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Increasing the Selection Power of a Radio Circuit
2i3
onant circuit of Fig. i. If the antenna itself were charged, like a condenser, by virtue of its capacitance, and allowed to discharge through the tuning coil to ground, it would vibrate
Current
100%
Fig. 4: A more highly damped oscillation train, characteristic of the non-persistent circuit of Fig. 3.
In Other words, the higher the resistance effective in a tuned radio circuit, the less sharp its tuning will be. Since the selection power, by means of which the circuit discriminates be
I I I I Antenna (Capacitance)
Tuning Coil (Inductance)
electrically at its natural frequency. The number of oscillation cycles before the current died away to negligible values would, as before, depend on the effective resistence of the circuit. Thus, if the switch were opened so as to put into the circuit the resistor shown in Fig. 5, the number of oscillations would be reduced just as in the case of the closed circuit.
If, now, we adjust the aerial circuit (by changing the inductance of the tuning coil) so that its resonant or natural frequency is 833,000 cycles per second, we will secure maximum antenna current from any station sending at that frequency. If the sending plant's wave is altered to a value above or below 833,000 cycles, the current in the receiving antenna will be reduced. We may plot, as in fig. 6, the amount of current which will be set up in the antenna as a sending station is adjusted to transmit successively all wave frequencies from 800,000 cycles to 875,000 cycles, both with and without the resistor in series.
The amount of resonant maximum current which will build up in any such circuit depends upon the degree with which the free or natural oscillations in that circuit cooperate with the arriving impulses to magnify their effects. Thus, the more persistent the natural oscillations are in the antenna, the greater and the sharper will be the rise of current as the resonant frequency is approached and reached. This is quite clearly shown by the curves of Fig. 6; when the resistance of the circuit is increased and consequently the persistence of its natural oscillations reduced, the resonant rise of current is neither so sharp nor so great.
Resistor
Switch
Fig. 5: An aerial-toground resonant circuit including a switch which may be opened (to the dotted position) for decreasing the persistence of oscillation
Earth
tween waves of different frequencies for the purpose of avoiding interference, depends directly upon the sharpness of tuning, it is obvi
100%
S 5
o
Ml 2
B %
50%
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Steep Rise of ^--^
Current in /
Persistent Circuit /
f j
V Slow Rise of Current (\ when Circuit has )\ Higher Resistance
_i \ 1 1 —
1 1 1 1—
800
810 820 830 840 850 860 Thousands of Cycles per Second
870
Fig. 6. Curves showing how the addition of resistance to the antenna circuit reduces the sharpness of tuning