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Text 4. Propagation of Radio Waves of Different Frequencies

As radio waves travel away from their point of origin, they be­come attenuated or weakened. This is due in part to the fact that the waves spread out.

In addition, however, energy may be absorbed from the waves by the ground or by ionized regions in the upper atmosphere, and the waves may also be reflected or refracted by the earth, by ionized regions in the upper atmosphere, or by conditions within the lower atmosphere. The resulting situation is quite complex and differs great­ly for radio waves of different frequencies.

Radiation of Electrical Energy

Every electrical circuit carrying alternating current radiates a certain amount of electrical energy in the form of electromagnetic waves, but the amount of energy thus radiated is extremely small unless all the dimensions of the circuit approach the order of magni­tude of a wave length. Thus, a power line carrying 60-cycle current with a 20-ft. spacing between conductors will radiate practically no energy because a wave length at 60 cycles is more than 3 000 miles, and 20 ft. is negligible in comparison.

On the other hand, a coil 20 ft. in diameter and carrying a 2 000-kc current will radiate a considerable amount of energy because 20 ft. is comparable with the 150-meter wave length of the radio wave.

From these considerations it is apparent that the size of radiator required is inversely proportional to the frequency.

High-frequency waves can therefore be produced by a small radia­tor, while low-frequency waves require a large antenna system for effective radiation.

Every radiator has directional characteristics as a result of which it sends out stronger waves in certain directions than in others.

Directional characteristics of antennas are used to concentrate the radiation toward the point to which it is desired to transmit, or to favor reception of energy arriving from a particular direction.

Generation and Control of Radio-Frequency Power

The radio-frequency power required by a radio transmitter is prac­tically always obtained from a vacuum-tube, oscillator or amplifier.

Vacuum tubes can convert direct-current power into alternating-cur- rent energy, for all frequencies from the very lowest up to 30 000 me, or even greater.

Under most conditions the efficiency with which this transforma­tion takes place is in the neighbourhood of 50 percent or higher. At frequencies up to well over 1,000 me, the amount of power that can be generated continuously by vacuum tubes is of the order of kilowatts.

Notes

Text 5. Reception of Radio Signals

In the reception of radio signals it is first necessary to abstract energy from the radio waves passing the receiving point.

After this has been done, the radio receiver must next separate the desired signal from other signals that may be present, and then reproduce the original intelligence from the radio waves. In addition, arrangements are ordinarily provided for amplification of the re­ceived energy so that the output of the radio receiver can be greater than the energy abstracted from the wave.

Any antenna system capable of radiating electrical energy is also able to abstract energy from a passing radio wave, because the elec­tromagnetic flux of the wave in cutting across the antenna conduc­tors induces a voltage that varies with time in exactly the same wave as the current flowing in the antenna radiating the wave.

The energy represented by the current flowing in the receiving antenna system is abstracted from the passing wave and will be greatest when the reactance of the antenna system has been reduced to a minimum by making the antenna circuit resonant to the frequency of the wave to be received.

Since every wave passing the receiving antenna induces its own voltage in the antenna conductor, it is necessary that the receiving equipment be capable of separating the desired signal from the un­wanted signals that are also inducing voltages in the antenna.

This separation is made on the basis of the difference in frequency between transmitting stations and is carried out by the use of reso­nant circuits which can be made to discriminate very strongly in favour of a particular frequency.

It has already been pointed out that, by making the antenna cir­cuit resonant to a particular frequency, the energy abstracted from radio waves of that frequency will be much greater than the energy from waves of other frequencies; this alone gives a certain amount of separation between signals.

Still greater selective action can be obtained by the use of addi­tional suitably adjusted resonant circuits located somewhere in the receiver in such a way as to reject all but the desired signals.

The ability to discriminate between radio waves of different frequen­cies is called selectivity and the process of adjusting circuits to reso­nance with the frequency of a desired signal is spoken of as tuning.

Although intelligible radio signals have been received from sta­tions thousands of miles distant, using only the energy abstracted from the radio wave by the receiving antenna, much more satisfacto­ry reception can be obtained if the received energy is amplified.

This amplification may be applied to the radio-frequency currents before detection, in which case it is called radio-frequency amplifica­tion; or it may be applied to the rectified currents after detection, which is called audio-frequency amplification.

The use of amplification makes possible the satisfactory reception of signals from waves that would otherwise be too weak to give an audible response. The only satisfactory method of amplifying radio signals that has been discovered is the use of vacuum tubes. Before such tubes were discovered, radio reception had available only the energy abstracted from the radio wave by the receiving antenna.

Notes

Text 6. Radio Receivers (I)

Transmission of intelligence by radio is based on modulation, this being a process by which the message to be transmitted is superim­posed at the sending end of a radio link as a modulating signal on a strong carrier wave, thereby changing the latter’s amplitude, frequen­cy or phase. The modulated carrier is radiated by a transmitting aerial as a wave of electromagnetic energy which propagates through space at the velocity of light. At the point of reception the modulated wave is picked up by a receiving aerial and is fed to the receiver input. In the receiver the signal is separated from the radio-frequency carrier and drives the receiver load, which may be a speaker, a recorder, a cathode-ray tube, etc. As an electromagnetic wave travels away from the transmitter it is weakened or attenuated. This is why radio receiv­ers should be capable of picking up relatively weak signals.

Radio serves a variety of purposes such as communication, broad­casting, navigation, radar and telecontrol.

Radio communication is the transmission and reception of mes­sages without wires or waveguides. It includes communication by radio telegraph, radio-telephone, radio teletypewriter, radio facsimi­le and television. It is the only method of communication between stationary and mobile objects (e. g. from ship to shore, from ground to aircraft, from ground to spaceships, etc.).

Radio broadcasting is radio transmission for general reception, including speech, music and commercial television.

Radio navigation is the use of radio facilities for determining the position and direction of ships and planes.

Radar (which is an acronym for Radio Detection and Ranging) is a technique for determining the range and bearings of objects (targets) by transmitting beamed high-power signals against reflective tar­gets, the reception of the reflected signals and the presentation of the resultant data on a dial or a cathode ray display.

Telecontrol is a technique for control of machinery by radio.

There exist two classes of receivers: communication and broad­cast receivers, the former being used in point-to-point radio tele­phone and telegraph service while the latter are designed for the reception of sound and visual programmes.

Notes

Text 8. Oscillators

An oscillator is a device which produces a regularly-recurrent waveform of voltage or current and it comprises two basic ele­ments — an amplifier and a frequency determining circuit.

The most common oscillator produces an output waveform, the latter approaching the sinusoidal very closely and comprises a valve and a tuned circuit. An amplifier valve with a parallel tuned circuit in its anode lead is known to give an amplification A at a resonance frequency of the tuned circuit, at this frequency the anode voltage being in phase opposition to the grid voltage. If a fraction l/A of the anode voltage is taken, reversed in phase, and fed to the grid, the grid voltage is obtained from the anode voltage. The attenuation and phase reversal can be obtained by including a small coil in the grid circuit and coupling it to the anode coil.

With such a circuit, oscillation starts up at a small amplitude and builds up until the non-linearities of the valve limit its growth. These non-linearities are found to play an essential part, one of the more important of them coming into action through the use of an automat­ic grid-bias current. The amplitude of the oscillation increasing, so does the grid voltage and hence the grid bias. The increasing bias reduces the mutual conductance of the valve and so the amplifica­tion, and an equilibrium condition is at length reached at which a steady amplitude of oscillation is generated/

Notes