Scattering performance of a plane wave by conducting finite helix

The scattering of a plane wave with a linear polarization by a conducting finite helix is presented. The analytical expresses of scattering field are obtained based on the boundary condition of a sheath helix model. Effects of a pitch angle and electrical size of the helix on the resonant behavior are investigated. The analysis shows that resonant behavior of the forward/backward scattering far field is only depended on the pitch electrical size and the radial electrical size, but it is independent of the turn of the helix.     

Extremely low-profile helix radiating a circularly polarized wave

It is shown that the combination of low pitch and a small number of turns realizes a low-profile helix as a radiating element of circular polarization. A two-turn helix of 4° pitch angle shows a bandwidth of 12% for a 3 dB axial-ratio criterion. The numerical results presented lead to new aspects of a low-pitch helix as an effective radiating element     

A study on helix pitch tolerance impact on TWT small-signal gain

Traveling-wave tubes (TWTs) consist of several sections with different helix pitch to provide the required small-signal gain. An extensive study aiming to improve the comprehension of the effect of the helix pitch manufacturing tolerances on the TWT small-signal gain has been completed. The obtained results demonstrate the relevant impact on the TWT small-signal gain when the helix pitch deviates from its nominal value and allow devising a tuning procedure for the realized tubes.     

Influence of helix parameters on the reflectance of microwave bychiral composites

We prepare planar chiral composite samples containing a random distribution of helixes of different parameters. The influence of the helix parameters on the reflectance of microwave energy by chiral composites is studied. The helix parameters include the helix diameter, the pitch, and the gauge diameter. The reflectance of the microwave by metal backed chiral samples is calculated from the complex permittivity, complex permeability, and complex chirality parameter measured with ordinary microwave circular waveguide measurement system in the frequency range of 8.5-11.5 GHz. The results show that the helix diameter, the pitch, and the gauge diameter have remarkable influence on the reflectance of microwave energy by chiral composites. There exist optimal helix parameters in order to obtain the lowest reflectance     

Optimization of the quadrifilar helix antenna

Experimental data on the quadrifilar helix antenna have led to an optimum pitch angle and an optimum ground plane size. The ground plane size is an important parameter, and its optimum diameter is about three times the diameter of the helix. The optimum pitch angle is about35deg. Beam patterns and ellipticity data are given for the optimized model. Bandwidth is about 4:1.     

钮销支撑螺旋线慢波结构色散特性及二次谐波抑制研究

寻求频带更宽、输出功率更大、体积更小的螺旋线行波管一直是研制工作的目标,二次谐波是影响宽带行波管工作的重要因素,本文对不同形式介质加载的螺旋线慢波结构进行了模拟,并对其中色散曲线最为平坦的钮销慢波结构进行了一维大信号模拟,证明改变螺距或夹持介质形状可有效展宽带宽、降低二次谐波功率。     

Optimization of helical coil applicators for hyperthermia

Numerical solutions were used to optimize helical coil applicators to maximize the ratio of axial heating to surface heating within coaxial muscle cylinders. It is shown that this optimization requires significantly larger values of pitch angle and frequency than any previously specified for experimental applicators. The maximum ratio of axial to surface heating is obtained with a linear helix defined to have a pitch angle of 90°. Calculated ratios of axial/surface heating with a linear helix are as large as 2.54 (at 352 MHz) for a muscle cylinder with a radius of 4 cm, and 1.26 (at 82 MHz) for one with a radius of 8 cm. The relationship of the linear helix applicator to the annual phased-array applicator and to the resonant cylindrical cavity applicator is discussed     

Variable pitch reconstruction using John's equation

We present an algorithm to reconstruct helical cone beam computed tomography (CT) data acquired at variable pitch. The algorithm extracts a halfscan segment of projections using an extended version of the advanced single slice rebinning (ASSR) algorithm. ASSR rebins constant pitch cone beam data to fan beam projections that approximately lie on a plane that is tilted to optimally fit the source helix. For variable pitch, the error between the tilted plane chosen by ASSR and the source helix increases, resulting in increased image artifacts. To reduce the artifacts, we choose a reconstruction plane, which is tilted and shifted relative to the source trajectory. We then correct rebinned fan beam data using John's equation to virtually move the source into the tilted and shifted reconstruction plane. Results obtained from simulated phantom images and scanner images demonstrate the applicability of the proposed algorithm.      

螺距对螺旋慢波系统色散和耦合阻抗的影响

螺距是慢波系统的一个重要参数,直接影响螺旋慢波系统的色散特性和耦合阻抗.严格求解螺距对慢波系统色散和耦合阻抗的影响是不方便的,而通过计算机模拟可以方便快速地研究螺距对慢波系统色散和耦合阻抗的影响.模拟结果表明,对于工程项目当螺距变化小于4%时,螺距对色散和耦合阻抗的影响是很小的.     

A study of the sheath helix with a conducting core and its application to the helical antenna

A theoretical study of the sheath helix antenna with a conducting core is made via the complex solutions of an associated determinantal equation. The variation of the characteristics of operation with respect to core size and pitch angle is established. A comparison of the complex solutions for the sheath helix and tape helix indicates that a good understanding of the helical antenna can be gained from the sheath model. In particular, explaining broadside radiation in terms of complex solutions is shown to give results in agreement with previously reported empirical data. However, this comparison also points out the limitations of the sheath model in predicting the operational parameters accurately.     

Effect of helix turn angle on the performance of a half wavelength quadrifilar antenna

The impedance and radiation pattern parameters of a /spl lambda//2 quadrifilar helix antenna (QHA) with turn angles in the range 0/spl deg/ to 235/spl deg/ are analyzed. It is shown that by selecting the helix turn angle to satisfy the minimum bandwidth and beamwidth requirements, an improved electrical performance and a reduction in the physical size of the antenna is obtained. This is demonstrated by comparing the performance of a conventional half turn QHA with structures having a smaller pitch length. The computed results are validated by experimental data at L-band.     

A method for reducing the second harmonic level in wideband traveling wave tubes

To reduce the second harmonic level and enhance the efficiency and stability against backward-wave self-excitation of wideband helical traveling-wave tubes containing a screen with longitudinally conducting edges, it is suggested that the gap between edge ends and a helix in the output section be gradually increased toward the energy output, the helix pitch being constant or slightly varied for optimizing the efficiency. It is shown that the increase in the gap reduces the second harmonic level and enhances the efficiency and the start current of backward-wave self-excitation.     

Computation of the Impedance of an Infinitely Long Helical Transmission Line by Numerical Methods (Short Papers)

A numerical method is given for the determination of the impedance of an infinitely long thin-wire helix. The propagation constant of the current for zero tangential electric field is found and used in a variational expression for impedance. Asymptotic values of resistance versus pitch are compared with resistances of infinitely long straight-wire antennas.     

A heuristic analysis for an inhomogeneously loaded tape helix used in a practical travelling wave tube

The analysis of an inhomogeneously loaded helix in the tape model, reported elsewhere, considering the effect of space harmonics as well as the different radial propagation constants over the different structure regions, is quite tedious and rigorous. So, to avoid the rigorous analysis here, a heuristic analysis has been developed by combining the results of the tape model analysis of a helix in free space with those of the sheath helix model analysis of a loaded helix. The results presented here in terms of dispersion characteristics have been compared with those results obtained from a rigorous approach (both for homogeneously and inhomogeneously loaded structures) in the tape helix model. However, the results obtained from this approach agree closely with those of the rigorous approach for a practical situation in which the discrete supports are smoothed out into a single continuous region. The approach is valid for an inhomogeneously loaded helix for optimum conditions of structure parameters, namely, helix pitch angle, helix to barrel gap, support material and inhomogeneity. Moreover, the results predicted by this heuristic approach have also been validated with those experimental results published in the literature and are found to be in good agreement.     

Leakage from balanced feeders

The findings of Martin on field reduction due to twisting of feeders are compared with theoretical and experimental results obtained by Baruah for fields at distances much greater than the pitch of the helix.     

Helix Leaky Waveguide

The propagation characteristics of leaky waves in a helix waveguide covered with a slitted cylinder are presented by a method of transverse network representation. The main interest is in helix waveguides with small pitch angles, characterized by a hybrid mode consisting of TE/sub 01/, and a small amount of TM/sub 01/ modes. The leaky wave discussed in this paper may then be regarded as a perturbation of the TM/sub 01/ wave by the slitted cylinder outside the helix. The radiation, metal, and dielectric losses are calculated numerically at a frequency of 50 GHz. The relation between the radiation loss and aperture angle of slit is very different from that of an ordinary leaky waveguide composed of a slitted cylinder without helix, especially when the distance between the helix and shield cylinder is about a quarter of the radial wavelength. The metal and dielectric losses are the same order as radiation loss, however the dielectric loss decreases as the power factor /spl epsiv/"/ /spl epsiv/' increases. The measured total attenuation constant averages about 5 dB/km, almost twice the theoretical value.     

一种具有新型分立介质支撑的翼片加载螺旋带慢 波结构的研究

本文研究了一种具有新型分立介质支撑的翼片加载螺旋带慢波系统,该种慢波系统具有较高的功率容量和较宽的带宽.通过用切比雪夫多项式来展开螺旋带上的面电流,用真空层来模拟螺旋带的厚度,用均匀分层介质来等效新型分立介质支撑,考虑到过渡连接金属块的影响,用场论的方法得到了非常实用的色散方程和耦合阻抗的表达式,同时进行了HFSS模拟,发现用场论的方法所得出的结果与用HFSS模拟的结果吻合良好.本文的结果对这种新型慢波结构的设计具有指导意义.     

提高宽带毫米波行波管效率研究

针对影响宽带毫米波行波管效率的主要因素,对提高行波管效率的相关实用化技术进行了研究。通过螺旋线线路渐变技术、降低高频损耗、多级降压收集极三个方面的研究,使现有某一宽带毫米波行波管总效率由原来的10%提高到20%以上。     

Low-profile helical array antenna fed from a radial waveguide

A low-profile array antenna composed of two-turn 4° pitch angle helices is designed for a frequency band of 11.7 GHz to 12.0 GHz. The feed wire of each helix is inserted into a radial waveguide through a small hole and excited by a traveling wave flowing in the transverse electromagnetic mode between the two parallel plates of the waveguide. The measured aperture efficiency shows a maximum value of 77% for a beam radiated in the normal direction and 69% for a 30° beam tilt     

Alcohol‐Responsive,Hydrogen‐Bonded,Cholesteric Liquid‐Crystal Networks

Hydrogen‐bridged, cholesteric liquid‐crystal (CLC) polymer networks are adopted as an optical sensor material to distinguish between ethanol and methanol. Fast uptake of the alcohols is facilitated by an incorporated porosity created by breaking the hydrogen bridges and by a previously removed non‐reactive liquid‐crystal agent. The discrimination between the alcohols is based on the diversity in molecular affinity of ethanol and methanol with the hydrogen‐bridged CLC polymer networks. The CLC networks are molecular‐helix‐based, one‐dimensional bandgap materials with a discrete reflection band in the visible part of the spectrum that depends on the pitch of the molecular helix. The changes in positions of the reflection bands of the CLC network accurately discriminate between the alcohol types and provide information on their ratio in case they are blended.