# How to measure the voltage drop between 2 points separated by a long distance?

In the project I am working on today, I need to measure the voltage drop between a portable probe and a fixed reference. The big challenge is these 2 elements could be distant from each other of up to a few miles.

The best solution we found today was to connect a wire to the fixed reference, in order to bring its potential to the probe, where the measurement is performed. But as this probe is constantly moving, this solution is not practical at all.

Do you have any idea of a way to do the job "wirelessly"?

• Out of curiosity, what is this probe measuring?
– dim
Jul 1, 2016 at 9:05
• The probe is directly measuring the voltage of the ground...which makes me realize the second part of my question doesn't make any sense. I removed it. Jul 1, 2016 at 9:10
• What is the reference connected to ? Jul 1, 2016 at 9:15
• @Edesign That's what I suspected and was exactly for this reason I was asking you... Well, the answer is "there is no way to do it without a wire", then, I'm afraid.
– dim
Jul 1, 2016 at 9:25
• If the interconnecting cable is regular coax maybe transmit a pulse and look at the returned reflection amplitude and determine how much loss there is in amplitude. Loss of amplitude will be related to resistive losses. Jul 1, 2016 at 9:25

Unfortunately, there is no way to do it without a wire.

You obviously can't use ground to bring your reference point. Because you are measuring voltage differences between the ground points, you know that ground voltage is not uniform.

Air can be used to transmit waves (conveying wireless data), but it can't be used to transmit an absolute voltage either.

So you can't use air, you can't use ground. You have to use a wire.

And since you seem to measure very low voltages, be careful regarding the wire section of this cable. Because it is very long, the current that the probe creates through this wire (although very low) may create a significant voltage drop that may affect the measurement (due to the wire resistance):

• Check the AWG of the cable
• Look at this table to get the resistance/1000m
• Calculate the total resistance depending on the wire length
• Get the probe leakage current through the wire (given in probe datasheet, or measure it using an ammmeter)
• Use ohm's law to check the resulting voltage drop is well below the voltages you actually measure.
• Thank you for this answer. Unfortunately, this is exactly what I was suspected. Jul 1, 2016 at 9:49
• @Edesign And, also out of curiosity, what are the voltage differences you typically measure when a few miles away from the reference point?
– dim
Jul 1, 2016 at 9:52
• Probably just a few mV. But this is sufficient for the application we have. Jul 1, 2016 at 9:55
• @Edesign Ok, then, if it's so little, you need to check a few things more. I edited my post in this regard.
– dim
Jul 1, 2016 at 10:04
• @Edesign I wasn't sure it made sense in your case. Honestly, I didn't have any idea what voltage differences in ground there could be from a point to another a few miles away. But given your inputs, you'd better check it actually. Probe current may be only in the ranges of nA/µA, but in your case, if the wire is small, that may start to be significant.
– dim
Jul 1, 2016 at 10:17

What if you used a TDR https://en.wikipedia.org/wiki/Time-domain_reflectometer to approximate the resistance of the wire and then use the input voltage to calculate the voltage drop? Also, if you know the length of the wire and you can look up the data sheet you can calculate the resistance of the run.

• This doesn't sound like a wireless solution as requested by the OP. Jul 1, 2016 at 16:45
• Thank you for the idea, but I already have a solution using a wire. My question was on a wireless solution only. Jul 4, 2016 at 7:21