Conventional slotted lines require very close machining tolerances and care in construction if accurate measurements of VSWR (voltage standing-wave ratio) are to be obtained. A much less critical slotted line may be obtained by use of the transformation w=tan z. The concentric line is converted to two parallel semiinfinite planes with a slightly elliptical center conductor. Simple modifications of this configuration will permit the construction of a slotted line section that is far less critical, with respect to mechanical dimensions, than its coaxial equivalent and will exhibit considerably less external energy radiation. Lines embodying this principle have been constructed, and were found to have excellent electrical properties with only moderate mechanical accuracies used in their construction. In a line covering the frequency range 500 to 4,000 Mc, a VSWR of only 1.006 was obtained for the basic slotted section and the necessary transition section to coaxial line.