### About

In this post, I’ll show you how to measure the length or find a break in a coaxial cable with the help of your oscilloscope and waveform generator. We can do this using signal reflection that occurs when a transmission line is not terminated correctly.

As you can see there is no reflection with a proper 50Ω termination.

But when we remove the termination or add a break in the line a signal reflection will appear. The first “step” is the initial signal while the second “step” is the reflection of the initial signal coming back. The time from the first step to the second one is the time it took the signal to travel to the end of the cable and back.

We can use the following equation

**Length = c * v * dT / 2**to figure out how long the cable is. The**c**is the speed of light,**v**is the velocity factor(the signal travels a bit slower than light through the cable this factor tells us how much slower, usually around 0.8 but it depends on your particular cable),**dT**time between the first and second step, finally need to take into account that the signal traveled to the end of the cable and back so to get the actual coax length we divide it by**2**.
If we put the numbers into the equation Length = 299 792 458 m/s * 0.8 * 18nS / 2 the length comes out to be 2.15m. The actual cable length is about 2m so the result is just a little off.