UNUNS Design of Broadband Ununs With Impedance Ratios Less than 1 : 4

The first appearance of the broadband transmission line transformer occurred in 1944, in a paper published by George Guanella [1] in the BraunBoveri Review. In that paper, he expounded the principles of this broadband impedance matching device and is considered the inventor. His goal was to design a broadband balun (balanced-to-unbalanced) for the HF band, matching the balanced impedance of 960 ohms in a vacuum tube amplifier to the unbalanced impedance of a 60 ohm coaxial cable; a 16:1 ratio. Since he did not have the magnetic materials of today he was unable to reach his goal. Even today it is a formidable task. Figure 1 shows Guanella’s approach for a 1:4 transmission line transformer, which is generally considered to be a balun. With terminal 2 grounded it becomes an unbalancedto-unbalanced (unun) transformer. As seen in the figure, his technique uses two transmission lines, connected in parallel on the input and in series on the output side, the transmission lines are coiled such that their common-mode “choking” action provides input-to-output isolation for the desired low frequency performance. Since each transmission line sees onehalf of the load RL its optimum characteristic impedance Z0 should be RL/2. Since Guanella adds voltages that have equal delays through the transmission lines, his technique is can be considered to be a member of the “equal-delay transformer” family. The method can be expanded by connecting three transmission lines in series parallel to obtain a ratio of 1:9, four lines to obtain a 1:16 ratio, etc. In 1959 Ruthroff [2] presented his classic paper in the Proceedings of the IRE. In it he employed two different versions of the balun and unun (Figure 2). Figure 2A is his unun and Figure 2B is his balun. As can been seen his unun has a direct connection from the input to the output. Since the transmission lines are coiled forming chokes the transmission lines are literally raised by a voltage equal to the input, resulting in a voltage twice the input and hence a 1:4 ratio. The characteristic impedance Z0 of the transmission line should be equal to one-half the load impedance RL. This technique has been described as a “bootstrap.” Clearly, it is a simpler circuit than Guanella’s but does not have the same high frequency response because it adds a delayed voltage to a direct one. The delay is excessive when the lines reach a significant fraction of a wavelength. In many cases, however, the transmission lines will be short enough provide sufficient bandwidth for the desired application. Broadband transformers with impedance ratios less than 1:4 are required for solid-state power amplifiers as well as signal dividing and combining circuits from HF to microwaves