Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 3, NO. 1, SPRING 1991

In the new branch line hybrids described in this article, the low-impedance, quarter wavelength lines of the conventional hybrids (see Figure 1) are replaced by optimized impedance inverters, and the high-impedance quarter wavelength lines by admittance inverters. The inverters provide an electrical phase shift of 90 degrees at the center frequency, as did the quarter-wave lines they replace in conventional hybrids.

Figure 1: Interconnection scheme for the balun.

The general form of an impedance (of K) inverter and an admittance (or J) inverter [1] along with the design equations are shown in Figures 2a and 2b respectively. The inverters are designed for values of K (or J) equal to the impedance (or admittance) of the lines that they replace in the conventional hybrid. The characteristic impedance of the lines used in the inverters may be selected in the range convenient for fabrication, for avoiding higher order modes, or simply for providing post production tuning possibilities, as desired. Here are several examples of the new type of branch line hybrids.

Figure 2a: Optimized impedance inverter to replace low Z ₀ quarter wave branch line coupler lines.

Figure 2b: Optimized admittance inverter to replace high Z ₀ quarter wave branch line coupler lines.

Using Impedance Inverters

The first example illustrates the design of a two-branch hybrid at 50 GHz, a frequency at which conventional branch line hybrid realization is normally inconvenient. The series arm (K=...