A Better Spark Transmitter Design

Spark Transmitter

   A simple coast station transmitter is shown above. It has higher power and a couple of modifications over the simple unit previously described. The modifications improve the efficiency by including a reactance regulator, a safety gap and a quenching gap.

    Power is supplied, not by a battery, but by a direct current motor driving an alternating current generator as shown in the top left of the schematic. Typically the generator would supply 200-500 volts at 500 Hz.
   The DC motor's rotor will draw an excessive amount of starting current (due to the lack of back EMF since the rotor isn't spinning)and so starting current is reduced by the boxed 'starting switch'.
   The starting switch box puts various amounts of current limiting resistance in series with the rotor. As the motor gains speed, and the back EMF increases, the operator moves the switch to the right in steps until it reaches its end of travel. At that point the motor is up to speed and all the resistance is switched out. Ten seconds was the usual start up time. Later versions of the starting switch required no operator action. The arm was pulled across the contacts by a solenoid, acting against the resistance of a dashpot. The variable resistors allow some control over the RPM of the motor, and hence the generator's frequency, and output voltage.

   The photo to the left shows the motor generator control panel from Estevan Point Wireless. The motor starting switch is located at the middle bottom. Mains and generator knife switches are in evidence. Good practice positioned the knife switch handles so that gravity would tend to open the circuit, not close it--hence placing the handle in the top position energized the circuit. Hand wheels are connected to the variable resistances. Motor/generator input voltage and current meters are set along the top. Of course the operator would have to manually start the gasoline or diesel engine generator set to supply electric current into the system.

   The simple fixed gap can be improved by building the gap too wide for the spark to jump. An insulted disk is rotated through this gap. The periphery of the disk has evenly distributed metallic buttons or teeth, which as it passes through the gap allows the built up charge in the capacitors to jump the gap. The effect is to create a musical note on the transmitted signal, improving readability at the receiving station. In the Bowerman photo the disk is seen face on and the buttons are plainly seen. The driving motor is off to the right.

   The simple rotary gap can be improved by building it onto the shaft of the station's alternator and making the number of electrodes equal to the number of poles on alternator. The spark can now be adjusted to occur at any point on the alternating voltage cycle by rotating the outside electrode with respect to the rotating electrodes. The best time to have the spark occur is when the alternating voltage from the transformer is at or the near zero point. At this point the charge on the capacitors are free to jump the gap without any 'interference' from the generator voltage. The synchronous spark gap gave a very distinct tone, easy to hear through atmospheric and man made interference.
    To the right is Jack Bowerman's photo of the Victoria station's synchronous spark gap. The toothed wheel is another form of a rotary disk. As can be seen it is mounted on the same shaft as the electric motor and generator and thus in synchronism with the generator's poles. The teeth are fitted into brass ring mounted on the circumference of an insulating disc. The high voltage appears at the top insulators and arcs as each pair of teeth rotates into position. The knob at the right of the gap assembly is a screw adjustment permitting some slight advance or retard between the fixed and rotating gaps, getting the generator poles in synch with the spark. This would be determined by the best sounding tone at a receiving station.

   At left is the American Marconi Company's "Oscillation Transformer" as used in their 2 KW transmitters. It is called the oscillation transformer as it and the distributed capacitance in the antenna wire determined the transmitter's radiated frequency--the antenna and coil created the oscillatory circuit. An analogy would be a Chinese gong: a hammer (the stored energy in the capacitors jumping the spark gap and hitting the antenna circuit) causing the antenna to 'ring/oscillate' (the energy alternately being stored in the distributed capacitance of the antenna and then in the transformer). This antenna oscillation dies out very quickly, in the order of hundredths of a second.
   The coupling between the aerial and transmitter circuit was varied by rotating the top coil as shown. Straight up and down would be maximum coupling, that is, maximum energy going into the antenna system. This was a simple way of controlling the power output of the system.

    In the schematic at the top of the page, a "short wave" switch changes the resonant frequency of the antenna system, and thus the radiated frequency. Generally two frequencies would be used--one would be 600 Meters and either double (1200 Meters) or half (300 Meters). If the capacitor was switched into the circuit, the frequency would be lowered. Since the antennas were never long enough to provide the correct resonant (radiated frequency), a loading coil would be required. This was simply an inductance in series with the antenna lead (aerial tuning inductance). Photos are from Fleming's Elementary Manual of Radio Telegraphy and Telephony 1915.    The operator could make adjustments by adjusting taps on the oscillation transformer or the aerial tuning coil and noting the affect on the RF ammeter. The object was to tune for maximum current.

   On the right is a typical high voltage transformer of the time The core has been lifted out of the case. Often the case would be full of insulating oil to reduce any internal high voltage arching. Note the 'safety spark gap' which, if the main spark gap became disconnected, the high voltage would jump this gap instead arcing inside the transformer.
   The quench gap improves the efficiency of the transmitter by almost a factor of two. The quenching gap is set in series with the main gap. The energy dumped into the coupling coil from the capacitors by jumping the spark gap transfers across to the antenna system via the antenna coupling coil. The energy in the antenna side is an alternating current/voltage, due to it being a resonant circuit, and as the cycle swings to the next peak some energy is coupled back across to the primary side and will be dissipated within the transmitter itself, unless there is a quenching gap. The returning energy doesn't have enough punch to jump the quench gap.

   Typical antenna arrangement of the time. Many wires, as high as possible, back and forth between the towers to make the antenna very capacitive. This distributed capacity, along with the inductance contained in the antenna tuning coils at the transmitter, formed a resonant circuit--creating the radiated frequency.
   The thought in those days was that antennas mounted as high as possible gave the best operating radius.

The Daily Colonist March 22, 1914
The Quenched Spark System
   The quenched spark system of wireless telegraphy, known on the Continent as the Telefunken system, is now being largely adopted in this country, in fact, there are over ninety British vessels fitted with this apparatus. The chief advantages of the system are well enumerated in a special publication issued on the subject by Messrs. Siemens Brothers & Co., Ltd., and which, in view of the valuable descriptive matter it contains, should be in the hands of all interested in wireless telegraphy.
   With the quenched spark system no soundproof cabin is found to be necessary--an important point on shipboard. Until recently ship owners have been obliged to provide a specially made sound insulated cabinet to contain the spark gap, the noise of which was very considerable and very objectionable. With the quenched spark installations, the working, even with large stations, is practically noiseless, and a shipowner fitting up an installation upon this system has no additional expense necessitated in building a special cabin. Any suitably situated cabin is able to accommodate the apparatus.
   Another important advantage with this system is that with all coupled transmitters, such as are now universally used, it is necessary to take into account the existence of not one wave but two, one being due to the electrical energy oscillating in the antenna, and one to that in the excitation circuit. Unless the oscillation in the excitation circuit can be rapidly quenched, as in the quenched spark installation, the component wave obtained from the aerial and excitation circuit waves is irregular, and has no musical tone. With the quenched spark installation, however, after a few oscillations the vibration of the excitation circuit is quenched, and all the resulting energy is radiated by the aerial in the form of practically an undamped wave. The advantage of this is that double the energy is radiated by a quenched spark than by an open spark. Also with these quenched spark radiations the electrical oscillations, on conversion to sound waves in the receiving station, give rise to a musical note, the pitch of which can be varied. This note being musical, it can be heard over natural electric discharges, or "atmospherics," even when these are as much as ten to fifteen times as intense as the incoming signs. It is interesting to note that the steamship Imperator has a quenched spark installation which radiates 7.5 kw of electrical energy, corresponding to and open spark installation of at least 12 kW.

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