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):