In this paper, a novel microstrip hairpin-line bandpass filter which employs a modified Minkowski fractal shape is proposed. Although conventional hairpin-line filters are popular for RF front ends, they suffer from undesired spurious responses located at the second harmonic, which causes asymmetry in the upper skirt band. By proper design, the second harmonic of fractal filters can be significantly suppressed through the use of fractal shape. To validate this novel geometry, the proposed filters are fabricated and measured. Simulated results are in good agreement with measured results.

In this paper, a rat-race hybrid coupler based on an open complementary split ring resonator (OCSRR) is presented. By embedding the OCSRR in the microstrip transmission line, slow-wave effect is introduced to achieve size reduction. The proposed rat-race coupler size is 37% smaller than the conventional rat-race coupler. Besides, the proposed coupler provides better third harmonic suppression up to 35 dB. The simulated results are compared with the measured data and good agreement is reported.

A planar miniaturized branch-line coupler with harmonic suppression property for UHF band applications is presented in this paper. By properly synthesizing the LC-tanks that employ artificial transmission lines, two pairs of quarter-wavelength branch-lines to respectively meet characteristic impedances of 35.4 and 50 ohms can be obtained with the coupler. For the operating band, it can achieve good 3 dB power division with a 90° phase difference in the outputs of the through and coupled arms. The coupler also has a small area of 20.5(L)18(W) mm2, corresponding to 0.11 λg0.1 λg at 922 MHz. Compared with conventional couplers, the proposed design not only offers a wide bandwidth of more than 230 MHz within 1° or 1 dB, but also works with additional harmonic suppression for achieving better performance. Therefore, the proposed branch-line coupler with a compact size is well suitable for power division application.

In this article, a simple structure of the Wilkinson power divider which can suppress the nth harmonics of the Wilkinson power divider is proposed. By replacing the quarter wavelength transmission lines of the conventional Wilkinson power divider with the equivalent P-type transmission lines, a compact power divider which can suppress the nth harmonic is achieved. Design equations of proposed P-type line are achieved by ABCD matrices. To verify the design approach, the proposed power divider is designed, simulated (by ADS, CST Studio, and Sonnet simulators), and fabricated at 1 GHz to suppress the fifth harmonic. The proposed structure is 46% of the conventional Wilkinson power divider, while maintaining the characteristics of the conventional Wilkinson power divider at the fundamental frequency. The insertion losses at the fifth harmonic are larger than 35 dB. Furthermore, the second to seventh harmonic are suppressed by least 10 dB. Here is an excellent agreement between simulated results and measured results.

A compact open-loop resonator bandpass filter is presented to suppress the spurious passband using compensated compact microstrip resonant cell (C-CMRC) feeding structure. Based on the inherently compact and stopband characteristics of the C-CMRC feeding, the proposed filters shows a better spurious rejection performance than the only open-loop resonator filter. The suppression is -57.4 dB, -49.5 dB, and -43.9 dB at the 2nd, 3rd and 4th harmonic signal separately. All the performance of proposed filters have been verified by the measured results.

A novel class of microstrip bandpass filter is configured using the impedance transformers and an improved stepped impedance resonator (SIR). This SIR is composed of a central narrow strip section with an aperture on ground and two wide strip sections at the two sides. This low-high-low SIR resonator has a promising capability in achieving an extremely large ratio of first two resonant frequencies for design of a bandpass filter with ultra-broad stopband. The two quarter-wavelength transformers with low and high impedances, referred as to impedance- and admittance-inverters, are modeled and utilized as alternative types of inductive and capacitive coupling elements with highly tightened degrees for wideband filter design. After extensive investigation is made on the two transformers and the proposed SIR, the two novel bandpass filters are constructed, designed and implemented. Two sets of predicted and measured frequency responses over a wide frequency range both quantitatively exhibit their several attractive features, such as ultra-broad stopband with deep rejection and broadened dominant passband with low insertion loss.

A side-coupled microstrip open-loop resonator is presented for design of harmonic-suppressed bandpass filters with compact size. In geometry, the open-ended microstrip feed line is put in close proximity to the loop resonator at the opposite side of an opened-gap. In design, its length is properly lengthened to establish the orthogonal even- and odd-symmetrical current distributions along the two coupled strip conductors. It thus results in cancellation the 1st parasitic resonance. The two-stage open-loop filter is first constructed and its performance is studied under varied feed line lengths. Furthermore, a four-stage filter block is optimally designed at 2.52 GHz and its circuit sample is fabricated with the overall length less than 60% of one guided wavelength. The measured insertion loss at the 1st harmonic is higher than 30 dB, the stopband covers the range from 2.8 GHz to 7.0 GHz, and the dominant pass bandwidth is about 9.0%.

In this letter, a novel design of a branch-line coupler with considerable reduction in its size and suppressed harmonic passband is proposed. By embedding a defected ground structure (DGS) unit cell under a microstrip line, compact branch-line couplers are easily achieved. The electrical length is scaled appropriately according to the slow-wave effect. In this case, the experimental coupling (S21 or S31) is comparable to that of conventional branch-line couplers. Also, experimental results indicate that DGS section is quite effective for the suppression of higher order harmonics.