**1. Introduction**

Nonreciprocal devices have been extensively used in modern microwave and millimeter systems [2–4, 14, 25, 27, 28]. In order to obtain the nonreciprocal effects, one needs to utilize the magnetized ferrite materials [2, 4, 14, 25, 27, 28] or active elements such as amplifiers [3]. Recently, the longitudinally magnetized ferrite coupled striplines or slotlines [2, 9, 14, 27] are being developed and employed to realize integrated nonreciprocal devices. Significant interest in these devices results from their advantages which are weak biasing magnetic field and wide operation bandwidth.

The basic part of ferrite coupled line (FCL) devices is longitudinally magnetized FCL section composed of two coupled lines placed on ferrite substrate [6, 21]. This structure was first proposed and experimentally verified by [6]. Next, Next, Mazur & Mrozowski in [21] using the coupled-mode method (CMM) developed the model of FCL section which explains the operation of this structure and gives basis steps in their design procedure. According to this model, in the ferrite section, a gyromagnetic coupling occurs, resulting in Faraday rotation effect. The wide operation bandwidth and high isolation are obtained, when the Faraday

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rotation phenomenon is optimal. This optimal effect is achieved when the ferrite material is placed in the region where the wave is linearly polarized and occurs in cylindrical waveguide with coaxially located ferrite rod or suspended stripline. In order to construct devices such as circulators [9, 23], gyrators [19, 22], or isolators [13], the FCL junction has to be cascaded with reciprocal sections providing input signals to the FCL which are either in phase or out of phase [20].

So far, studies concerning FCL devices have been focused mainly on structures realized in a planar line technology [1, 4, 9]. Such structures allow one to obtain fully integrated FCL devices. However, due to the significant length of the ferrite section, the main drawbacks are high insertion losses occurring in ferrite material and large dimensions of the structure.

There were several attempts to improve performance and to reduce total dimensions of planar FCL devices. Promising results concerning low insertion losses and high isolation were obtained for the nonreciprocal devices employing a ferrite coupled slotline [9] and stripline junctions [23, 24]. For the fabricated devices, obtained insertion losses were not lower than 3 dB and isolation was better than 12 dB [9, 18, 23]. Moreover, in order to reduce the dimensions of the planar FCL devices in [4], the circulator with appropriate matching networks at the ports ensuring multiple reflections was proposed. For presented device, the FCL junction length reduction by a factor of two was obtained. The drawback of this structure was high value of insertion losses caused by multiple transmission of signal through the lossy ferrite junction. Also similar length reduction of FCL junction was achieved with the use of periodic left-handed/ferrite coupled line (LH-FCL) structures [1]. However, for the simulated circulator utilizing LH-FCL section, the insertion losses were not lower than 4 dB.

The better performance in comparison to currently proposed planar configurations was obtained for nonreciprocal devices utilizing cylindrical ferrite coupled line (CFCL) junction [10]. Due to the similar geometry to the circular waveguide with coaxially located ferrite rod, such structure allows to obtain close-to-optimal Faraday rotation effect. Moreover, in such configuration, stronger gyromagnetic coupling occurs which is a result of high magnetic field concentration in the ferrite medium. These make possible to design shorter ferrite junctions ensuring lower insertion losses in comparison to planar ones. This junction was successfully applied to realization of nonreciprocal devices such as isolators and circulators [11, 12].

This chapter presents the authors' recent research on the nonreciprocal devices utilizing longitudinally magnetized FCL junction. The operation principle of FCL junction is explained, and the hybrid techniques of analysis are shown. Numerical and experimental results concerning the nonreciprocal devices using different configurations of FCL junctions are presented and discussed.
