Question How can HIFREQ be used to compute the Voltage Drop along a conductor?
Answer The GPR difference between the mid-points of the two conductor segments on either side of an impedance gives only the scalar potential difference between those two potentials. The full voltage difference that you have to use to verify the V = Z I formula is obtained by integration of the electric field between those two points (along a path following the conductors) and includes a vector potential term in addition to the scalar potential difference.
One way to account for the vector potential term is to put a computation profile with a few points (maybe 10) stretched between those two points, then request the computation of the vector potential. Right after the table listing the vector potential values in the F09 file, there should be a table listing the voltage difference between all points on the profile and the origin of the profile. Unfortunately, it was realized recently that this quantity is computed to insufficient accuracy for many applications. However, the so-called "Induced" voltage in the table corresponds to the vector potential term and appears to be unaffected by this accuracy problem. You can therefore use it safely. It should be added to the GPR difference between the mid-point of the segment lying towards the end of the original conductor (P2) and the mid-point of the segment lying towards the origin of the original conductor (P1). The profile should be oriented from P1 to P2.