Miniaturized Multi-Band Smartphone Antenna Using Non-Foster Based Active Matching Below 1 GHz

In this paper, a miniature triple-band antenna using active-passive matching method is presented for 5G mobile terminals. The antenna consists of two driven monopoles and one parasitic ground branch, which generates multiple modes to cover the 1710-2690 MHz and 3400-3600 MHz bands. Given that the passive antenna cannot resonate in the 698-960 MHz band, active matching technique based on non-Foster circuit is proposed. It can produce negative capacitor by using transistors and positive feedback topology. The impedances of the non-Foster circuit and the passive antenna are conjugated, leading to a good match over a wide bandwidth. Lumped components are added to ensure the stability of the active antenna. Combining the active and passive matching, a two-port antenna is designed to cover the three bands simultaneously. Filters are implemented to separate the low-band and high-band signals. The measured -6 dB bandwidth can cover the 698-960 MHz, 1820-2690 MHz, and 3170-3700 MHz bands with a small ground clearance of 25×4 mm². Comparing to passive matching, the proposed active-passive hybrid matching has a gain improvement of about 20 dB in the low band, which is attractive in the miniaturization of mobile terminal antenna design.