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Would a light bulb light up if wired to the end on an antenna ?

If you would connect a light bulb on one of its poles to a quarter wave antenna like in the schematics below, would the bulb light up because of current flowing in the antenna (and voltage at the end of the antenna) ? (given sufficient power supplied to antenna)

light bulb connected one side to antenna: schematics

JeanMi
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    a) no, because the current has to flow *through* the bulb, and b) that is exactly the point where, through inevitable boundary conditions (end of conductor) the current is always 0. – Marcus Müller Jan 05 '23 at 15:53
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    A neon bulb placed in a strong enough rf field will glow by itself (see https://www.rfcafe.com/references/qst/neon-bulbs-qst-july-1953.htm for example). A friend used one when working 160m using a wire antenna supported by a weather balloon, serving as a warning marker near the top. – Jon Custer Jan 05 '23 at 16:14
  • In Britain the Droitwich BBC long wave transmitter (198 kHz, 500 kW). I have heard of nearby farmers getting free light for their barns from coils supplying fluorescent tubes. Also of metal connections (e.g. bolted joints or pipe connections) 'singing' audibly (with the broadcast program) and showing visible corona. Also the famous story of the old lady who went to the doctor saying she could 'hear voices' which turned out to be from a metal dental filling. The clue was she said 'they always give the news at 6 o'clock'. In a quietened room the doctor put his ear to her mouth and heard it too. – Michael Harvey Jan 06 '23 at 08:12

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no, because the current has to flow through the bulb, and the end of the dipole is exactly the point where, through inevitable boundary conditions (end of conductor) the current is always 0.

What you can do of course is attach a working antenna to a light bulb with a matched impedance. That would maximimize the power extracted from the wave front.

Marcus Müller
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  • Would there be a configuration where the light bulb could light up while only being connected on one of its pole? – JeanMi Jan 05 '23 at 16:08
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    no, because that would *always* put it on the end of the conductor. Where there is no conductor, no current can flow. – Marcus Müller Jan 05 '23 at 16:10
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When thinking about antennas you have to keep two views in mind simultaneously:

When looking at the big picture, you have the magic of current flowing into an open piece of wire and radiating away. Maxwell's equations are needed to explain what's happening, and we know all sorts of approximations - especially near quarter and half wave wires.

But when looking at any small part of the antenna system, say less than $\lambda/10$ in size, then its just a regular electric circuit and Kirchoff's rules apply (of course just derived from Maxwell), but much simpler. So your DC or 60 Hz intuition is good enough for any small part of the circuit.

Your question doesn't say what frequency you're thinking of. If the frequency is high enough that the light bulb itself is $\lambda/4$ tall, say 1296 MHz and up, then it might work as an antenna and carry current. But if it's at 14 MHz, then the second case applies. You know a bulb won't light up if one side is connected to 120 V mains, the same applies here.

tomnexus
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Yes, somewhat. The end of the antenna is the worst place to put a bulb, in terms of getting it to light, but it can still happen if there's enough power going into the antenna, for two reasons:

  1. The lightbulb has a physical size, which means that attaching it to the antenna extends the antenna. This is best demonstrated with a 4' linear fluorescent tube, but even if we're talking about a small globe, or even a tiny LED, the center of the light is some distance from the very end of the conductive path, and will have some current through it.

  2. Capacitive coupling to the environment ("end effect") means that the current isn't really 0 at the ends of a dipole either — some AC current can flow through the capacitor formed between the end of the antenna element and whatever conductive stuff is in the environment.

Both effects together mean that there will be some current available to light the bulb, and it doesn't really take very much as long as your goal is "there is a visible glow", not "light up a room".

hobbs - KC2G
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Try a neon bulb, which lights up in the presence of an electric field

Phil Freedenberg
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