This example, in its time, was cutting edge technology but today is regarded as some
sort of 'hand cranked' apparatus.
The step up transformer takes the battery voltage and boosts it to many thousands of volts. Since transformers do not work on direct current some form
of interrupter is required. It takes the form of a simple clapper on the end of the coil, actuated by the magnetic field, due to the primary current flow. In practice the interruptions occur,
for explanation purposes, about 200 times a second. The collapsing magnetic field caused by the interruption to primary current flow cuts through the secondary winding turns inducing a voltage. The voltage induced is directly
proportional to the "turns ratio" between the two windings.
In the case of the apparatus shown in the image, the core is a bundle of soft iron 22
SWG wires with a diameter of 2" and a
length of 18". Primary winding is 300-400 turns of #12 Standard Wire Guage (SWG) copper wire wrapped on the core. It is wound bobbin style
around the soft iron core. The spark coil secondary is not wound smoothly back and forth (bobbin style) across the primary windings, but is
done using many insulated disk shaped coils, joined together as illustrated on the left. This prevented high voltage breakdown within the transformer as the
high voltages generated were now distributed along the width of the secondary coil, and not through the thickness.
Hook a couple of capacitors and a coil of wire as shown in the top
diagrams. The
capacitors shown here are glass jars, a layer of tin foil inside
and a similar layer on the outside. The glass serves as an insulating medium
between the two layer and, as a result, can handle the high voltages. The value
of the capacitor would typically be in the order of 0.1 mfd. The
radio frequency (RF) coil/transformer will couple the energy to the antenna system.
Applying battery to the primary circuit causes current to flow and, due to
the interrupter, be cycled off and on about 200 times a second. The voltage
generated, due to transformer's step up action, on the secondary side is several
times higher when the interrupter opens the circuit since the rate of change of
the magnetic field is much faster collapsing. When the interrupter closes the
circuit the magnetic field's rate of change is much slower due to the inductive
effect of the primary coil. The resulting high voltage on the secondary
alternates at the same frequency as the interrupter buzzes.
If no coil and capacitors were across the gap, the spark could
be several inches long--hence references to a 10" spark coil transmitter in
the old books. In practice the capacitors initially act as a short
circuit across the coil's secondary, the voltage rising as the value of the
capacitors and winding resistance of the spark coil's secondary allow.
The applied high voltage isn't available for the length of time required,
due to the quick action of the interrupter, and as a result the capacitors do
not fully charge. Thus high
voltage is quite a bit less than the peak supplied by the transformer and as
a result the spark gap must be
decreased to the order of 1/4 to 1/2 inch before a spark is obtained.
The antenna system has distributed capacity to earth and
with the inductance of the coupling coil creates a resonant circuit.
The amount of inductance is adjusted, by means of taps, to resonate at a
particular frequency. In those days the wavelength would be somewhere
in the region of 600 to 2000 meters (500 kHz to 150 kHz).
On the rise of the high voltage the capacitors charge
up via the antenna coil until the flash over voltage of the spark gap is
realized. The spark appears, for all intents and purposes, as
short circuit. The capacitors are now able to discharge their energy
into the resonant antenna via the coupling coil, then the coil back into the capacitors via the still
arcing gap. This back and forth transfer of the energy will continue
until the arc finishes, or the resonant circuit's energy is dissipated in
overcoming the resistance losses. In practice the resonant circuit's
energy will diminish before the arc is extinguished due to the fall of
secondary voltage.
Thus the resonant antenna produces a burst of radio frequency
energy at each arc of the spark gap. (A crude analogy would be a
mallet [the energy pulse] hitting a gong [creating a diminishing tone].) The power induced into the antenna
circuit
is proportional to the voltage charge on the capacitors which, in practice, is governed by the
spark gap spacing. A larger spacing allows the voltage on the capacitors to reach a higher value before the spark gap is overcome.
Of course the arcing gap shorts out the high
voltage transformer too. The fix is to install some small inductances on
the transformer low voltage leads. Energy induced into the antenna from
the discharging capacitors will also be coupled back into the spark transmitter.
The fix for this is to install a second spark gap, called a quench gap in series
with the capacitor side of the RF transformer. The 'main bang' of the
transmitter will energy will easily jump the quench gap, but any energy coming
the other way will be much less and be unable to jump the quench gap.
A simple amplitude modulation receiver, such as a crystal set, tuned to the frequency of the transmitter, would hear a tone, the
frequency of which is equal to that of the arc (400 Hz not 200 Hz), which in turn is governed by the frequency of the interrupter.
This unit would be quite similar to the transmitters carried on the first wireless equipped vessels. The only changes would be the insertion of
a telegraph key in series with the battery which the operator would manipulate to form the Morse characters, and suitable coupling of the RF
coil to an antenna.
This transmitter is inefficient, even by turn of the century standards. Go to the
"Next" page to see a coast station transmitter write up.
As an aside, if the RF coil and capacitor are removed, put a spark plug in place of the spark gap, we will have
a car ignition system. That explains why a passing car can often be heard on a radio--its ignition system is a wireless transmitter. Note that the missing
coil and capacitor are still there--the ignition wiring has its own distributed inductance and capacitance.
This link lets you hear what a spark transmitter sounded like. The whole
recording explains itself.
The professional images are from "An Elementary Manual of Radio-Telegraphy and Radio-Telephony, 3rd edition by J.A.Fleming/Longmans, Green & Co.
London 1916).
