Text 22. Bearing

The determination of bearing is not quite so easy as finding range, but it is, nevertheless, much simpler than the determination of ele­vation. Target bearing is determined by the use of special aerial ar­rays. Proud owners of portable broadcast receivers who regard their loop aerial as a “special” array, may find the bearing of the local B.B.C. station by suitable orientation of their equipment. Radar, requiring a somewhat more refined method, with corresponding accuracy, finds itself unable to rely on simple loops and generally uses such a system as that of the Slowcock array. This sys­tem consists essentially of two collinear hori­zontal half-wave dipoles with centres one wave­length apart connected to the receiver as shown

in Fig. 8. When the line of the aerials is perpendicular to the hori­zontal direction of arrival of the reflected pulse, zero signal is present­ed to the receiver, even if the wave is arriving obliquely from above. If now the receiver moves slightly off bearing, the direction of arrival of the reflected signal departs from the zero-signal position and a small signal is presented to the receiving circuits, the phase of this signal reversing as the direction of it passes through that of “on bearing” or zero signal. Comparison of the phase of this signal with a “standard” signal received on another canal then makes it possible for the opera­tor to determine to which side of the receiving array the target has moved and so enables him to return quickly to the zero signal position. In practice it is not convenient to work to a minimum, or null point, especially when the signal is being observed on a cathode-ray tube, and so special methods of presentation are adopted which in the main con­sist of either a maximum signal or the equality of two signals for the correct on-bearing position of the receiving array. There are many variations of this type of aerial array for special purposes, but most radar equipments depend on the above simple fundamental system.

Notes

bearing [Ъеэпц]	— опора; навантаження
determination [di,t3:mi'nei/n]	— визначення
array [з'геї]	— ряд; приведення до ладу
loop [lu:p] aerial	— рамочна антена
nul	— неіснуючий
obliquely 0'bli:kli]	— косо
target ['taigit]	— ціль

Text 23. Transistors, the Basic Mechanism

Transistor action is the control of currents or voltages in one junction by the currents or voltages in another junction. This control is possible because of the minority and majority carriers flowing across the junctions being controllable. Transistor action can occur when we have two or more rectifying junctions in close proximity. In fact, all transistors make use of rectification at junctions within materials in one way or another.

There is a large variety of transistorlike devices, some examples being the transistor diode, which may be of the alloy, surface-barri- er, grown-junction, melt-quench, diffused-base, drift, or point-con­tact types. There are tetrode transistors, field-effect transistors and many more. It seems likely that there will soon be at least as many transistors types as there are vacuum tubes types.

All transistors have some features in common. They are generally small in size. The actual working volume of a transistor seldom exceeds a cubic millimeter. This is true even for high-power devices. There is a heat-dissipation problem to be faced in almost all transistor applica­tions. In high-power applications, particularly, the heat-dissipating struc­ture is many times the size of the device itself. The characteristics of transistors depend upon the operating temperature to a great extent, and special consideration must be given to the stability of transistor circuits as a function of temperature. The power dissipated in circuits will be large enough to cause large changes in the temperature of the transistor. In the case of the vacuum tube, temperature is not usually a primary circuit stability consideration since the tube is usually hot anyway because of the use of thermionic cathodes. As an aid in solving the temperature-sensitivity problem, new high-temperature materials are being developed. Germanium is good for the devices operating be­low about 90° C. Silicon can be used to about 200° C. There is no reason to believe that better materials cannot be developed.

Notes

basic ['beisxk] — основний

junction ['d3Ai)k/n] — з’єднання

minority [mai'noriti] — меншість

majority [ma'djonti] — більшість

proximity [prok'simiti] — близкість

rectification [^ectifi'kei/n] — детектування

alloy ['геїоі] — сплав

surface-barrier ['satfis baeris] — поверхова перешкода grown-junction — зрощуваний перехід

to quench [kwentf] — гасити

to melt — плавити

diffused-base [di'fju:zd] — дифузійна база

drift — зсув

to exceed [ik'si:d] — перевищувати

dissipation ^disi'pei/n] — розсіювання