Measuring the speed of electrons?

bigal_scorpio said:

I just want something cheap and cheerful, to quote your tag "good enough", it would be used infrequently and mainly to check if there is enough cat5 cable in the box for a particular job so I would only need a rough guide, maybe 5% or even 10% would probably be an accepable error since the boxes have 305 meters to begin with.

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Last time I had a box of cat5 it had the start and end number on the box. Then on the cable was a number every meter. Subtract the number on the cable from the box end number = cable length left in box.

Mike.

Mikebits said:

Depending on the cable length, the phase change may go through several iterations and I see no way to know which iteration your looking at.
The TDR is the best method I know of.

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Yes that does occurr to me as well, but you could choose a wavelength that would give enough range of measurement for the lengths of cable you're likely to measure. If a sensible wavelength is chosen then you'd need a very long cable indeed for a full 360 degree phase shift...A TDR is of course the best option, but the equipment is expensive and the original question concerns is a budget project method of testing.Brian

Last time I had a box of cat5 it had the start and end number on the box.

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(Slap my forehead) of course!

And I've used the same markings to 'measure' the length of existing runs where I want to add more parallel cables.