Impedance Matching of a Loaded Microstrip Transmission Line by Parasitic Elements

Transcription

1 Impedance Matching of a Loaded Microstrip Transmission Line by Parasitic Elements H. Matzner 1, S. Ouzan 1, H. Moalem 1, and I. Arie 1 1 HIT Holon Institute of Technology, Department of Communication Engineering, 52 Golomb St., Holon 58102, Israel, haim@hait.ac.il, An impedance matching of a loaded microstrip transmission line by parasitic elements is presented. A 1:2 microstrip splitter is taken as a test case, and a comparison is made between a standard λ/4 matching to our λ/4 parasitic matching. Simulation and measurement show that the quality of the standard λ/4 matching is better, while the impedance bandwidth is almost the same. The advantages of the parasitic matching technique are shortly discussed. A simple microstrip splitter is presented in figure 1. The microstrip transmission lines are printed on a FR4 substrate having ε r 4.7, 1.6 mm thickness and having width of 2.95 mm. The characteristic impedance of the microstrip transmission lines is 50 Ω. Keywords-component; impedance matching; parasitic matching elements. I. INTRODUCTION Impedance matching is needed in most RF and microwave systems. Generally, in the various matching techniques the conductors of the matching subsystem must be physically connected to the conductors of the transmission lines or waveguides of the matched system. In the present work we propose an easier impedance matching technique for a loaded microstrip transmission line, by using only parasitic elements. That is, in our case there is no physical connection between the conductors of the matching subsystem and the conductors of the main system. A 1:2 microstrip splitter is chosen as test case to check the quality of the proposed parasitic matching technique. The return loss of the splitter is checked, by a simulation and a measurement, in three cases: without matching, with a standard λ/4 microstrip transformer, and with a parasitic λ/4 The structure of the paper is as follows: in chapter II we present the configurations of the unmatched splitter, the splitter matched by standard λ/4 microstrip transformer, and the splitter matched by the parasitic λ/4 In chapter III the simulated return loss of the 3 configurations is displayed, and in chapter IV measurement results are shown. Chapter V is devoted to conclusions and discussion. II. CONFIGURATIONS OF THE UNMATCHED AND THE MATCHED SPLITTER A. The 1:2 Microstrip Splitter. Figure 1. A 1:2 microstrip splitter. The 50 Ω transmission line are printed on a 1.6 mm thick FR4 substrate having er 4.7. The widths of the lines are 2.95 mm. The splitter is unmatched. The load seen by the main line equals to Z L = = 25 Ω, hence the reflection coefficient at the load is given by Γ = 1/3 and Γ = (25 50) / ( ) (1) SWR = (1 + 1/3) / (1 1/3) = 2 (2) B. A Standard λ/4 transformer matching. As shown in [1], in order to perform the λ/4 transformer matching, we have to find a distance d measured from the load to the right edge of the transformer, which is a transmission line having a characteristic impedance Z q, with length λ/4. The solutions are

2 Z q1 = Z 0 [(1 - Γ ) / (1 + Γ )] 1/2 ; d 1 = λ (θ + π) / 4 π (3) Z q2 = Z 0 [(1 + Γ ) / (1 - Γ )] 1/2 ; d 2 = d 1 ± λ / 4 (4) Side views of the microstrip line and the new line are shown in figures 3 and 4. The new transmission line has been designed and simulated by the CST Microwave Studio in order to have a characteristic impedance Ω. Its wavelength at f = 1 GHz was found by the CST software as well. Where Γ = Γ exp (j θ). For a real load the standard solution is the first one, for which d = 0 and Z q = (Z 0 Z L ) 1/2 = Ω (5) The width of the transformer is 5 mm and its length is 40 mm at f = 1 GHz. The splitter containing the λ/4 microstrip transformer is shown in figure 2. Figure 4. A new transmission line, based on the standard microstrip transmission line shown in figure 3. The characteristic impedance of this transmission line is Ω. A piece of length λ/4 of this line is used for the parasitic matching. Figure 2. A 1:2 microstrip splitter containing a λ/4 standard microstrip The length of the transforner is 40 mm at f = 1 GHz, its width is 5 mm and its characteristic impedance is Ω. In order to perform the parasitic matching, we simply take a piece having the same structure as the structure of the upper part of the transmission line shown in figure 4, and place it on the microstrip line at the calculated matching position. One can check the optimal position of the piece by let it slide a little bit around the matching position. When the desired matching quality is achieved, the matching piece can optionally be fixed to the line by dielectric screws. An upper view of the parasitic matching is shown in figure 5. C. λ/4 Parasitic Matching Configuration In order to perform the parasitic matching we first produce a new transmission line, by adding another dielectric layer and another parasitic conductor above the added dielectric layer. Figure 3. A standard microstrip transmission line, having characteristic impedance equals to 50 Ω, used in the microstrip splitter. Figure 5. The parasitic λ/4 technique. The characteristic impedance of the microstrip line having on it the parasitic piece is Ω. The length of the λ/4 piece is 39 mm at f = 1 GHz.

3 Figure 6: S 11 simulation of the unmatched splitter. Figure 7: S 11 simulation of the splitter, matched by the standard l/4 microstrip

4 Figure 8: S 11 simulation of the splitter matched by the λ/4 parasitic III. SIMULATIONS OF THE SPLITTER Simulations of the unmatched splitter, the splitter matched by the standard λ/4 microstrip transformer and a simulation of the splitter matched by the parasitic λ/4 transformer are presented in figures 6, 7, 8 respectively. It is shown that the quality of the matching by the standard λ/4 microstrip transformer technique is quite better: the bandwidth for return loss less than -10 db is between 0.3 and 1.6 GHz, and the minimal value of the return loss is -22 db. In the case of the λ/4 parasitic transformer the bandwidth for return loss less than -10 db is between 0.3 and 1.35 GHz, and the minimal value of the return loss is -18 db. IV. MEASUREMENTS OF THE SPLITTER Similar results are obtained by measurements. S 11 measurement of the unmatched splitter, the splitter matched by the standard λ/4 microstrip transformer technique and the splitter matched by the λ/4 parasitic transformer is shown in figures 9, 10 and 11 respectively. It is seen that the bandwidths of the splitters are quite the same, while the average value of the return loss in the case of the standard λ/4 microstrip transformer matching is lower than that obtained by the λ/4 parasitic Figure 9: S 11 measurement of the unmatched splitter.

5 IV.CONCLUSIONS A kind of a parasitic λ/4 transformer matching technique, suitable for microstrip and similar transmission lines has been presented. It is shown that a very good matching level has been obtained, with the present method, albeit with a little bit lower quality than the matching quality achieved by the standard λ/4 microstrip transformer matching technique. The advantage of the present method is that one can check, after the simulation, the matching level of the real circuit, without yet fixing the parasitic matching element to the circuit. One can try to find the optimal position of the matching element, or try other parasitic matching elements. When the matching level is obtained, the matching piece can be fixed to the main circuit Figure 11: S11 measurement of the splitter matched by the parasitic λ/4 Figure 10: S 11 measurement of the splitter matched by the standard λ/4 microstrip by dielectric screws. This method is appropriate for emergency cases, to improve the matching level, or for fine tuning, after the circuit has been produces. It is also interesting to investigate the efficiency of other parasitic matching elements like stubs, tapers etc. [1] P. M. Pozar, Microwave Engineering, John Wiley & Sons, 3 rd Ed., 2005, chapter 5. [2] N.N. Rao, Elements of Engineering Electromagnetics, 5 th Ed., Prentice0Hall, 2000, chapter 7.