Impedance transforming directional couplers with increased achievable transformation ratio

Abstract

Novel impedance transforming directional couplers are proposed in which the achievable impedance transformation ratio is increased above the previously reported limit related to coupling of the coupled-line sections. The proposed couplers consist of two coupled-line sections between which uncoupled sections of left-handed lines are connected. The presented concept has been verified by circuit simulations as well as by measurements of the manufactured 3-dB coupled-line impedance transforming directional coupler operating at f0 = 1.2 GHz and featuring twofold increased impedance transformation ratio R = 4.

Broadband coupled-line directional couplers with high impedance transformation ratio

“…From the comparison it is clearly seen that the proposed technique allows for realization of high transformation ratio R in wide operational bandwidth. In contrast, in the previously published couplers either the bandwidth is limited by the chosen technique for high R realization [12], or the transformation ratio R is limited by the bandwidth enhancement [15].

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“…Recently a concept has been presented in [12, 13] where a new class of directional couplers has been proposed which offer equal power split between the coupled and direct ports. What is more, such couplers offer impedance transformation in a way that, for certain frequency range, the input ports are matched to one particular impedance ZT1 (typically 50 Ω) and the ports to which the power is delivered (coupled and direct) are matched to the other chosen impedance ZT2.…”

“…What is more, such couplers offer impedance transformation in a way that, for certain frequency range, the input ports are matched to one particular impedance ZT1 (typically 50 Ω) and the ports to which the power is delivered (coupled and direct) are matched to the other chosen impedance ZT2. Hence, the impedance transformation R of such couplers has been defined as It has also been shown that in such couplers strong limitation on the possible transformation ratio R exists [12, 13], i.e. where k is the coupling coefficient of the coupled-line section and in the case of 3-dB coupled-line directional couplers for which k = 0.707 the maximum available transformation ratio R = 2.…”

“… where k is the coupling coefficient of the coupled-line section and in the case of 3-dB coupled-line directional couplers for which k = 0.707 the maximum available transformation ratio R = 2. The problem of the overcoming this limitation has been shown in [12], where the application of left-handed transmission line section allows for lowering the required coupling of coupled lines, thus allows for enhancing R . However, only narrowband operation can be achieved in this case.…”

“…In this paper, we propose a novel type of 3-dB impedance transforming directional couplers, which are based on the recently published coupled-line directional couplers composed of coupled-line sections having different electrical lengths [16] and impedance transforming couplers described in [12–15]. The proposed coupler is superior to the previously published impedance transforming directional couplers, since it offers wide operational bandwidth together with very high impedance transformation ratio R .…”