High Directivity CTL-Couplers and a New Technique for the Measurement of CTLl-Coupler Parameters

A very powerful method to measure the parameters of a one-section Coupled Transmission Line (CTL) coupler is presented. Three transmission measurements at the frequency of maximum coupling is sufficient to give the even- and odd-mode impedances (Z_e,Z₀) and the even- and odd-mode lengths (Theta_e,Theta_{0). Explicit equations are presented for the necessary calculations. Data for some stripline couplers measured with the new method are presented. The data show a very significant length difference between the even and odd modes, although boards with the same dielectric constant (measured according to the MIL standard) were used. Improved dielectric-constant measurement techniques for thin boards have been developed. The presented methods give more relevant information of dielectric constant and Q value of the thin boards. A new method for wide-band improvement of CTL-coupler directivity when Theta}sub e/ ≠ Theta₀ has been developed. The theory is presented with all necessary equations and it covers both Theta_e > Theta₀ (microstrip) and Theta_e < Theta₀. The new method makes use of short compensating sections in the coupler to achieve a directivity pole at an arbitrary frequency. The wanted bandwidth determines the optimum location of the pole. Several examples with design data are given.