Measuring Coax at (nearly) the Speed of Light - Ham Radio Q&A

Joe joins me today to talk a bit about Velocity Factor of cable and demonstrates a way to measure the length of coaxial cable using a signal generator and oscilloscope.

Colophon:

[peterdekeles.]

Always happy to learn something new. Received by J-pole last night. (super fast shipping, Thanks) Opening this morning

[JamienM]

Great video. I just had a demo at work on how to use a TDR to detect faults in ROV umbilicals. You explanation simplified the theory for me! Thanks

[markv]

Hi Mike and Joe. I still have my old Riser Bond 2901C Digital TDR that I carry in the work vehicle. Great video presentation! I do get asked about terminating the cable to measure and it is advised not to terminate, the end open.

[sevenoseven]

With the setup you have you can see if old coax is still 50 ohms by adding a variable resistor to an so 239 chassis connector. By turning the variable resistor you can eliminate the reflected wave displayed on the scope. Then remove the variable resistor and measure the ohm value. It will tell you the impedance of the coax.

[tombauer]

Just ordered an AA-230 today because it has a TDR. Also ordered OSL calibration kit from Amazon. You guys might consider a video on why and how we would use these devices to analyze an antenna calibrating the coax out of the equation. Great job. Glad I found your channel. 73, KE0ABA

[daviddickey]

Electrons and most subatomic particles dont travel at speed of light in a vacuum. Certainly photons do, maybe all particles with no rest mass - but point being that most don't. Photons just do without effort, other particles such as electrons have to have lots of energy applied to them to even get close, but can never quite make it.

Also, signal analyzers and oscilloscopes are a must have for debugging electronic equipment and a cheap (though probably less precise alternative) is to use the raspberry pi for both of those functions.

[arthurdent]

Hi, Mike(s?). Very interesting episode. I am always happy to see something on TDR. I find it to be a very under-reported subject.

[margaqrt]

Was the end of the 25' tested coax terminated with an open, short or 50 ohm dummy load?

[delray-qjqb]

guys with all respect - the drift speed of electrons in conductors on the order of a quarter of a millimeter per second. very easy to check by googling it. what travels so fast is electromagnetic field.

[crazynordlander]

I have a Fluke 620 cable tester that gives the length of a cable and if it is miswired. It will give you the feet at where the fault is. Is it possible that this meter has a Time Domain Reflectometer built into it or is using some other form of witchcraft to determine cable length? Great video. I hope you two weren't messing with the Space-Time Continuum...that can really fowl things up...

[Bernd_Steiner_Ammerland]

Great video, but how comes the wave after the antenna back to the speed of light?

[DialMMicrocontrollr]

Will a butt connection with another length of coax also show a "fault" at the end of the first cable length?