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I could relatively easily take the feeder down and use a tape measure but it’s more of a challenge to learn how to measure the length using my oscilloscope -Red Pitaya - but I have to admit I have no idea how to go about it.

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  • $\begingroup$ I should add that I can reach the antenna feed point, it' the logistics of "unrouting" the feeder that make me want to explore a "electronic" method $\endgroup$ – forestDM Mar 26 at 17:05
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  • $\begingroup$ Use a scope to measure the length and impedance of coax $\endgroup$ – Pete NU9W Mar 27 at 13:12
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    $\begingroup$ Isn't that a Time Domain Reflectometer? $\endgroup$ – Duston Mar 27 at 13:18
  • $\begingroup$ Pete, NU9W's, Youtube link was really enlightening. BUT one essential bit of info I missed from my original question is: That it's 300Ω ribbon I am wanting to measure. As I understand it this will: 1/. affect the velocity factor (nearer .85) $\endgroup$ – forestDM Mar 29 at 15:14
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A quarter wavelength of coax (shortened by its velocity factor) is a notch filter if the far end is left open.

So leave the far end open; and try feeding the coax from a frequency generator voltage source plus a series resistor at the near end; and sweep the frequency across a slightly wider range than is appropriate for an adjusted quarter wavelength to be your min to max estimate of your coax length. Use two scope probes on points before and after the series resistor, and look for the frequency causing the maximum drop in the ratio of waveform levels between the two scope inputs.

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