What kind of impedance does this coax cable have?
Sometimes you dig around in your parts bin, desperately searching for a bit of coax cable to make a new connector to your fantastic whizzbang widget. If you find a piece of cable without markings, how do you know if it’s 75Ω, 50Ω or some other impedance? Well, here are two easy ways of finding out.
Measure twice, calculate once
To calculate the cable impedance, you only need to use the simple formula:
But how do you get the inductance and capacitance of your bit of cable? Just take a piece of the cable (just a short piece is enough) and measure the capacitance between the center core lead and the shield of the cable on one end of it. You need an accurate unit with four wire measurement capability which in my case is an excellent and cheap DER EE DE-5000 handheld LCR Meter.
Now let’s hook the cable up to the LCR-Meter.
As you can see, the measured Capacitance of the cable is 10 pF.
Cable Inductance
Next up is the inductance. You have to remove some of the insulation on the other side of the cable and short out the center pin to the shield. Then use the LCR Meter and measure the inductance at the open end of the cable.
So now lets short out the other end of the cable and measure.
Here, the inductance of the cable is 100 nH.
You should now have two values, the Capacitance in pF and the Inductance in nH.
So let’s plug in the values into my trusty calculator:
What? That can’t be right?
This result had me stumped for a while. I was certain that the cable I’ve used for illustration was 75Ω. So I remeasured and came to the same results. A quick Google search shows that the cable, Bedea RG-62 is in fact 93Ω. So close enough. As you can see, the results are close enough to distinguish between 50Ω, 75Ω, 100Ω or any other type of coaxial cable. So now you have a quick way of checking the impedance of cables you bought from a Ham-Fest or just some random cable hidden in the cable drawer.
Another Way of Measure Cable Impedance
I have written an article about measuring the velocity factor of a cable. You can use the same technique to send a short pulse through the wire and check the reflection. By adding a potentiometer at one end of the cable and sending a short pulse through it, you could adjust the potentiometer until the reflected signal is at the lowest value. By measuring the Ω value of the potentiometer when you get the least reflected signal back to the oscilloscope, you then know the impedance of the cable.
The cable is normally 75 ohm. No problem at the cable. But the measured cable length is so short. Measure the cable with min. 100 meters. (just a short piece is enough) is not enough. To measure just a short piece of cable, you have to have a very very good and expensive LCR meter measurement device. Not a hand LCR meter with cheaper one.
can we use a multimeter to measure the impedance of an impedance matching pad.i have minicircuits bmp-5075+ matching pad but when i tried to find the impedance using multimeter ,75ohm reads 130ohm and 50 ohm reads 88ohms . why this too much deviation ?
No, you can't use a Multimeter to measure impedance because impedance is resistance at a frequency. So you need an LCR meter.
this mesures coax cable but what about normal electricity wire how can measure it impedance
this measure applied to only coax cables but what about electricity cables such as power cable how can i measure it
Very good and helpful article Jack! Thank you!
Thank you, very useful. Worked well using my Peak Atlas LCR45 meter, of course the cable was the wrong impedance for what I wanted to use it for!
I've had success with this on standard size coax cables but the not with the smaller PCB IPEX to SMA cables. I've tested 3 different "Chinese" brands which are supposed to be 50ohm but I'm receiving values in the range of 140-170 ohms. I'm following the procedure laid out here . Am I doing something wrong? Assuming the impedance is in the range of 150ohms, can I assume that signal loss is negligable since these connectors are extremely short?