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CONDENSERS AND CAPACITY
There are two main types of condensers used in all classes of electrical and radio work, viz., fixed condensers and variable condensers. We shall discuss variable condensers used in radio reception for both tuning and reaction circuits.
In the case of a radio receiver, it is necessary to be able to vary the tuning so that signals of different wavelengths or frequencies may be received. This can be accomplished in one of two ways, by varying either the value of the inductance, or the capacity of the condenser, comprising the tuned circuit.
Most variable condensers have an air dielectric, but there are a few in which the dielectric consists of other insulating material. The former are always more efficient because air is the most perfect dielectric material known, causing the very minimum waste of energy. On the other hand, the solid dielectrics, although having lower efficiencies, have a higher "dielectric constant", which means that a smaller area of electrodes (vanes) is required to make a condensor of any given capacity. The physical size of a condenser with solid dielectric is enormously less than that of an air dielectric condenser of equal capacity. Another disadvantage of solid dielectric condensers is that their minimum capacity is comparatively high.
It is unwise to employ a solid dielectric variable condenser for tuning the circuit of any set in which a high degree of sensitivity is required. Such a condenser is nevertheless very useful for tuning a set of the portable type where space and weight are important considerations, and where maximum efficiency is frequently a matter of compromise.
WHAT IS RADAR?
The word "radar" means Radio Detecting and Ranging. Radar is a very special kind of location by radio. Radar equipment is capable of determining by radio the presence of objects, their direction and recognizing their character.
All types of radar sets principally consist of some six essential components: 1) a transmitter, 2) a receiver, 3) an antenna system, 4) an indicator, 5) a timer, 6) a power supply.
A radar set detects objects by sending out short powerful pulses of ultra-high frequency radio waves energy. As the transmitted energy strikes target it returns back to a receiver. Why wireless waves should return to the place from which they were sent out? Radar uses very short wireless waves. These wireless wav1^ behave in a good many ways like those of light. If light waves meet a suitable smooth surface, lying at right angles to their path, they are bent right-round and reflected back.
Very short wireless waves are bent and scattered by a surface of a radar target. But a small part of the radiation comes back to the receiver.
The receiver picks up the returning echo through its antenna and translates it into a visual readable signal on a fluorescent screen.
Date: 2015-10-21; view: 307; Нарушение авторских прав