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In the video and pictures below, this guy is trying to replicate Hertz's experiment to generate electromagnetic fields from a dipole antenna. How did a spark generate electromagnetic fields? I thought it is the capacitors that generated the electromagnetic fields.

https://www.youtube.com/watch?v=9gDFll6Ge7g

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    $\begingroup$ Your old AM radio is a good lightning detector... $\endgroup$
    – Jon Custer
    Commented May 31, 2023 at 13:50
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    $\begingroup$ Note that there is a difference between an EM field and an EM wave. EM fields exist wherever there is a moving charge. An EM wave is a propagating disturbance in any EM field. $\endgroup$
    – Todd Wilcox
    Commented May 31, 2023 at 14:19
  • $\begingroup$ @ToddWilcox did EM fields extend infinitely to every directions? Or did they end up forming loops? $\endgroup$
    – SnoopyKid
    Commented Jun 1, 2023 at 10:32
  • $\begingroup$ @SnoopyKid "Field" means "a set of values in every point in space and time". The field is always there (everywhere). What we analyse is the various disturbances in those fields - for example, a moving charge will cause waves in the electromagnetic field (certain very specific self-propagating repeating patterns), while a static charge will be surrounded by a gradient "hill" (or "hole"). $\endgroup$
    – Luaan
    Commented Jun 1, 2023 at 12:40

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EM waves are generated by accelerating charges. The transmitter is a circuit that supports an oscillating current (which requires that the charges are accelerating during the oscillations). The spark is part of that circuit, but is not the cause of the transmissions.

The capacitors increase the amount of charge that can be placed into the circuit before it discharges. The transmitter in the video has the foil pieces acting as capacitors.

The spark gap is acting as a switch. If the antenna were directly connected, it would be very difficult to place much charge on it. By having a gap, when the antenna begins charging, more charge can be delivered into the capacitance of the antenna. Then when the voltage is high enough, the spark forms and allows the current to flow, which oscillates in the antenna until drained.

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    $\begingroup$ Note that a radio transmitter is essentially a switch that turns on and off at the signal frequency, interrupting a current from a power supply. The spark gap is just a cruder switch than a transistor. $\endgroup$
    – John Doty
    Commented May 31, 2023 at 17:43
  • $\begingroup$ @BowlOfRed so electromagnetic fields emanating from moving charges look like this? researchgate.net/publication/319198017/figure/fig1/… $\endgroup$
    – SnoopyKid
    Commented Jun 1, 2023 at 10:26
  • $\begingroup$ That's one particular model. Whether they "look like that" depends on what you're hoping to get from the model. That would be a different question. $\endgroup$
    – BowlOfRed
    Commented Jun 1, 2023 at 15:55
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The first primitive radio transmitters were spark gaps. They generate radio waves from high voltage sparks between two conductors. The high energy excites electrons to higher energy levels where photons are emitted in all directions. All electrons emit photons when they're accelerated or moved in any way, especially when there energized in a spark gap.

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Electromagnetic fields are caused by the movement of charged particles. Modern devices use conductive antennas of a size matching a desired wavelength to emit controlled, band-limited signals of moderate power.

Although an electrical current propagates at the speed of light, the individual carriers in a conductor (electrons) move quite slowly, and we witness the bulk effect of the movement through the conductor. In a spark gap, however, the electrons are separated from their conductive environment and forced to travel through air or space. Without the conduction band of conducting material, they accelerate somewhat freely from one electrode to another, gaining considerable velocity and creating a relatively powerful electromagnetic field.

This field is inherently modified by the volume of charge required to maintain the arc. The capacitors are quickly exhausted, depriving the arc of its sustaining power, and the arc stops momentarily as the capacitors charge. This is a chaotic process and results in the generated field having a considerable bandwidth in addition to its high strength. The fields thus generated are detectable with minimal technology, unlike modern radio signals which are detected only by devices bandwidth-limited and purpose-built (unless you're driving past a high power AM transmitter, in which case your fillings may suffice).

Although the spark-gap transmitter has its place in radio history, particularly Marconi's famous transatlantic message, it is clear that only one transmitter can effectively operate within its range. Spectrum management has addressed this issue.

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  • $\begingroup$ I'm curious whether it is the current flows or the electrons, that move in the opposite direction to current, that generate magnetic fields? $\endgroup$
    – SnoopyKid
    Commented Jun 1, 2023 at 10:32
  • $\begingroup$ @SnoopyKid There is no difference in reality, just in the model. It is the moving charge that generates the electromagnetic wave (not field - you need to be careful with scientific terminology). The "mismatch" between electron flow and current flow is entirely about choosing one arbitrary convention over another - unfortunately, the "sign" of the current was chosen before we've known it's the motion of the negative electrons that "is" electric current. But regardless, it's just convention - nothing to do with reality itself. If you change all the signs, everything works exactly the same. $\endgroup$
    – Luaan
    Commented Jun 1, 2023 at 12:50

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