Ultra-wideband filters based on multimode resonators

Frequency-selective devices with ultra-high frequencies bandpass filters, in particular, are among the most important elements of tropospheric and space communication systems and special radio equipment. The characteristics of all the studied microwave devices were obtained by numerical electrodynamic analysis of their 3D models. While modeling, substrates with the same permittivity ε = 9,8, thickness h = 1 mm were used to compare the obtained data objectively. The first construction consists of three electromagnetically coupled pin microstrip quarter-wave resonators. The considered ultra-wideband filter is configured in such a way that the maximum level of power reflection in its passband does not exceed 14 dB, while its order N equals 9. The device has relatively high frequency-selective properties, however, the asymmetric shape of its frequency response is an obvious disadvantage. To eliminate the above-mentioned drawback of the filter, it is necessary to organize the inductive and capacitive coupling between its multimode resonators so that they relatively compensate each other at certain frequencies. The second construction meets these requirements: its external resonators, as well as those in the first device, are quarter-wave three-mode, and the internal resonator is half-wave two-mode. Such an ultra-wideband filter has a high steepness of both slopes of the frequency response, due not only to the eighth order, but also to the attenuation poles located on both sides of the passband. At the same time, compared to the filter implemented only based on three-mode resonators, it is also 16% smaller. With a considerable increase in the number of two-mode and three-mode hairpin resonators used in the designs of microstrip filters, the number of possible combinations of these resonators grows significantly. At the same time, the frequency-selective properties of such devices can differ. The third construction includes a pair of external three-mode resonators and a pair of internal two-mode resonators, so the increase in the number of filter sections in this case is due to the increase in the number of two-mode half-wave resonators in the design. At the same time, such an ultra-wideband filter of the tenth order also has high frequency-selective properties, but its significant improvement, in comparison with the selective properties of the second filter, is not observed. The fourth construction is implemented with a central two-mode resonator, on both sides of which there is a pair of three-mode resonators, so the increase in the number of sections to five in this case is due to the increase in the number of three-mode resonators. Such an ultra-wideband filter of the fourteenth order stands out from all the devices considered above by its higher frequency-selective properties. So, it is worth noting the greater steepness of the low-frequency slope of its passband, as well as the shape of its frequency response, which is quite close to symmetrical. Therefore, microstrip filters based on hairpin resonators were investigated. The developed devices have not only extended passbands, but also high frequency-selective properties. At the same time, the results of the research revealed that the designs of filters with a central two-mode resonator with three-mode ones on both sides of which, single or paired, are distinguished by frequency response shape close to symmetrical, due to the presence of attenuation poles on it.