This is a follow-up question related to my non-contact voltage detection circuit (here). I wanted to be able to detect voltage in a wire a few feet away from the Arduino, so I added a length of RG174 coaxial cable (with the shield grounded) between the 4-inch pickup antenna and the circuit.

I was expecting the coaxial cable to prevent pickup of any other signals, but otherwise no further effects.

The actual results were that the cable did act as a shield, but it substantially reduced the effect of the pickup antenna. To be able to detect the AC field, I had to length it by a few inches and wrap the antenna several times around the 120v wire.

Why did this happen, and is there some way to be able to calculate these variations and be able to determine them before-hand?

Diagram (excuse the MS Paint coaxial cable):

Coaxial Diagram

  • \$\begingroup\$ When you say the shield is "grounded", what do you mean? That it's connected to GND on the Arduino? What's that connected to? \$\endgroup\$
    – Phil Frost
    Jan 1, 2014 at 13:00
  • \$\begingroup\$ The shield is connected to the Arduino-provided GND. \$\endgroup\$ Jan 1, 2014 at 17:39

2 Answers 2


The coax cable shielded the signal line between the antenna and the receiving circuit, which is how it reduces extraneous pickup. Note that this does nothing about extraneous signals picked up by the antenna itself.

The reason it attenuated signals from the "antenna" is because it adds some capacitance from the signal line to ground. It looks like what you are calling a antenna is really just a capacitive pickup. The capacitance from the source signal to this pickup will be small, so the small capacitance in the coax cable is large enough to form a significant voltage divider.


Olin is quite correct - coax has capacitance and this forms a potential divider with the capacitance you use to "pick-up" your signal. The longer the coax cable, the more attenuation you'll get.

If you want to do this properly, you ought to consider using a "head amplifier". This would be a small op-amp circuit places close to where you are trying to measure the AC power signal. Just a unity gain buffer amp will do. Choose an op-amp with low input capacitance probably sub 5pF and it should work better. It'll also buffer your arduino from what could be a potentially damaging voltage source.

  • \$\begingroup\$ Do the 4011's not provide the necessary buffering to protect the Arduino? \$\endgroup\$ Jan 1, 2014 at 17:40
  • \$\begingroup\$ Yes that should be sufficient. \$\endgroup\$
    – Andy aka
    Jan 1, 2014 at 18:45

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