Чтение общенаучной литературы. Кытманова О.А. - 16 стр.

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17
current sources, it is necessary that the radiating system offer a relatively low
impedance to the radio-frequency energy to be transmitted. This is accomplished
by tuning the antenna circuit to resonance with the frequency to be radiated, which
makes the impedance of the antenna circuit low and enables' a relatively small
applied voltage to produce a very large antenna current and hence a high radiated
energy. This is the only reason for tuning the transmitting antenna, as the mere
tuning of the radiating systems to the frequency being transmitted does not
increase the radiated power per ampere of current. The tuning is effected by
inserting an inductance or a condenser in series with the antenna, as circumstances
require. Thus in the flat-top antenna the antenna has a capacity resistance and so is
tuned by the use of the inductance coil.
Cathode-Ray Tubes
Cathode-ray tubes are widely used in various branches of radio engineering such
as oscillography, radiolocation, television, etc. In the narrow part of the tube the
cathode K, focusing system and beam-deflecting system are mounted. Deposited
on the inner surface of the glass face-plate is a luminescent screen S. The cathode
is of the indirectly heated oxide-coated type. It is fabricated in the form of a
cylinder with the oxide coating on its end cap. The cathode is mounted inside a
control electrode (modulator) CE in which an aperture is provided. The brightness
of the spot on the tube screen can be varied by changing the negative potential on
the control electrode with respect to the cathode thus changing the electron-beam
current. Moving along the tube axis after passing the
control electrode is the electron stream which
encounters two anodes A! and A2, both of which are
cylindrical in shape. The accelerating field provided by
the two anodes ensures the motion of electrons towards
the screen and simultaneously focusses the stream into
a narrow beam. Electron beam focussing can be
accomplished with the aid of either an electric or
magnetic field. In the first case focussing is termed
electrostatic and takes place in the electric field
between A1 and A2. An electron E moving at some
angle to the device axis is deflected by the electric
field set up between the anodes. Proper selection of the voltage difference on these
electrodes . ensures focussing of the beam on one . spot on the tube screen.
Magnetic beam focussing is achieved by a focussing coil mounted onto the tube
neck. Deflection of the electron beam is accompanied in the same manner as
focussing that is either by an electric field or by a magnetic field. The electrostatic
system of beam deflection consists of two pairs of vertical and horizontal
deflecting plates. An electron passing between two parallel plates to which a
certain voltage is applied, it will be deflected towards the positively charged plate.
There being two pairs of mutually normal plates, the electron beam can be
deflected in horizontal and vertical planes.

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