________________________________________________________________________________________ 1 Warsaw University of Technology, Institute of Electronic Systems, Nowowiejska 15/19, 00-665 Warsaw, Poland, e-mail: aabr@ise.pw.edu.pl, tel.: +48 22 2347710, fax: +48 22 8252300. High Directivity Couplers Realized in Microstrip Line Technology A. Abramowicz 1 A. Golaszewski 1 L. Kowalczyk 1 Abstract  This paper describes the design and realization of high directivity planar couplers realized in microstrip line technology. The basic idea is that coupling between two transmission lines is present at two points only. The coupling is realized by capacitances soldered to the lines or realized as interdigital capacitors. The presented type of couplers is in close relation to two hole waveguide coupler. The proposed couplers are narrowband ones. Several couplers with 3, 6, 10 and 15 dB couplings have been designed and realized. Lumped SMD capacitors have been used. In each case directivity is better than 30 dB. Couplers are planar, compact and if required can be easily tuned for needed parameters. 1 INTRODUCTION The microstrip based circuits are widely used in RF and microwave frequency range. This technology has a lot of advantages with ease of mounting passive and active elements at the top. Among other devices directional couplers are often realized in microstrip technology. Structures that use microstrip coupled lines have two modes of propagation called even and odd one. The velocity of the signal traveling through mcoupled microstrip lines is different for even and odd modes, which degrades performance of microstrip couplers. The high isolation and directivity is not achieved within the coupler's operation frequency range. Usually the maximum achieved directivity is limited to 20 dB only [1]. This is a disadvantage compared to stripline couplers with directivity over 40 dB. In the past the problem of limited directivity of microstrip couplers has been thoroughly studied and several method to overcome the problem have been given. Some of them required additional layers of dielectric placed above the microstrip lines [2]. Other methods use additional compensating elements soldered to the lines [3-6]. Especially compensating capacitances have been quite popular [3-5]. Even exotic changes of the coupled lines shape can be a solution [7]. In this paper we propose a new method for realizing a high directivity microstrip coupler in completely planar form. Our idea follows the waveguide coupler realization in which holes in common wall between two waveguides provide electromagnetic coupling between waves in the waveguides [8]. In the simplest realization two holes located at the quarter of wavelength are enough to create the coupler. The coupling is through holes only and between holes the waveguides are not coupled. In microstrip technology the waveguides are replaced with microstrip lines located at certain distance big enough to provide no or a very small coupling betwen them [9]. The two holes are replaced with two capacitors soldered to microstrip lines at both sides of the coupler. The distance between capacitors depends on the coupling level and is close to a quarter of wavelength in microstrip line for small couplings. For strong coupling e.g. 3 dB the coupler is much shorter and its length is lower than a half of a quarter of wavelength in microstrip line. Described couplers are narrowband thus cannot replace the traditional ones but there is plenty of applications where narrow band couplers suit perfectly well. In the paper the theory and design formulas for presented type of the coupler is given. Realized examples show couplers with high directivity exceeding 30 dB and at each coupler port a good match on the level of 25 dB. The couplers have been realized within the project for LLRF system in XFEL accelerator. The goal have been stated as a realization of directional couplers with directivity better than 30 dB operating in a band of 30 MHz around frequency of 1.3 GHz. Frequency 1.3 GHz is used in FLASH and XFEL (European X –Ray Free Electron Laser) in DESY, Hamburg. One of the subsystems installed in XFEL needed several high directivity couplers that can be embedded in the PC board realized with microstrip lines. The project included four models of directional couplers with 3 dB, 6 dB, 10 dB and 15 dB coupling. The couplers has been designed with NI AWR Microwave Office. The parameter of primary importance is coupler directivity, coupling values are of secondary importance. The directivity of at least 30 dB has been needed within small bandwidth around 1.3 GHz. The couplers have been etched on Rogers RO4350B laminate of 0.254 mm thickness. The size of each coupler is 40 mm by 20 mm. The proposed solution is unique in microstrip technology. It is simple and cheap. 2 COUPLER WITHOUT COUPLED LINES The proposed new type of a microstrip coupler is presented in Fig. 1. The coupler consists of two uncoupled lines of electrical length θ and 50 Ohm impedance (it is possible to use weakly coupled lines too) and two capacitors. The coupled port is denoted 2 and isolated port is denoted 3. Port 4 is transmission port. The capacitors provide coupling between lines and if properly selected and located and proper distance the directional coupler can be created. The lines between capacitors can be of any impedance but it is quite natural to use lines of impedances equal to port impedances. In our case the impedance have been 50 Ohms. Unfortunately design