The comparison of measured coupling and isolation of proposed coupler with conventional coupler is shown in Figure 8. For a conventional coupler, the measured maximum isolation is only 23 dB, whereas for coupler with three slots it is 60.5 dB. Thus, the directivity of the microstrip coupler is improved up to 43 dB (with better than 30-dB directivity over 60-MHz bandwidth), using a coupler with three slots. In external compensation techniques like [3,4], the values of the compensating elements are calculated using the iteration process, and the length of the coupler must be redesigned. This difficulty is avoided in the proposed coupler. The isolation of the coupler can be calculated in the design process itself. The design process is simple when compared to [13][14][15]. The limitation of the proposed coupler is its narrow band. It can be used for narrow band applications such as RFID. For the proposed coupler S 41 is not equal to S 32 , and it cannot be used as a reciprocal device, as the structure is asymmetric. The calculated, simulated, and measured isolation of conventional coupler and proposed couplers are listed in Table 1.
CONCLUSIONIn this article, a microstrip coupler with high directivity has been presented by varying the width of the coupler. Using this procedure, a 15-dB microstrip coupler is designed and fabricated at 0.9 GHz. A high directivity of 43 dB is obtained. The simulation results are verified with measurements. When compared to other methods, the structure of the coupler is planar, its construction is simpler, and it does not require any iteration design process. ABSTRACT: In this article, a tunable branch-line coupler with a novel structure is designed to operate at 5.7GHz. A design method is based on miniaturization method with adjusting the resonance frequency. The proposed coupler without taped line feed structures operated at 5.62 GHz, and it has a size of 55.8 mm 2 while its performance is reasonable. To improve the performance, the taped line feed structures are added. The wideband response exhibits that the proposed coupler acts properly with the spurious harmonics up to 11.4 GHz. In addition, at 5.7 GHz, the good insertion loss (2.3 dB) and coupling factor (2.6 dB) are obtained. The proposed coupler is simulated, fabricated, and measured. The results show that there is a good agreement between the simulation and measurement results.
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