938 



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. 



