the spectral lines reveals the distribution of molecules and dynamics of clouds. Figure 12.30 shows a measured spectrum of a molecular cloud in Orion [15].

Pulsars, originally called pulsating stars, are swiftly rotating neutron stars. They emit radiation in two narrow beams that sweep space like the beams from a lighthouse. The periods of short pulses received from pulsars range from one-thousandth of a second to a few seconds. The periods are very stable but are in some cases increasing as the neutron star slows down.

Radio galaxies and quasars (quasistellar objects) are radio sources outside our galaxy. The nearest normal galaxies have been mapped using radio waves. The radio emission of an ordinary galaxy is only a small fraction of its optical emission. In radio galaxies, the intensities of radio and optical emissions are of the same order. Quasars look like point sources. Their optical spectra show such large red shifts or recession velocities that the most distant quasars must be located near the edge of the known universe, or they are seen as they looked in the early universe. The intensities of both optical and radio emissions change with periods as small as one day, indicating that the quasars are small. The huge radiation of such a small object can be explained only by assuming that matter is falling down a black hole.

12.9 Sensors for Industrial Applications

Microwaves can be used for many kinds of measurements in industry [16–19]. The electrical properties of matter determine how radio waves propagate in it and reflect from interfaces. Electrical properties in turn depend on physical properties such as moisture, density, composition, and temperature. The

Figure 12.30 Spectral lines of an interstellar molecular cloud. (After: [15].)

354 Radio Engineering for Wireless Communication and Sensor Applications

advantages of microwave sensors are that measurements can be carried out without touching the object and that microwaves penetrate into the material. Different types of sensors and their applicability are treated here.

12.9.1 Transmission Sensors

A typical transmission sensor has two horn antennas, a transmitter, and a receiver. Waves pass through the object placed between the antennas. The phase shift due to the change in propagation velocity and the attenuation due to loss in the material are measured. Transmission sensors are simple and can be used for the measurement of materials moving on a conveyor belt, for example, or in a large tube. They are used to measure the moisture content of grain, coal, sand, and so on.

12.9.2 Resonators

A microwave resonator may be a cavity, strip-line, slot-line, parallel-wire line, or coaxial-line resonator. Resonators based on a slot or parallel-wire line may be used to measure liquid, granular, or pasty materials. Stripline resonators are suitable for measuring sheets and material layers. Cavity resonators may be used as gas analyzers and to measure bars and materials flowing in tubes. Resonators may be used for many kinds of measurements: the moisture content of paper (see Figure 12.31), veneer, and air, the thickness of paper mass, the fiber orientation in paper, the thickness of plastic bars

Figure 12.31 Strip-line resonator array for measuring paper moisture profile. (After: [17].)

and metal sheets, the burning energy value of peat, the water content of snow, and so on.

12.9.3 Reflection Sensors

The complex reflection coefficient of an object depends on the relative permittivity er and its distribution within a few skin depths from the surface. If the object is layered, the thickness and permittivity of the layers may be solved from the frequency response of the reflection coefficient. An open end of a coaxial cable or waveguide, which is pressed on the surface of the object, is a simple reflection sensor. The reflection coefficient depends on the end capacitance and conductance, which in turn are functions of er . Measurement of ground moisture and testing of materials are applications of reflection sensors.

12.9.4 Radar Sensors

Radar sensors can measure the amplitude of reflection, propagation time, or Doppler shift. Applications of radar sensors are numerous: door openers, movement detectors in burglar alarms, surface height detectors in vessels (applicable also when there is danger of explosion or foam on the surface), measuring power line vibrations, detecting rot in trees (see Figure 12.32), detecting pipes, cables, ancient relics, and mines in the ground, measuring marsh depth, and more.

Figure 12.32 Impulse radar used to measure rot in trees.