English: Huge spiral "pancake"
inductors in a high power
longwave alternator transmitter at "Paris Radio Central" transatlantic
wireless telegraphy station, Sainte Assise, France, in 1922. These were called "oscillation transformers". The primary coil was connected to the huge rotating radio frequency alternator, which produced the radio signal. The secondary was connected to the overhead wire aerial system, over a mile long, supported on sixteen 850 foot high towers, which radiated the radio waves. The inductance of the secondary coil resonated with the capacitance of the antenna to form the
tank circuit of the transmitter, to control the output frequency. It transmitted on a wavelength of 14,300 meters, or 21 kHz. The French station, which became operational July 4, 1922, had two 500 kW
Alexanderson alternator transmitters and two 250 kW alternator transmitters, and was one of the most powerful station in the world at the time. The station's transmitters were designed to handle telegram traffic at a rate of 100 words per minute, and all 6 of them had a combined capacity of 36,000 words per hour.
The spiral shape of the coils reduced resistive losses at radio frequency due to proximity effect. The copper conductors are made in the form of wide strips to increase their surface area in order to reduce their resistance at radio frequencies, since radio frequency currents flow along the surface of conductors due to skin effect.
Caption: "
Large tuning inductances required to operate on long wavelengths"