Чтение общенаучной литературы. Кытманова О.А. - 15 стр.

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16
discriminated against, and in general will depend somewhat on the carrier
frequency.
Fidelity represents the extent to which the receiver reproduces the different
modulation frequencies without frequency distortion. The fidelity of a radio
receiver is expressed in curves, which give the variation in audio-frequency output
voltage as the modulation frequency of the signal is varied. In order to facilitate
comparison, the output in? expressed in terms of the ratio of actual output to the
output obtained when the modulation frequency is 400 cycles.
Radiation of Electrical Energy
Every electrical circuit carrying alternating current radiates a certain amount of
electrical energy in the form of electromagnetic waves, the amount of energy thus
radiated being extremely small unless all the dimensions of the current approach
the order of magnitude of a wavelength. Thus a power line carrying 60 cycles
current with 20 ft. spacing between conductors will radiate practically no energy
because of a wavelength at 60 cycles being more than 3000 miles and 20 ft. is
negligible in comparison. On the other hand, a coil 20 ft. in diameter and carrying
a 2000 kc current will radiate a considerable amount of energy because 20 ft. is
comparable with the 150-meter wavelength of the radio wave. The common radio
antenna consisting of a vertical wire with a flattop structure is essentially a
condenser in which one plate is the ground, the other one being the flat top. Such
an arrangement will be a good radiator of electrical energy when the ratio of height
to wavelength is appreciable, i. e., at least 1 : 100, and preferably 1 ; 10. Similarly
a coil will be a good radiator of electrical energy provided the size of the coil be
sufficiently great. The usual loop antenna consists of a coil and will be an efficient
radiator to the extent that the ratio of loop diameter to wavelength is appreciable.
It is apparent from above considerations that the size of radiator required is
inversely proportional to frequency.
High-frequency waves can therefore be produced by a small radiator, low-
frequency waves requiring a high-antenna system for effective radiation. The
practical result of this fact is that the antennas of low-frequency transmitting
stations are sometimes suspended from towers over 500 ft. high and yet are less
efficient radiators than an antenna of one-tenth, this height operating at a very high
radio frequency.
Every radiator has directional characteristics as a result of which it sends out
stronger waves in certain directions than in others. Thus, while a vertical wire
radiates the same amount of energy in directions that are perpendicular to the wire,
the radiation in vertical plane varies from a maximum in a horizontal direction to
zero in a vertical direction. Directional characteristics of an antenna are taken
advantage of " to concentrate the radiation toward the point to which it is desirable
to transmit.
The amount of energy sent out from any radiating system is proportional to the
square of the radio-frequency current that flows in the radiator. Due to all the
common sources of radio-frequency energy being relatively low voltage high

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