[TCML] Triggered rotary spark gap
quarkster at att.net
Fri Aug 29 00:36:47 MDT 2008
While researching early high-power spark-gap radio transmitters, I came across an interesting design: a triggered rotary spark gap.
This was used in a 200kW spark-gap transmitter installed by Marconi at Caernarvon, Wales in 1916. This transmitter used four "synchronized" rotary spark gap modules to provide precisely timed current pulses to the transmitter's tank circuits.
The novelty was in the design of the RSG modules. Each module consisted of three plain rotating metal disks, without any electrodes, teeth, or projections of any kind. There was one large diameter disk, and two smaller disks. The axes of the smaller disks were oriented at 90 degrees to the axis of the large disk, with one small disk located at each side of the large disk.
All three disks were motor driven at a few hundred RPM (the actual RPM was not critical). The spark would jump from the OD of one of the smaller disks to one side of the large disk, and then from the opposite side of the large disk to the OD of the second small disk.
The spacing between the disks was adjusted so the 5kV supply was not able to jump the air gap. However, there were trigger electrodes located between each small disk and the large disk. The trigger electrodes were connected to the output of a special induction coil. The induction coil was pulsed by a small, conventional "toothed" rotary spark gap driven by a variable speed motor. This gap was only switching 300ma into the induction coil's primary. Every time the induction coil received a pulse of current from its RSG, the main high-power RSG was triggered. The variable speed motor driving the induction coil RSG allowed the break rate of the main gap modules to be "adjusted" for best transmitter performance.
The main gap modules were switching 200kW, so quenching was critical. Since all three disks were rotating, a fresh, cool metal surface was presented for each firing. The ionized air was purged from the gap by airflow caused by the boundry layer effect of the rotating disks (same principle as used in the Tesla turbine).
An inductor in the HV power supply line also helped the gap to quench after the tank cap had dumped its energy into the tank circuit.
Details of the original Marconi triggered RSG can be found at:
Scroll down to "Caernarvon 200kW timed discharger", and see Fig 9.
More information about the Tesla