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DESIGN AND SIMULATION OF 505.8 MHz STRIP LINE DIRECTIONAL COUPLER* M. Ahlawat# and R.S. Shinde, RRCAT Indore, INDIA Abstract A strip line directional coupler is designed and simulated using finite difference time domain technique (FDTD), for low insertion loss and high directivity of 40 dB, optimized up to 10 kW RF power at 505.8 MHz. This will be be used for circulator testing, and other high frequency applications for reflection measurement in transmission lines. INTRODUCTION RF power reflected back from RF cavity is attenuated using circulator (situated between RF source and cavity), which may otherwise damage the RF source (or amplifier). By means of this device, reflected RF power is circulated to high power matched termination, and hence protecting the RF source. For characterization of circulator, which has been developed & is further being improved for higher powers at Ferrite Lab, AMTD, RRCAT, a high directivity (better than reflection from circulator) coupler is required. It is also important for taking proper safety measures for attenuation of any reflection coming from the circulator itself. To meet the requirement of circulator testing and mismatch measurement transmission lines, a narrowband stripline dual directional coupler with high directivity by utilizing phase superimposition technique, has been designed. THEORY AND DESIGN Directional coupler, in general is a passive RF device which couples a fraction of RF power flowing through its main line in one direction only[6]. In dual directional coupler power flowing in one direction is coupled to one port (of the auxiliary line), and in opposite direction to another port. Stripline, microstrip and waveguide technologies are used depending on frequency of application and RF power level. Considering its size constraints, strip line geometry has been implemented for Indus 2 RF frequency of 505.8 MHz. Parallel line coupled stripline directional couplers are wideband type and have low directivity[1][5]. To meet the requirement of the high directivity, phase superimposition technique has been implemented for constructive and destructive interference at coupled and at isolated port respectively. First of all stripline geometry including stripline conductor width and thickness, dielectric medium and its thickness are optimized to achieve the transmission line system of 50 characteristic impedance, which is given by equation Eq-1[2]. Characteristic impedance is proportional to spacing between ground planes, and *Work supported by RRCAT (DAE) Indore #[email protected] inversely proportional to strip line conductor width & square root of dielectric constant of the medium. Z0 = 30𝜋 𝑤𝑒 ( √𝜀𝑟 𝑏 −1 + 0.441) where 𝑤𝑒 𝑏 = 𝑤 𝑏 0 -{ 𝑤 2 (0.35 − ) 𝑏 𝑓𝑜𝑟 𝑓𝑜𝑟 𝑤 𝑏 𝑤 𝑏 > 0.35 < 0.35 } (1) where ε𝑟 relative permittivity, 𝑤𝑒 effective strip line conductor width, w width of strip line conductor, b spacing between two ground planes. Dielectric thickness in combination with strip line width and thickness plays an important role in impedance matching and determining the breakdown voltage, peak power & the power handling capacity of the directional coupler. In addition to optimizing these parameters, Impedance matching techniques like quarter wave (λ/4) transformer are used, if heavy mismatch between systems characteristic (around 50 Ω) impedance and typical characteristic impedance of the strip line structure, which may be case at very high powers. Dielectric thickness may be allowed to be within a range of values, as strip line conductor width and thickness can be adjusted for achieving impedance matching. Increased dielectric medium thickness gives higher peak and average power handling capability but the reduced equivalent transmission line capacitance and hence higher the characteristic impedance (as characteristic impedance inversely proportional to the square root of capacitance, Z0 = √𝐿⁄𝐶 ). Increased the strip line conductor width reduces the inductance per unit length due to increased magnetic field path length, and increases capacitance pet unit length which results in the reduced characteristic impedance. Increased strip line conductor width and thickness provides the higher average power handling capacity. Secondly and most critical, it is the schematic design for the phase superimposition of the two RF components coupled from the main line. It is having 3 types of lines main line, auxiliary line and coupling lines. Distance of open end half guide wave length λg/2 coupling lines from the main line, and the dielectric thickness determines the amount of RF coupling. Constructive and destructive interferences are decided by length of the path travelled by coupled RF signal before superimposition at the auxiliary line. However design is based on the stripline geometry, but works like wave guide directional coupler. RF power is coupled to the coupling lines only at two points separated by half the guide wave length[4], rather than along whole the line length as in the side-coupled strip line couplers. Length of the coupling lines as in fig-1 is also taken as half the guide wavelength g/2, so that it will not affect the auxiliary line impedance. Analysis is done by the phase calculations as in waveguide directional coupler, and not the "even and odd coupling mode" method used for stripline couplers[3]. In this way, by considering main power from port 1 to port 2, coupled RF signals are in same phase in forward direction to port 4 (coupled), and out of phase in backward direction to the port 3 (isolated). ELECTROMAGNETIC MODELING Electromagnetic modelling and the thermal analysis of the designed circulator has been done using CST Microwave studio utilizing FDTD technique[7], with transient & thermal solver. Symmetrical stripline geometry with 140 mm long, 10 mm wide & 0.2 mm thick stripline centre conductor, and 320 mm long, 10 mm thick, 150 width two dielectric slabs of alumina (and epoxy) has been optimized for the 50 characteristic impedance. Two coupling lines separated by g/2 70 mm are used to tap the RF power from the main line through its open ends situated at a distance of 10 mm. Coupling of RF power takes place by means of the electrical field between centre conductor and ground planes. Schematic of the stripline dual directional coupler is as shown in Fig-1. Full geometry having 50,000 mesh cells, air as background material, and open boundary conditions is excited using discrete ports. Figure 2: Smith chart of the S11-reflection from the port-1 of directional coupler, showing good impedance matching throughout the geometry. S parameter observed for the optimized directional coupler are as in figure 3, showing insertion loss, reflection, isolation, coupling, and hence giving directivity better than 40 dB . Figure 3: S-Parameters i.e. reflection S11, Insertion loss S21, coupling S41 and isolation S31, observed for the strip line directional coupler. Figure 1: Schematic diagram of the stripline dual directional coupler. Impedance matching in the whole geometry is important criteria for smooth operation of the directional coupler. Matching condition i.e. reflection parameter S11 is observed using the smith chart shown in fig-2. As apparent from the smith chart, all ports are having good matching (49.36 Ω port impedance against 50 Ω excitation source) and hence negligible reflection. Whole strip line geometry is enclosed in the copper casing to avoid the leakage of RF signal to surrounding. Electrical &Thermal analysis Port 1 has been excited using high voltage port up to 700 V giving out ~ 10 kW RF power. As in fig - 4, maximum electrical stress observed in the strip line geometry is observed is 704 V/mm, which is far below breakdown voltage of alumina 30 kV/mm, hence electrically very stable. However maximum voltage gradient applied across the dielectric medium is limited by the surrounding air (3 kV/mm) and humidity contents, but still can perform stably up to higher power by taking proper safety measures. RESULTS AND DISCUSSION Scattering parameters for the optimized strip line directional couplers are insertion loss S21 ~ -0.1 dB, reflection S11 ~ 27 dB , coupling S41 ~ 27 db, and isolation S31 ~ 65 dB, so directivity ~ 40 dB are observed, which are satisfactory enough for the measurement of reflection from the circulator (~ 30 dB). High directivity of the narrowband directional coupler is achieved by frequency selective phaser sum of the two coupled RF signal components. Since internal dielectric material is able to support higher peak breakdown voltage, so by selecting proper port termination, and taking high voltages measures, safety factor (typically 2), and cooling arrangement for the centre stripline conductor, directional coupler may be further upgraded for higher power levels. ACKNOWLEDGEMENT Figure 4: Vector electrical field profile of the directional coupler. As is evident from the thermal profile, maximum of 13 °C temperature rise has been observed (306 K). Up to this temperature there is no significant change in dielectric properties of the mediums used copper and alumina, so can perform thermally stable. Thermal profile of the directional coupler at 10 kW input power is observed as in fig-5. Figure 5: Thermal profile across the geometry of directional coupler. Thermal simulation has been carried out by using forced cooling of outer casing to ambient temperature of 293 K. As evident from the temperature profile of strip line directional coupler maximum rise is observed nearby the strip line conductor due to higher electric field (fig. 4). Stripline conductor cooling is mainly controlled by the heat transfer through the dielectric medium. Due to additionally cooling through port contacts, actually temperature rise will be much lesser than the simulated. We are indebted to Dr P.D. Gupta, director RRCAT for constant encouragement for development of ferrite circulator and associated components . REFERENCES [1] Howe. Harlan Jr, Stripline circuit design, 2nd edition, 1974. Artech House. [2] Pozar D.M., Microwave Engineering (2011). John Wiley and sons. [3] Experiment 5 Coupler design. http://www.hit.ac.il/.upload/engineering/microwave_ -_experiment_5_-_coupler_design.pdf [4] George Kennedy, Bernard Davis, Electronic communication systems, (2005), Tata Mc Graw-Hill. [5] Directional coupler, Microwave 101.com. http://www.microwaves101.com/encyclopedia/directi onalcouplers.cfm. [6] Power divider and directional coupler, Wikipedia. http://en.wikipedia.org/wiki/Power_dividers_and_dir ectional_couplers. [7] CST (computer simulation technology) Website. http://www.cst.com/Content/Events/workshop_docu ments/2012/1116-PR-Microwave-Circuits-andComponents.pdf