| With the rapid development of wireless communication technology, maritime satellite communication plays an important role in our life. Since BGAN has been put into use in China, it experienced the test of the special application, such as the relief of ice and snow disaster, Wenchuan earthquake and Xinjiang He tian earthquake disaster, and the Somali escort, was highly recognized by the state. As an extension of the existing public network and emergency communication system, the maritime satellite communication system will play a great role in the communication and emergency communication in remote areas of our country. In August 2014, the establishment of the Beijing Office of the International Maritime Satellite Organization will further deepen the long-term cooperation with China. October 2015, national president Xi Jinping visited INMARSAT which is a British satellite communications company, enough to prove that the China attaches importance to the satellite communications, this visit may lead to attention of the market for the entire satellite communications industry.In this paper, the RF front end devices in mobile terminal of BGAN communication system were studied, a broadband circularly polarized antenna, a duplexer and a low noise amplifier were designed. In which, the broadband circularly polarized antenna was designed using a aperture coupling orthogonal feed stacked patch antenna structure, the stacked patch antenna is a square symmetric two layer structure with increased air layer in the middle, so the impedance matching in broad bandwidth was realized. In order to radiate the circular polarization wave, the antenna was fed by the method of aperture coupling orthogonal dual-feed. The aperture of feed is Hour-Glass type, farther incresed impedance matching bandwidth and the coupling energy. The microstrip line with open-stub was used as feeding line because using it to adjusting match easily. In order to increase the gain of the antenna, a 2 x 2 antenna array using above stacked patch antenna as unit antenna was made up. As a result of orthogonal double-fed for unit antenna, the 2 x 2 antenna array needs a eight equal power divider to feed. The eight equal power divider was designed by seven bisected Wilkinson power dividers, its bandwidth is wide, and with good characteristics. The phase relationship needed for the circular polarization radiation was achieved by broadband phase shifter, by introducing parallel open line and short line to achieve 90 degrees of broadband phase shift. The duplexer was designed by connecting two filters which worked at the uplink and downlink frequency band, respectively. The filter were designed by loading a stepped-impedance resonator on open-loop resonator, and introduced a controlled zero in frequency band,so that implemented the miniaturization and high out-of-band rejection. The low noise amplifier was designed by ATF54143 transistor of Avago, the gain flatness at the request of low noise figure increased by the method of low-pass filter matching in former level and high-pass filter matching in after level, reached the requirements of the design.In this paper, the method of theoretical design combining with electromagnetic simulation software optimizing was adopted. According to the designed results, the antenna,multiplexer and low noise amplifier were fabricated and measured. The measurement results shown that:the most 3dB axis ratio angle range of the broadband circular polarization antenna array is 120°, its gain is 10.9dBi, and with good directivity characteristics. In the BGAN frequency band, the return losses of the duplexer is more than lOdB, the isolation is more than 30dB; the gain of the low noise amplifier is greater than 14dB, and with a flat curve, the noise figure is 1.4dB, with a good performance, so all the design requirements are basically achieved. |
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