A method includes positioning a coil antenna in a first pipe concentrically arranged within a second pipe, the coil antenna having a first terminal, a second terminal, and one or more wires that exhibit a winding density proceeding radially between the first and second terminals according to a given function. The first and second terminals are then excited with an alternating current or voltage, and a signal is measured between the first and second terminals. A characteristic of at least one of the first and second pipes is then calculated based on the signal.