The quadrifilar helix antenna (QHA) is a circular polarized antenna consisting of four helical arms excited by quadrature phase. The radiation pattern, when the whole helix is like a short rod with a proper axial length, the number of turns is one-half, and the end of arms connects to each other for generating a one-wavelength resonance between two arms, is omni-directional with a cardioid shape. We propose a miniature dielectric-loaded QHA which is one of the suitable ways for miniaturized antenna without changing the geometry. The proportion of scale-down size depends on the permittivity of dielectric. The higher permittivity implies the smaller antenna. Hence, the size of the proposed antenna is shrunk with a ceramic load which is a material mostly employed in antenna design by reason of low loss and high permittivity. The configuration then is very compact and can easily be integrated into a handset.
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Fig. 2-1 Antenna structure (a) Overall structure including the matching capacitor
(b) The bottom view of the antenna rotated for self-phasing
The geometry of the proposed antenna is depicted in Fig. 2-1. The antenna dimensions are presented for operating frequency around 1.575GHz. The overall structure contains a dielectric-loaded antenna with printed metal strip and a small planar circuit board with a simple feeding and matching network. The dielectric entity is a ceramic rod with a hole through the center and a relative permittivity about 40. Significantly, the planar board has a protrusion used to fix together with the rod. The metal strip has four arms printed on the side wall and a metal ring on the top of side wall whereas the extension of arms on the bottom connects to the feed points of the antenna fed by a pair of lines. The geometry of these arms is designed for right-hand circular polarization (RHCP) radiation towards left-hand helix. The top of the rod is not printed in order to reduce the time for manufacture procedures but the performance keeps similar to the traditional one.
Due to the high-permittivity dielectric material, the antenna is fabricated to a very small size with 4.5 mm in radius and 14.8 mm in height which is less than 0.08λ.
The difference between the electrical lengths of two BHAs for circular polarization is achieved by making a little modification on geometry or offset of feed points. Fig. 2-1 (b) shows the rotated feed point locations involving two BHAs with different path lengths. While properly adjusting these two parameters, the circular polarization is generated. Moreover, RHCP and LHCP are determined by the relationship between L1 and L2 with fixed placement in two BHAs.
The radiation resistor of the proposed 1.575GHz QHA is less than 2Ω in such small size of the antenna. The loss tangent of ceramic rod is so small that the dielectric loss is ignored. It is needed to be very careful to match a small antenna for less loss in a matching circuit. There is an unavoidable short feeding line for the antenna with parasitic inductor in this structure. By using only one SMD (Surface Mount Device) capacitor and properly adjusting the parasitic inductor, the antenna can be matched. With a suitable value of capacitor, the antenna is acceptably efficient. The matching circuit on the PCB is shown in Fig 2-1(a). The placement and value of the capacitor associated with the geometry of feeding line can be selected to match the wanted impedance. The simple matching circuit is done in small size and almost not occupies any space on the PCB. Fig. 2-2 shows the simulated input impedance of QHA including a very short feeding line that is considered as a parasitic inductor. Besides, the impedance of matched QHA simulated by the model of SMD capacitor is also depicted. The capacitor used is Murata GRM1885C1H5R6DZ01 with the value of 5.6 pF. The resistor of input impedance is about 1.3Ω. Fig. 2-3, shows the simulated radiation
pattern having the properties of cardioid shape and omni-direction at the frequency 1.575GHz in the
(i) Dashed line: the QHA with the parasitic inductor (ii) Solid line: the QHA matched by adding SMD capacitor
0 1.01.0-1.0 10.0
(i) Dashed line: the QHA with the parasitic inductor (ii) Solid line: the QHA matched by adding SMD capacitor
3. Application of 2-D Non-Uniform Fast Fourier Transform Algorithm
Application of two-dimensional nonuniform fast fourier transform (2-D NUFFT) technique to analysis of shielded microstrip circuits.
Fig. 3-1 2-D NUFFT algorithm: exponential function at a nonuniform sample point (xt , ys ) is approximated by Fourier bases at (q +1)×(q +1) uniform oversampled grids (Xi ; Yj ) in a square neighborhood or by those in an octagonal neighborhood.
Fig. 3-2 Hairpin resonator in shielded box and its mesh scheme in analysis. Structure parameters are εr = 10.2, L1 = 0.7, L2 = 1.01, L3 = 2.74, L4 = 8, L5 = 6, w1 = 1, w2 = 1.19, g1 = 0.2, and g2 = 0.8. All dimensions are in millimeters.
Fig. 3-3 Measured and calculated S-parameters of the hairpin resonator.
4. Simulation and Modeling of Nano-scale High Frequency Devices and Components 4-1 Schrödinger and Poisson’s solver for nanodevice applications:
Numerical solution of the Schrödinger and Poisson equations (SPEs) plays an important role in semiconductor simulation. We present a robust iterative method to compute the self-consistent solution of the SPEs in nanoscale metal-oxide-semiconductor (MOS) structures. Based on the global convergence of the monotone iterative (MI) method in solving the quantum corrected nonlinear Poisson equation (PE), this iterative method is successfully implemented and tested on the
single-, double-, and surrounding-gate (SG, DG, and AG) MOS structures. Compared with other approaches, shown in Fig. 4, various numerical simulations are demonstrated to show the accuracy and efficiency of the method. (Computer Physics Communications. Vol. 169, No. 1-3, July 2005, pp. 309-312.)
Fig 4. The plot (a) of the left figure is the computed electron density and (b) is the potential of the three MOS structures. The inset figures of the left figure are the cross-sectional views of the MOS structures. The right one is the maximum norm errors of the computed potential versus the number of iteration for the three algorithms applied to SG, DG, and AG MOS structures, respectively.