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I've been trying to measure the speed of an electromagnetic wave with my oscilloscope. I used :

  • a 2 CH + SG function oscilloscope (Hantek DSO 2D15), SG set on square wave signal, amplitude 5V, frequency 10KHz
  • 2 probes (x10 mode), compensated, with both wires of the same length
  • a couple of copper wires (2m each)

First of all, I plugged an aligator-clip ended probe into the signal generator, and plugged both CH1 and CH2 probes into the signal out, in order to mesure the delay (for instance, when tension starts to fall). With a time/div set to 2ns, I almost got zero delay. I inverted CH1 and CH2, same results.

Then I plugged a 2m length black wire into the black alligator probe coming from the SG, same thing with the red on with a 2m length red wire, and chose one of the 2 probes to connect it at the end of the wires. I started again the SG and...TADA ! A +/- 8 ns delay appeared. I tried to switch CH1 and CH2, delay still observed. As expected, the probe at the end of the wires is a bit late compared to the signal.

BUT : the light travels at about 30cm/ns (in vacuum). Then, to travel for 2+2=4 meters, it would have taken more than 13ns. And if we consider the material, the shape of the wires, the expected speed would have been no more than 80% * lightspeed.

In any case, the signal travels TOO FAST and I'm aware that lightspeed is a limit (as far as we know) so we can't "overspeed" it :)

So I suppose there is either a mistake in the way I reason or in my equipment.

If we consider the first possibility, for a reason I can't explain, the wave might travel for only 2 meters, what would give 6,67ns (for light in vacuum) and if we take into account the fact that the wave isn't light but electrons and travels 20% slower, then my results are pretty accurate.

So, what is wrong with my 2 2m cables ?

EDIT : have I somehow experienced a kind of reflexion in my setup ? Is the quality of the signal generator affecting the results ("slow" rise time and fall time)

EDIT2 : here is a diagram of my very simple setup (made with Tinkercad) simple measuring setup speed em wave The delay I get with the channel measuring the circuit with the 2x 2m wires (in this diagram, CH1) is below the value of the delay I figured out

Thanks

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    $\begingroup$ Have you tried this with coax cables instead of bare single wires? It's not easy to control (or estimate) the propagation velocity along randomly arranged bare wires. Also a schematic or block diagram of your setup would make it easier for us to understand what you did and give you a useful answer. $\endgroup$
    – The Photon
    Feb 22, 2022 at 17:19
  • $\begingroup$ Seconding the recommendation to try a coaxial cable. If that’s not immediately an option, you might see some improvement if you take two of your single wires and construct a twisted-pair cable from them. Some web searching will help you find the impedance and signal propagation speed for twisted-pair cables based on the geometry you come up with. $\endgroup$
    – rob ♦
    Feb 22, 2022 at 17:33
  • $\begingroup$ Yes I'll do that, I didn't have coaxial cables long enough to achieve my experiment. And of course I'd better twist the cables, such RJ45 cables are. But that doesn't explain why a get a higher speed than the speed of light 😅 on the contrary, the EM wave's velocity should have been reduced a lot...Yep I'll try to draw a diagram 🙂 $\endgroup$
    – Fafanellu
    Feb 22, 2022 at 19:57
  • $\begingroup$ I don't know your exact set-up, and I await your diagram. However, it sounds to me that you shouldn't be treating your 2 wires as if they posed a 4 meter "run" for the e-m waves, but only a 2 meter run. Again, I await your diagram. $\endgroup$
    – Math Keeps Me Busy
    Feb 22, 2022 at 20:07
  • $\begingroup$ Diagram added ! and yes my setup behaves exactly as if I had a 2m-length cable instead of a 4m-length... $\endgroup$
    – Fafanellu
    Feb 23, 2022 at 8:06

5 Answers 5

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This is a transmission line problem. You seem to be expecting your current to travel up your 2m red wire and then back down your 2m black wire, for a total of 4m signal propagation distance and a minimum of 12 nanoseconds propagation time.

But you have in fact built a two-meter cable. Your voltage pulse travels along your cable as an electric field pointing from the negative wire to the positive wire. The changing electric field in the gap between the wires induces a magnetic field in the gap as well. And that changing magnetic field induces an electric field within the wires, pushing a positive current along the positive wire and pulling a negative current along the negative wire. The light-speed propagation time along your two-meter cable would be about six nanoseconds, which is comfortably shorter than what you’ve observed.

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  • $\begingroup$ Thanks a lot for this explanation ! So if I use a short cable (let's say for the +) and a 4m cable (for the -), will I get this "problem" too ? According to your explanation, there is also an interaction between fields that belong to the wires. So if I increase the gap between the wires (which in my setup, not my diagram, are already separated), should I get a bigger delay ? I'll test it 🙂 $\endgroup$
    – Fafanellu
    Feb 23, 2022 at 16:07
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    $\begingroup$ For a given cable geometry, you can compute the cable impedance and the signal propagation speed. If the cable impedance is not constant (as for a cable with a big floppy loop in one conductor, which I think you are suggesting in your comment), frequency-dependent reflections will mean that your nice square signal from your function generator will not reach your oscilloscope as a nice square signal. Consider that your signal generator output and your scope input might be connected internally to a common ground, so in the DC limit you could throw the black wire away. $\endgroup$
    – rob ♦
    Feb 23, 2022 at 17:03
  • $\begingroup$ Your final sentence makes sense ! In order to test that, I could use only one "long" wire, first the - one, and if the delay is close to 0ns that will confirm the ground are common (which would be a normal thing). Then to confirm that, use the "long" cable only for the + one, and see if the delay is close to the delay I got with both of the "long" wires. I'll test that and maybe I'll edit the original post. Thanks for your help ! $\endgroup$
    – Fafanellu
    Feb 23, 2022 at 18:00
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Also please note that if the copper wires were covered in insulation, then the speed of an electromagnetic pulse in the wire will be slowed even more. This is called velocity factor and must be taken into proper account when shortwave radio antennas are manufactured, since the insulation alters the resonant length of an antenna. More info on this is available on the amateur radio stack exchange.

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  • $\begingroup$ Yes, sure, that's why I was wondering why my signal traveled ABOVE the speed of light 😅 $\endgroup$
    – Fafanellu
    Feb 22, 2022 at 19:47
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the speed of light in vacuum ist not 30cm/s nor is it 30cm/ms, $c=3*10^8m/s$ for 4m in vacuum light would take $t=\frac{4}{3}*10^{-8}s$

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  • $\begingroup$ my mistake, I wanted to write 30cm/ns. I edited the original post to correct it $\endgroup$
    – Fafanellu
    Feb 22, 2022 at 12:06
  • $\begingroup$ so how did you expect to measure it with 10kHz $\endgroup$
    – trula
    Feb 22, 2022 at 12:14
  • $\begingroup$ hence, following what you wrote, to cross those 4 meters, light in vacuum would take 13ns, which isn't what I got. I could accept if my result had been above that, not below :D $\endgroup$
    – Fafanellu
    Feb 22, 2022 at 12:14
  • $\begingroup$ By considering significant times (rise or fall). By the way, I repeated the experience with faster frequencies, with almost the same results... $\endgroup$
    – Fafanellu
    Feb 22, 2022 at 12:16
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I think you'll find this interesting.

This fellow did the experiment you're doing...

https://youtu.be/2Vrhk5OjBP8

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  • $\begingroup$ Yep but with 1000 meters he might get a non-negligible resistance/impedance. I'm watching it in order to get inspired... $\endgroup$
    – Fafanellu
    Feb 22, 2022 at 19:52
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It appeared that CH1 and CH2 shared the same ground and that is the reason why my setup behaved as if the EM wave traveled along a 2m wire ! Moreover, I did some research, and found that many oscilloscopes (actually most of them) have inputs that share a common ground. However, some oscilloscopes have differential inputs. Anyway, sometimes things are simpler than we think. Now I'll try to find out how do the geometry and the metal of the wire affect the propagation speed

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  • $\begingroup$ I believe you will get the same results even if the two oscilloscope channels has separate grounds. Two parallel wires each 2 meters long makes 1 transmission line 2 meters long. The electromagnetic wave does not travel 4 meters in that case. $\endgroup$
    – Math Keeps Me Busy
    Feb 25, 2022 at 9:59
  • $\begingroup$ Even if I separate the wires, let's say, with a 50 cm gap ? I'd love to test it, but with my oscilloscope I don't think that's possible 🤔 $\endgroup$
    – Fafanellu
    Feb 25, 2022 at 14:13
  • $\begingroup$ Think of power coming from a utility power plant, traveling down a transmission line of high tension wires. The wires might be multiple meters apart. The electromagnetic waves travel from the power plant down the transmission line to a transformer (and ultimately to a consumer). The waves don't travel down one wire and back the other. $\endgroup$
    – Math Keeps Me Busy
    Feb 25, 2022 at 14:26
  • $\begingroup$ In this video : youtube.com/… we have an accurate explanation of what happens when doing a similar mental experience $\endgroup$
    – Fafanellu
    Aug 28, 2022 at 14:15

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