An Ultra-Wideband Low-Profile Dipole Patch Antenna for VHF-Band Probing Radars
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摘要: 该文提出了一种具有超宽带(Ultra-Wideband, UWB)性能和低剖面特征的偶极贴片天线。天线的偶极贴片层采用Gielis曲线形状的金属结构,通过曲折技术减小了天线的横向尺寸。在辐射层末端加载金属短路壁并与接地面连接,共同形成背腔,从而提升了天线的增益。为进一步优化性能,设计了人工磁导体(Artificial Magnetic Conductor, AMC)单元,并在辐射贴片下方布置AMC阵列。该结构不仅改善了阻抗匹配,还显著降低了天线的剖面高度。此外,在贴片末端加载吸收电阻,进一步改善了电压驻波比(Voltage Standing Wave Ratio, VSWR)。对天线模型进行了加工,并在暗室中测量,仿真与测量结果表明,在电尺寸仅为0.38λL × 0.18λL × 0.07λL(其中λL对应最低频率工作波长)的情况下,该天线实现了100 MHz–366 MHz的带宽,并表现出良好的定向辐射特性,峰值增益达6 dBi。Abstract:
Objective In radar systems, the limitations of traditional narrowband antennas in terms of data transmission rate and resolution have become increasingly apparent. Consequently, a wide variety of ultra-wideband (UWB) antennas have been proposed, exhibiting excellent range resolution and strong anti-interference capability. However, at low-frequency bands, existing UWB antennas often suffer from excessively large physical dimensions, which makes antennas bulky while ensuring mechanical strength and posing significant challenges for airborne or vehicle-mounted radar assembly. However, compact antennas that are easy to install suffer from low gain, which fails to meet the penetration depth requirements for deep subsurface detection. Balancing antenna size and radiation gain over an ultra-wideband range to fulfill the needs of VHF wideband probing radars remains a critical challenge. To jointly optimize bandwidth, size, and gain, this paper presents a planar dipole antenna loaded with an artificial magnetic conductor (AMC) structure and metallic shorting walls. The proposed antenna achieves stable radiation performance over a wide frequency range while maintaining a low-profile configuration and a structurally simple design. As a result, it provides an effective solution for VHF-band detection radar antennas. Methods By comparing the reflection phase characteristics of AMC unit cells with different geometrical shapes, square-shaped unit cells were selected to form a 9×7 AMC reflective layer. Owing to its in-phase reflection property, the AMC structure overcomes the conventional limitation that requires a quarter-wavelength separation between the antenna and a metallic ground plane, thereby significantly reducing the antenna profile height. The dipole patch employs an optimized meandered current-bending technique, which effectively reduces the horizontal size of the antenna. Meanwhile, metallic shorting walls are vertically loaded at both ends of the antenna. According to the image theory, equivalent currents will be generated on the outer surface of the metal walls when the antenna operates, thereby effectively extending the equivalent electrical length, leading to improved low-frequency performance without increasing the physical size. In addition, two vertical metallic walls are connected to the ground plane on both sides of the antenna, jointly forming a reflective back cavity. This structure enhances the unidirectional radiation characteristic and improves the antenna gain. As a systematic co-design to break the physical constraints, four 125-Ω resistors are bridged between the antenna feed point and the metallic sidewalls. This resistive loading effectively suppresses low-frequency strong resonances and broadens the impedance bandwidth at the cost of acceptable Ohmic losses. Results and Discussions A prototype antenna with favorable simulated performance was fabricated and measured in a microwave anechoic chamber. The measured impedance bandwidth (VSWR < 2) ranges from 50 MHz to 400 MHz, which agrees well with the simulated bandwidth of 84 MHz to 366MHz. The measured impedance matching performance is slightly better than the simulated result, mainly due to the reduced reflections caused by cable losses and power divider losses in the feeding network. The measured antenna gain follows the same trend as the simulated gain, with deviations within an acceptable range of 1dBi. Radiation pattern measurements demonstrate that at 100 MHz, 200 MHz, and 300 MHz, the measured co-polarization patterns agree well with the simulated results, and the maximum radiation direction is consistently normal to the antenna plane, validating the effectiveness of the proposed antenna design. As shown in figure 5, on the radiating patch layer, the current mainly flows along the +x direction, thereby generating a radiating electric field along the +z direction. The current distribution on the AMC unit can be equivalent to a current loop oriented along the +z direction. At this frequency, the x-direction current and the parasitic current loop of the AMC jointly enhance the radiation gain of the antenna. This proves the mechanism by which the AMC structure improves antenna gain. However, when the operating frequency increases to 400 MHz, the electrical size of the antenna reaches approximately $ 1.6\lambda $, resulting in main lobe splitting and a shift of the maximum radiation direction toward 90°. Although high-frequency beam splitting introduces spatial clutter, it is a reasonable physical trade-off for achieving the 0.07 λL ultra-low profile, and the overall UWB characteristics still provide high time-domain resolution for detection radar systems. At 400 MHz, the measured H-plane co-polarization level is slightly higher than the simulated result, which may be attributed to the coupling effect between the feeding cable and the vertically mounted antenna. Conclusions This paper proposes a low-profile ultra-wideband planar dipole antenna. By introducing an AMC layer and resistive loading, the impedance matching performance is significantly improved while maintaining a compact antenna size. In addition, a reflective back cavity is incorporated to enhance backward reflection, thereby increasing the realized antenna gain. A prototype of the proposed antenna was fabricated and experimentally characterized, and the measured results show good agreement with the simulations. The antenna achieves a measured impedance bandwidth of 100 MHz–366 MHz under the condition of VSWR < 2, while maintaining a compact size of 0.38λL × 0.18λL × 0.07λL. The maximum measured gain within the operating band reaches 6 dBi. This co-design demonstrates a comprehensive advantage in balancing key performance indicators such as wide bandwidth, low profile and relatively high gain, providing a practical and valuable implementation approach for the engineering design of low-frequency band detection radar antennas. -
Key words:
- Artificial magnetic conductor /
- Low-profile /
- Ultrawideband /
- Gain enhancement
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表 1 天线结构参数值
参数 值 (cm) 参数 值 L 118 Sx1 0.23 W 60 Sx2 0.3 H1 40 SY 0.3 H2 165 m1 7 a 11.6 m2 7 d 12.2 n1 0.8 cx -3.3 n2 1.6 cy 2.8 n3 1.6 -
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