Electronic – How to a DC spark create EM waves


I have a question please about sparks used in the early history of radio. I have read the similar questions but I still need your help please.

I have learned from sources such as https://neurophysics.ucsd.edu/courses/physics_1b/SerwayCP7_Ch21.pdf and http://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/Kennedy%20N4GG.pdf that AC, whether in wire or spark, creates EM waves. This is because in AC the electrons are constantly accelerating and decelerating, and acceleration of a charged particle is required to generate EM waves.

I am using 2 HV generators to generate sparks and a neon bulb to test for presence of EM waves.

(1) The first generator is a small enclosed plastic cylinder. I give it 5VDC input with low amps. I couldn't find specs for output except it claims to generate 400KV output which I doubt. This generator continually produces an intermittent spark which causes the neon bulb to flash in sync.

(2) The second generator is a ZVS flyback transformer ("12v-36V Zero Voltage Switching(ZVS) Tesla Coil Flyback Driver Circuit"). I give it 24VDC at 10A. This generator continuously produces a solid spark which causes the neon bulb to stay lit continuously. The specs for this generator state that it generates "High voltage direct current output, voltage input voltage of about 1000 times."
If this is true then it seems to be that the DC spark is creating EM waves, which does not make sense to me. How is the neon bulb lighting up?

Best Answer

We usually use the math of sine waves ("AC") to analyze electromagnetic radiation, and there are good reasons for this. However, the first treatment of how charge generates radiation that my generation of MIT physics students encountered was different.

Consider an isolated charge, not accelerating. Its electric field lines simply point away from it uniformly. Now, move it to one side suddenly. Afterward, its electric field lines still point away uniformly, but from a different place. It's not hard to show, using Maxwell's equations, that the boundary between the space where the field lines originate from the first position and the second will expand as a sphere at the speed of light. On the boundary, the field lines are kinked. That's an expanding, non-sinusoidal electromagnetic wave.

Your spark gap isn't exactly this, but it is closely related: a bunch of charge suddenly moves across the gap. The return current through your power supply is a complication, but the basic result, an impulsive non-sinusoidal electromagnetic wave, is similar.

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