Antennas

The antenna of a transmitter should not be close to the transmitter itself. Commercial broadcasting at medium frequencies generally requires a very large antenna, which is best located at an isolated point far from cities, whereas the broadcasting studio is usually in the heart of the city. FM, television, and other very-high-frequency broadcasts must have very high antennas if appreciably long range is to be achieved, and it may not be convenient to locate such a high antenna near the broadcasting studio. In all such cases, the signals may be transmitted by wires. Ordinary telephone lines are satisfactory for most commercial radiobroadcasts; if high fidelity or very high frequencies are required, coaxial cables are used.

High-Fidelity Systems

Fidelity is the equality of response of the receiver to various audio-frequency signals modulated on the carrier. Extremely high fidelity, which means a flat frequency response (equal amplification of all audio frequencies) over the entire audible range from about 20 Hz to 20 kHz, is extremely difficult to obtain. A high-fidelity system is no stronger than its weakest link, and the links include not only all the circuits in the receiver, but also the speaker, the acoustic properties of the room in which the speaker is located, and the transmitter to which the receiver is tuned. Most AM radio stations do not reproduce faithfully sounds below 100 Hz or above 5 kHz; FM stations generally have a frequency range of 50 Hz to 15 kHz, the upper limit being set by Federal Communications Commission regulations.

control grid - управляющая сетка

screen grid - экранирующая сетка

suppressor grid - защитная сетка

swing - размах

plate circuits - анодная цепь

impedance - полное сопротивление

filament - волокно, нить накала

in series - последовательно

pentagrid - пентагрид

ripple - пульсация

smoothing - сглаживание

choke-input - дроссельный вход

Тексты для студентов-заочников группы (“РТ” 2 к. 4с.)

GAS-FIELD TRIODE (THYRATRON)

There is no gas at all within conventional diodes, triodes, tetrodes and other tubes. But a gas-filled triode (thyratron) is a tube in which the amount of a gas is sufficient to determine the electrical characteristics of the valve when ionization takes place. In the gas-filled triode the control grid potential determines the plate voltage ionization occurs at. So the more negative the control grid as far as the cathode is concerned, the higher the plate voltage must be before ionization takes place. Ionization taking place, the plasma is produced. The plasma is referred to as a region containing substantially equal numbers of positive ions and electrons, so that the net charge in this region equals zero. If ionization takes place as a result of collision between positive ions and electrons, the arc is formed. The grid loses control over the arc as it is immersed in plasma and is separated from the arc by the positive ion sheath. Grid control can be reestablished when the plate potential is reduced to a value below that is necessary to maintain the arc. It's possible to extinguish the arc by lowering the plate voltage. Then the grid once more becomes the controlling factor and again determines when ionization will occur.