직렬공진 급전구조를 이용한 광대역 소형 안테나

Abstract

Designing an internal antenna within a mobile device has become an extremely challenging task as a result of the ever-growing demands for miniaturization, multi-band operation, and broadband operation characteristics. It has been also suggested that the radiation properties of a small antenna are intrinsically limited by its dimensions. To improve antenna bandwidths, a wide variety of techniques have been introduced, which can be classified into three approaches based on decomposing an entire antenna system into four parts, including feeder, resonator, radiator, and ground plane. As an approach of controlling the coupling between feeder and resonator, a series- or parallel-resonant feed structure is a reasonably convenient and flexible technique. Yet despite various types of wideband antennas adopting the series-resonant feed structures have been implemented since a long time ago, the coupling mechanisms of these antennas still remain inadequately clarified. The objective of this dissertation is to thoroughly study the coupling mechanisms of antennas with series-resonant feed structures, based upon reaction concept and two-port network analysis. Consequently, it is indicated that the coupling between feeder and series resonator can be controlled by adjusting the impedance level of series resonator. Several wideband small antennas employing the series-resonant feed structures are proposed as instances, after performing the theoretical analysis on this coupling mechanism, to demonstrate its capability of being optimized to gain the enhancement of antenna bandwidths. A loop-type GradiANT having an enhanced wideband property is introduced for USB dongle applications. By choosing an appropriate ratio of the inductance to the capacitance, i.e. a suitable impedance level, of a series-resonant feeding circuit that includes a chip inductor and a chip capacitor, the impedance bandwidth of VSWR less than 2 is realized to be 26.4% at 2.2 GHz, which is sufficient bandwidth to cover the WCDMA, WiMax 2.3GHz, and Wi-Fi 2.4GHz bands. A compact triple-band GradiANT intended for mobile terminals is implemented subsequently, within a considerably small clearance of 12 mm × 6 mm. By adopting two series-resonant feeding loops and three inductive couplers exciting the characteristic modes on a chassis, impedance bandwidths of 21 MHz, 210 MHz, and 1820 MHz are achieved, covering GPS, Wi-Fi 2.4GHz and 5GHz bands, respectively, at VSWR less than 2. Then, a dual-band 2D PIFA achieving significantly enhanced impedance bandwidth is newly designed. It employs a dual-band series-resonant feed structure consisting of two series resonators. The dual-band impedance bandwidth is thus enhanced based upon the appropriate impedance levels of these two resonators, each of which independently optimizes the coupling between feeder and resonator in the corresponding frequency band. The effectively enhanced antenna bandwidth sufficiently covers the extensively used frequency bands of 698–960 MHz and 1710–2690 MHz in mobile handsets, with VSWR less than 3.| 안테나의 소형화, 다중대역, 광대역 특성에 대한 수요가 늘어남에 따라, 모바일 단말기안의 내장형 안테나를 설계하는 것은 매우 도전적인 과제가 되고 있다. 또한, 소형 안테나의 방사 특성이 안테나의 사이즈에 본질적으로 제한되어 있다고 이미 주장이 되어왔다. 안테나의 대역폭을 향상시키기 위해 매우 다양한 기술들이 소개되었으며 대략 3가지 방식으로 분류될 수 있다. 사실 이 3가지 방식은 전체의 안테나 시스템이 4개 부분, 즉 급전선, 공진기, 방사체, 그라운드 부분으로 나눠져 있다는 가정에 기반을 두고 분류된 것이다. 그 중에 급전선과 공진기 사이의 결합을 제어하는 방식으로서 제안된 직렬 또는 병렬 공진 급전구조는 상당히 편리하고 신축성있는 기술이다. 하지만, 직렬공진 급전구조를 적용한 다양한 형태를 가진 광대역 안테나들이 이미 오래전부터 구현되었으나, 이러한 안테나 내부의 결합 원리에 대한 설명이 아직도 불충분하다. 본 학위논문은 reaction 개념과 2-port 네트워크 분석을 바탕으로 직렬공진 급전구조를 이용한 안테나 내부의 결합 원리에 대해 철저히 연구하는 것에 목적이 있다. 결론적으로, 급전선과 공진기의 결합은 직렬공진기의 임피던스 레벨을 조절함으로써 제어될 수 있다고 나타난다. 이 결합을 최적화시키는 원리에 대한 이론적 분석을 수행한 뒤, 안테나 대역폭이 확장되는 효과를 실증하기 위하여 직렬공진 급전구조를 이용한 몇 가지 광대역 소형 안테나를 예시로 제안한다. 먼저, USB 동글에 적용을 위한 향상된 광대역 특성을 가진 루프 형태의 GradiANT가 소개된다. 칩 인덕터와 칩 캐패시터가 하나씩 포함되어 있는 직렬공진 급전회로의 적절한 인덕턴스와 커패시턴스의 비율 (즉, 임피던스 레벨)을 선택함으로써, VSWR이 2보다 작을 때의 임피던스 대역폭은 2.2 GHz에서 26.4%가 실현되며, WCDMA, WiMax 2.3GHz, 그리고 Wi-Fi 2.4GHz 주파수 대역들을 충분히 커버할 수 있다. 이어서, 모바일 단말기에 적용을 위해 12 mm × 6 mm인 매우 작은 클리어런스 안에서 초소형 삼중대역 GradiANT가 구현된다. 직렬공진 급전 루프 2개와 그라운드 특성모드를 구동시키는 인덕티브 커플러 3개를 이용하여, VSWR이 2보다 작을 때의 임피던스 대역폭은 21 MHz, 210 MHz, 그리고 1820 MHz가 실현되며, 각각 GPS, Wi-Fi 2.4GHz와 5GHz 주파수 대역들을 커버한다. 그리고 나서, 임피던스 대역폭이 엄청나게 확장된 이중대역 PIFA가 새로 설계된다. 이 안테나는 직렬공진기 2개로 구성된 이중대역 직렬공진 급전구조를 이용한다. 이 2개 직렬공진기가 각각 적절한 임피던스 레벨을 가짐으로써, 해당 주파수 대역에서 급전선과 공진기의 결합이 동립적으로 최적화될 수 있기 때문에 이중대역 임피던스 대역폭의 확장효과가 실현된다. VSWR이 3보다 작을 때, 이 효과적으로 향상된 안테나 대역폭은 모바일 단말기에서 널리 사용되고 있는 주파수 대역인 698–960 MHz와 1710–2690 MHz를 충분히 커버할 수 있다.; Designing an internal antenna within a mobile device has become an extremely challenging task as a result of the ever-growing demands for miniaturization, multi-band operation, and broadband operation characteristics. It has been also suggested that the radiation properties of a small antenna are intrinsically limited by its dimensions. To improve antenna bandwidths, a wide variety of techniques have been introduced, which can be classified into three approaches based on decomposing an entire antenna system into four parts, including feeder, resonator, radiator, and ground plane. As an approach of controlling the coupling between feeder and resonator, a series- or parallel-resonant feed structure is a reasonably convenient and flexible technique. Yet despite various types of wideband antennas adopting the series-resonant feed structures have been implemented since a long time ago, the coupling mechanisms of these antennas still remain inadequately clarified. The objective of this dissertation is to thoroughly study the coupling mechanisms of antennas with series-resonant feed structures, based upon reaction concept and two-port network analysis. Consequently, it is indicated that the coupling between feeder and series resonator can be controlled by adjusting the impedance level of series resonator. Several wideband small antennas employing the series-resonant feed structures are proposed as instances, after performing the theoretical analysis on this coupling mechanism, to demonstrate its capability of being optimized to gain the enhancement of antenna bandwidths. A loop-type GradiANT having an enhanced wideband property is introduced for USB dongle applications. By choosing an appropriate ratio of the inductance to the capacitance, i.e. a suitable impedance level, of a series-resonant feeding circuit that includes a chip inductor and a chip capacitor, the impedance bandwidth of VSWR less than 2 is realized to be 26.4% at 2.2 GHz, which is sufficient bandwidth to cover the WCDMA, WiMax 2.3GHz, and Wi-Fi 2.4GHz bands. A compact triple-band GradiANT intended for mobile terminals is implemented subsequently, within a considerably small clearance of 12 mm × 6 mm. By adopting two series-resonant feeding loops and three inductive couplers exciting the characteristic modes on a chassis, impedance bandwidths of 21 MHz, 210 MHz, and 1820 MHz are achieved, covering GPS, Wi-Fi 2.4GHz and 5GHz bands, respectively, at VSWR less than 2. Then, a dual-band 2D PIFA achieving significantly enhanced impedance bandwidth is newly designed. It employs a dual-band series-resonant feed structure consisting of two series resonators. The dual-band impedance bandwidth is thus enhanced based upon the appropriate impedance levels of these two resonators, each of which independently optimizes the coupling between feeder and resonator in the corresponding frequency band. The effectively enhanced antenna bandwidth sufficiently covers the extensively used frequency bands of 698–960 MHz and 1710–2690 MHz in mobile handsets, with VSWR less than 3.