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(e.g. a battery, or a high voltage
transformer) charges a capacitor (C1 ) through a resistor until the spark gap
discharges(due to ionization), then a pulse of current passes through the capacitor
(C2). 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 because due
to sudden flow of large current, free electrons of antenna become capable of
leaving metallic surface and starts moving to some extent so as to generate EM
waves.
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(due to short circuiting). 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
A simple spark gap consists of two conducting electrodes
separated by a gap immersed within a gas. When a sufficiently high voltage is
applied, a spark will bridge the gap, ionizing the gas and drastically reducing
its electrical resistance. An electric current then flows until the path of
ionized gas is broken or the current is reduced below a minimum value called
the 'holding current'. This usually occurs when the voltage across the gap
drops sufficiently, but the process may also be assisted by cooling the spark channel
or by physically separating the electrodes. This breaks the conduction through ionized gas, allowing the capacitor to
recharge, and permitting the recharging/discharging cycle to repeat. The action
of ionizing the gas is quite sudden and violent , and it creates a sharp sound
. The spark gap also liberates light and heat.