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

In this article, 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.