A new type of open-wire line is described for which many disadvantages, notably radiation and unbalance, are overcome by replacing one of the physical conductors by the image of the other in a semi-infinite, highly conducting plane.
The characteristics of a line driven at both ends and loaded at the center are examined; it is shown that the impedances measured on both sides of such a line are equal and simply related to the load impedance, provided that electrical symmetry is maintained about the load point. The double-ended drive requires no dielectric supports and permits the line conductor to continue past the load.
The construction and operation of an image line with double-ended drive is described. The characteristics of the new line compare very favorably with those of a good coaxial measuring line.
Finally, use of the line in the measurement of the apparent impedance of a cylindrical dipole antenna as a function of the line spacing is described.