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Text 14. Basic Tube Types

Vacuum Diode

The vacuum diode is a two-electrode vacuum tube. One electrode acts as an emitter of electrons and is called the “cathode”. The other electrode acts as a collector of electrons and is called the “anode” or “plate”. The emitter may be either directly or indirectly heated. In physical form the vacuum diode may vary from a small metal tube to a large glass rectifier tube.

The most useful property of the diode is that it passes current only in one direction. This property makes the diode useful as a de­tector and as a rectifier for d. c. power supplies.

Vacuum Triode

A vacuum triode is a three-electrode tube containing an emitting electrode called the “cathode”, a control electrode called the “grid”, and a current-collecting electrode called the “anode” or “plate”.

The emitting electrode may be an indirectly heated oxide cathode, an oxide-coated filament, or a filament of tungsten.

The control electrode, usually in the form of a grid of fine wire, surrounds the emitter and is in turn surrounded by the plate in the commonest form of triode. By virtue of its proximity to the cathode the grid is able to influence the electrostatic field at the cathode to a greater extent than can the plate, and thus it is able to control the flow of current from the cathode. The grid is usually operated ona slight negative potential so that the electrons will pass between the grid wires without hitting the wires themselves.

Triodes have their greatest use as power amplifiers. They are also used extensively in control applications wherever a small voltage is wanted to control an appreciable amount of current.

Notes

anode ['aenoud]

анод

plate

пластина

d. c. (direct current)

постійний струм

tungsten ['tArjstan]

вольфрам

filament ['filamant]

нитка, волосок накала

to emit [I'mit]

виділяти, випромінювати

grid

сітка

Text 20. Fundamentals of Radar

How Radar Works

The design of a radar begins with consideration of its intended use, that is, the function to be performed by the radar as a whole. The uses generally divide into three categories:

  1. Warning and surveillance of activity, including identification.

  2. Aids to the direction of weapons, that is, gunfire control and searchlight control.

  3. Observation of terrain echoes or beacons for navigation and cont­rol of bombing.

There is nothing mysterious or complex about radiolocation. It rests on the foundations of ordinary radio theory, and is a technique based on the transmission, reception, and interpretation of radiofre­quency pulses. Considered as a whole, it must be admitted that even the most elementary of radar equipment is difficult to visualize, but this is simply due to the fact that so many (normally) curious circuits and pieces of apparatus are gathered together under one roof. No particular circuit or detail of the equipment is in itself especially difficult to understand, and once the elements are known the com­plete assembly is no longer mentally unmanageable.

The word “radar” is derived from the phrase “radio direction- finding and range”, and it may be more expressive than the older “radiolocation”, or it may not. Finding the position of an aircraft or a ship by means of radio covers a very wide field of electronic appli­cation, covers, in fact, the whole area of radio direction-finding (R. D. F.) from the elementary bearing-loop to the principle of the reflected pulse which represents the latest principle of the technique. The term will be used to cover only those methods of detection which depend upon the reflected pulse, the characteristic (by popular opin­ion) which distinguishes radar from all other methods of position- finding in that no cooperation is required on the part of the target. We shall not dwell, therefore, upon the older and more familiar meth­ods which depend upon the reception at two or more points of a signal transmitted by the body under location itself.

The actual equipments in use which employ the reflected pulse principle are greatly varied from the point of view of physical ap­pearance, but their basic principles are the same.

First, let us tabulate and briefly analyse the problem to be met. The aim of radar is to find the position of a target with respect to a fixed point on the ground — say the position of an aeroplane or a barrage balloon with respect to the radar equipment situated in a field a mile or so away: Three quantities must be

measured in order to define the position of the aeroplane or the barrage bal­loon: first, the slant range, the length of the most direct line drawn from the radar site to the target; second, the angle of bearing, i.e. which point of the compass the target occupies; third, the angle of elevation. Fig. 6 should make these points clear for you. When the target is an aeroplane, these three quantities are continuously varying so that the problem of position-finding is somewhat complicated by the fact that the radar equipment has to “fol­low” as well as find. In the case of barrage balloon, things are not quite so difficult, and the three important factors may be found at leisure.

Notes

fundamentals l^fAnda'mentlz]

  • основа; основні принципи

radar ['reids]

  • радіолокатор

surveillance [ssi'velisns]

  • нагляд

intended [in'tendid]

  • навмисний

identification [ai^entifi'kei/n]

  • розпізнавання, ототожнення

searchlight [!sa:t/lait]

  • прожектор

terrain ['terein]

  • місцевість, територія

beacon ['bi:kn]

  • маяк

puise [pAls]

  • вібрація

target ['ta:git] t

  • ціль

o dwell

  • докладно зупинятися

barrage ['bæræj]

  • аеростат загородження

ballon at leisure ['lejs]

  • на дозвіллі

i. e. = that is

- а саме; тобто