The spark transmitter is very simple in operation, but it presented significant technical problems mostly due to very large induced EMF when the spark struck, which caused breakdown of the insulation in the primary transformer. To overcome this the construction of even low-power sets was very solid. The damped wave output was very wasteful of bandwidth, and this limited the number of stations that could communicate effectively without interfering with each other.
In its simplest form, a spark-gap transmitter consists of a spark gap connected across an oscillatory circuit consisting of a capacitor and an inductor in series or parallel. In a typical transmitter circuit, a high voltage source (shown in the schematic as a battery, but usually a high voltage transformer) charges a capacitor (C1 in figure) through a resistor until the spark gap discharges, then a pulse of current passes through the capacitor (C2 in figure). The inductor and capacitor after the gap form a resonant circuit. After being excited by the current pulse, the oscillation rapidly decays because energy is radiated from the antenna. Because of the rapid onset and decay of the oscillation, the RF pulse occupies a large band of frequencies.
The function of the spark gap is to present a high resistance to the circuit initially to allow the capacitor to charge. When the breakdown voltage of the gap is reached, it then presents a low resistance to the circuit causing the capacitor to discharge. The discharge through the conducting spark takes the form of a damped oscillation, at a frequency determined by the resonant frequency of the LC circuit.
In its simplest form, a spark-gap transmitter consists of a spark gap connected across an oscillatory circuit consisting of a capacitor and an inductor in series or parallel. In a typical transmitter circuit, a high voltage source (shown in the schematic as a battery, but usually a high voltage transformer) charges a capacitor (C1 in figure) through a resistor until the spark gap discharges, then a pulse of current passes through the capacitor (C2 in figure). The inductor and capacitor after the gap form a resonant circuit. After being excited by the current pulse, the oscillation rapidly decays because energy is radiated from the antenna. Because of the rapid onset and decay of the oscillation, the RF pulse occupies a large band of frequencies.
The function of the spark gap is to present a high resistance to the circuit initially to allow the capacitor to charge. When the breakdown voltage of the gap is reached, it then presents a low resistance to the circuit causing the capacitor to discharge. The discharge through the conducting spark takes the form of a damped oscillation, at a frequency determined by the resonant frequency of the LC circuit.
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