A Wideband Balun — How Does it Work?

Whenever a balanced antenna (i.e., dipole, loop or spiral) is used, the issue of how to feed the antenna becomes relevant. Because a balanced antenna requires a balanced feed, a balun is needed. A balanced antenna fed by a two-wire transmission line is a balanced system with respect to the lines, provided that the two feed points on the balanced antenna have the same orientation and placement with respect to the lines. If the balanced (symmetrical) antenna is connected to a coaxial transmission line, the transition from the feed line to the balanced antenna is an unbalanced (asymmetrical) driven system. The balun is inserted between the feed line and the antenna in order to provide a transition between the coplanar waveguide (CPW) and the coplanar strip line (CPS). The balun produces a symmetrical radiation pattern however, this article does not deal with the radiation from a connected antenna. In some applications, it is necessary to connect the feed terminals on the balanced antennas to an unbalanced coaxial cable that requires not only a balanced-to-unbalanced transformation circuit, but also an impedance match due to the different characteristic impedances of the antenna and the cable. In the literature, different types of baluns are described [1, 2]. In this article, the focus is solely on a planar balun [3] because it has some very good qualities, such as a low insertion loss and wide bandwidth. The balun is planar, which makes it realizable using conventional photo techniques. To aid the balun design and characterization, the electromagnetic simulation program IE3D Version 6.03, a method of moment computer program developed by Zeland Software [4], was used to predict the performance of the balun structure. The measured results of the constructed balun structure were then compared to the simulated results obtained from IE3D. Baluns whose characteristics remain virtually unchanged over an exceptionally large bandwidth have a multitude of uses. For example, they can be used for ground penetrating radar where the use of a wideband antenna is necessary [5, 6].