Directional couplers are invaluable components for measuring the power levels of signals in microwave systems. Especially when high power levels are involved, a properly designed directional coupler provides a lower-power coupled signal that is within the power measurement range of a power meter or spectrum analyzer. By applying the finite-element method (FEM) of analysis, it is possible to effectively analyze and design asymmetrical rectangular (coaxial-microstrip and coaxial-stripline) directional couplers for high-power applications, taking into account the effect of metallic diaphragms that partially separate the coupler inner conductors.

The FEM of analysis^1,2 is a simple, accurate, and efficient tool for analyzing asymmetrical rectangular (coaxial-microstrip and coaxial-stripline) directional couplers for high-power applications. In the particular designs of these studies, metallic diaphragms are used to partially separate the inners conductors. These metallic diaphragms make it possible to easily control the coupling factor. These couplers can be realized without major difficulties and feature simple, low-cost mechanical construction. As an example of the analysis and design approach, broadband directional couplers with 20-dB coupling will be realized.

Directional couplers are key components in many RF applications, in particular for measurement applications. A variety of different couplers are available from a number of manufacturers, including stripline couplers, waveguide couplers and coaxial configurations. Stripline or microstrip couplers are well suited for broadband applications; ³

unfortunately their significant losses can prevent their use when high-power handling is required. Waveguide Bethehole couplers are used in high-power applications, ⁴ but are not a practical solution for broadband use. Although many techniques have been proposed for this purpose, ^5,6 the primary mode is limited at low frequencies by the cutoff frequency while the higher-order modes limit the upper-frequency extension. When bandwidth requirements are not critical, coaxial directional couplers using air dielectrics are a traditional solution, ⁷ and would be ideal for their low-loss performance and high power-handling capabilities, with their transverse-electromagnetic (TEM) field configurations ensuring zero cutoff frequency.

Another solution for high-power coupler requirements as proposed in ref. 8 is a coaxial-to-microstrip coupler (Fig. 1). An additional type of coupler proposed in this report is a rectangular coaxial-to-stripline coupler (Fig. 2). For these two couplers, the rectangular asymmetrical coaxial main line is coupled to a microstrip or stripline transmission line through an aperture on the ground plane of the dielectric substrate.

The electrical properties of the loss-less and inhomogeneous couplers of Figs. 1 and 2 can be described in terms of their primary parameters (the matrices for [L] and [C]), ² where:

Various numerical techniques can be used to determine the accurate primary parameters of the couplers. In this report, the FEM has been used for the analysis and the design of the couplers shown in Figs. 1 and 2.^1,2

For asymmetrical inner conductors, ² and using this numerical model, capacitances C_i(ε_r) are computed for:

(All of the other conductors are grounded.)

Setting V₁ = V₂ = 1 V yields a capacitance C3, so that the coupling capacitance C_m is calculated by the following relation:

Inductances L_i are given in terms of capacitances, as in the case of a single quasistatic line, ² and the mutual inductance, L_m, is calculated from the following relation: