TABLE OF CONTENTS The 1:1 balun is well known to radio amateurs and antenna professionals, since it is widely used to match coaxial cables to dipole antennas and to Yagi beams that incorporate matching networks which raise the input impedance to that of the cable. The purpose of the balun is to minimize RF currents on the outer shield of the coaxial cable which would otherwise distort radiation patterns (particularly the front-to-back ratio of Yagi beams) and also cause problems because of RF penetration into the operating room. The balun accomplishes this by its balanced feed and choking action at the low-impedance point (and hence high-current point) on an induced antenna current due to antenna asymmetry. Many successful forms of the 1:1 balun have been used. They include: (A) the bazooka, which uses 1/4- ? decoupling stubs, (B) some 10 turns of the coaxial line with a diameter of 6 to 8 inches, (C) ferrite beads over the coaxial line, and (D) ferrite-core or air-core Ruthroff designs.
The most popular form of the 1:1 balun has probably been the Ruthroff design which is shown in Fig 9-1. Fig 9-1A is the schematic for the toroidal version and Fig 9-1B for the rod version. The single winding of the toroidal transformer, shown as an extension of the top winding of the coiled transmission line, is usually wound on its own part of the toroid. The low-frequency model for the 1:1 baluns in Fig 9-1 is shown in Fig...
