotational speed without contact with the machine part being checked. In hospitals, they can be used for continuous monitoring of severely ill patients, observing chest movement and breathing. They can also be used by the shipbuilding industry to measure the speed of a ship's slide when it is launched. Mini radars are also used as a navigational aid for manoeuvring in small harbour areas. This small device is increasingly in demand in many industries and its place is undoubtedly assured in the future.