Page:Popular Science Monthly Volume 88.djvu/966

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��Popular Science Monthly

��transmitter may be considered. If the condenser, inductance and resistance of Fig. I are replaced by the antenna circuit of Fig. 5, it is easy to see that the constants of the two circuits may be made substantially the same. If the total antenna resistance is 2 ohms, the resonance curve of Fig. 3 will indicate the variation of antenna current with frequency; while, if the resistance is 10 ohms, Fig. 4 will be correct. In the former case over twice the current will flow between antenna and ground than in the latter; if the antennas are of the same height, that having the lower resistance will radiate energy- over four times as effectively. However, in order to keep the current at its maximum value in the low resistance antenna, it is necessary^ to regulate the frequency of the alternator much more closely than is needed in the second case. Thus, in an alternator sender, low resistance and consequent high natural persistence may be a practical disadvantage; it is sometimes necessary to compromise between highest electrical efficiency and greatest operating convenience.

In all the above cases the source of radio-frequency power is an alternator, and the currents and waves involved are of the continuous or sustained type. In such circuits the damping does not effect the sharpness of radiated waves, but only their amplitude and the ease with which the greatest intensity may be secured and maintained. In spark- discharge circuits, which depend upon their natural constants to determine not only the amplitude and frequency, but also the decrement of the oscillations within them, the circuit damping be- comes of the greatest importance. The details of this branch of the subject are so involved that it is not possible to treat them fully in a series of elementary articles such as these; only certain fun- damental facts can be presented.

From the experiments in connection with the circuits of Figs, i and 5, it is evident that the maximum transfer of energy from the alternator to the circuit in which it is connected can occur only when there is minimum impedance (or at the tuned point), and maximum persistence (which corresponds to the condition of least effective resistance).

��This broad principle is applicable to all cases of resonant transfer of energy; the largest exchange occurs when the excit- ing oscillations and the excited circuit are of the same frequency and of the greatest persistence. It makes little difference whether the energy is trans- ferred magnetically, as in an inductive coupler, or by electromagnetic waves extending over long distances; agree- ment of frequency and persistence are essential. It is well to note that if the exciting oscillation is damped there is no gain secured by increasing the persistence of the excited circuit beyond a certain point; reduction of resistance to the amount which gives this best condition is helpful, however.

That this general principle applies to radio receivers as well as to transmitters may be seen by consideration of Fig. 6. In this diagram, A and B represent respectively the closed and open circuits of a spark-type transmitter, and C and D mark the antenna and secondary circuits of a receiver located some distance from A and B. If the condenser of A is charged and allowed to discharge across the gap, electrical oscillations will be set up in the closed circuit. These will have their frequency determined by the effective values of the capacity and inductance of the circuit, and their damping will depend upon the induct- ance, capacity and effective resistance. If the circuit B has the proper natural frequency, it will be excited violently by the voltages impressed across the inductive coupling, and a comparatively large current will be set up in it; this antenna current will have the frequency of the two circuits A and B, and a damping dependent mainly upon the effective resistance of the aerial circuit. Waves of this same frequency, and of the damping of B, will be radiated and will pass over the earth's surface to the receiving antenna C. If C has the correct tuned frequency, currents will be set up in it ; if the effective resistance is of the proper value, these currents will have the largest amplitude. In the same way as at the transmitter, maxi- mum transfer to the circuit D will take place if this final circuit is not only tuned, but is also of the proper persis- tence.

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