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) 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