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By controlling the time to apply magnetic field to a Fe3O4 magnetic fluid with polymer acrylic resin and by evaporating the solvent, linear chain-like structures were obtained and fixed. Optical microscopy and transmission electronic microscopy (TEM) were used to observe the assembly structures of the fixed magnetic nanoparticles. We found that the chainlike structures with size even down to hundreds nanometers had smaller substructures. A picture of aggregation between linear chain-like substructures is proposed to understand the formation of the macrostructure in magnetic fluid induced by a magnetic field. Two basic aggregations, the coarsening and the connection of the substructures, were observed.

A wideband circularly polarized (CP) cross-dipole antenna with parasitic elements is proposed. The proposed antenna consists of a pair of driven crossed strip dipoles, two kinds of parasitic elements (inner and outer parasitic elements), and a pair of vacant-quarter printed rings. The driven crossed strip dipoles can generate a CP mode due to the sequential phases given by the vacant-quarter printed rings. Another CP mode can also be stimulated by placing these parasitic elements to the edge of crossed strip dipoles. By combining with two CP modes of the cross-dipoles, the proposed antenna achieves a wideband CP performance. The measured results indicate that the proposed antenna exhibits wideband CP characteristics with an impedance bandwidth (IBW) of 77.6% (1.11 -2.52 GHz) and axial-ratio bandwidth (ARBW) of 66% (1.25 -2.48 GHz). The proposed antenna would be suitable for CP applications in wireless communication systems, such as the global positioning system (GPS; 1.575 GHz) and wireless broadband access service (WiBro; 2.3 -2.39 GHz).INDEX TERMS Broadband, circularly polarized (CP), crossed dipoles, parasitic loading, sequentially rotated configuration.

This paper develops an active magnetic near field probe (H-field probe) by using a fourlayer printed circuit board (PCB) technique. Two-turn detection structure and a low noise amplifier are used to improve probe's frequency response. The two-turn detection structure can maximize the use of PCB stack resources, and a 14.5 dB gain amplifier can increase signal output capability. The probe can then be used for electromagnetic compatibility (EMC) test from 150 kHz to 3 GHz. Compared with traditional active shielded-loop H-field probe (TAHP) with loop area of 500 𝜇m 2 , the frequency response of the proposed probe can improve 20 dB at 0.5 GHz. Different from the method of comparing the sensitivity of the probe only through the frequency response, this paper measures and analyzes the real sensitivity of the probing device composed of the proposed probe. Under the condition of the same receiver parameter setting, the sensitivity at 0.5 GHz is increased by 11.7 dB compared with commercial passive probing device and 20.2 dB at 0.5 GHz compared with the probing device with TAHP device. It reaches −32.4 dB µA. In addition, the proposed probe has acceptable spatial resolution of 1.28 mm at 1 GHz, which is more suitable for printed circuit board level EMI analysis.