Broadband Miniature Balun
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
The balun (balanced/unbalanced line) has design and operation similar to prior full-size baluns but is miniature in size and is effective at high frequencies, such as 300-1000 MHz, to be unaffected by resonances caused by stray capacitances and inductances. Therefore, the balun has a broadband frequency response. A conceptual drawing of the balun is shown in Fig. 1. The two legs of the coax loop form what is essentially a one-turn transformer. The shield of the coax makes up the primary winding, and the center conductor is the secondary. The result is that the secondary has the same impedance as the primary (1:1 turns ratio), but it is isolated from the system ground (the shield of the input cable). In this way, the transformation is made from an unbalanced line to balanced. The length of each leg of the balun is important.