Finite-difference time-domain model for the feeding gap of coaxial probe driven antennas

In the finite-difference time-domain (FDTD) method, a simple and realistic feed model for coaxial probe driven antennas is proposed here. The feed zone of the antenna may be considered as an equivalent source in view of the antenna theory and a load port in view of the transmission line theory. The proposed feed model is constructed by combining the infinitesimal-gap source condition of the antenna and the equivalent load condition of the feed line. It leads to perform no additional FDTD cell modelling of the line. The transient reflected voltage and the input impedance of cylindrical monopole antennas fed by coaxial lines are calculated numerically and then compared with the accurate measurement and a full fine-grid. The FDTD results of the proposed model have a good agreement with the measured data and the fine-grid results.     

Characterization of a polarization-independent transmission trap detector

A six-element polarization-independent transmission trap detector with coaxial input and output beams has been constructed and full characterized. The measured optical parameters are compared with their values, predicted by Fresnel equations. Measured transmittances are in agreement with the predicted values within 2 x 10(-5) in the wavelength region from 450 to 650nm. The spectral responsivity of the transmission trap detector is in agreement with the predicted values within 0.035% at 543.5- and 633.0-nm vacuum wavelengths. The spatial uniformity of the responsivity is +/-0.03% across the active area of approximately 5 x 6 mm(2), measured with a laser beam of 1-mm diameter. The angular uniformity of the transmission trap detector is better than +/-0.01% for +/-3 degrees rotation around two perpendicular axes.     

同轴传输反射法测量高损耗材料微波介电常数

同轴传输反射方法可以用来测量高损耗材料的微波介电常数。该方法将环形样品嵌入同轴线内,通过测量样品两端的散射系数来确定材料的微波介电常数。文中介绍了测量原理及测量系统,测量了一种石蜡基混合样品的微波介电常数,并通过改进的Bruggeman数学模型,推算出其中陶瓷材料的介电常数。     

Dielectric Measurements in a Shielded Open Circuit Coaxial Line

A coaxial transmission line terminated by a shielded open circuit is convenient for dielectric measurements over a frequency range from quasi-static to microwave in a single sample holder. The transmission line analysis including the open circuit termination covered by lossy dielectric is outlined. Examples of measurements of known liquids are given. Errors may arise because the support bead of a coaxial connector may excite various-higher modes depending on the perfection of the symmetry and on the product frequency times refractive index of the sample. Criteria are given for the onset of higher modes.     

New Coaxial RF-DC Ammeter

A unique new type of RF ammeter is described. The ammeter combines broad frequency coverage with broad current range and is designed for use in a 50-ohm coaxial transmission line. The ammeter consists of an elliptic-cylinder silver reflector of infrared energy with a cylindrical thick-film heater along one focus and a heat-sensing thermopile along the other focus. Advantages over older ammeter designs include a shielded construction with inherent electrical isolation between the RF and dc circuits, a large dc output (approximately 1-100 mV) corresponding to a decade dynamic range (0.25-2.5 amperes), and a frequency range from dc to about 1 GHz. This coaxial ammeter is believed to be the first to have very low standing-wave ratio (SWR) (1.03) in a 50-ohm transmission line. Ammeters of lower current range can be built, but at a penalty of increased SWR due to the larger heater resistance required.     

Calculating the coupling factor in a multilayer coaxial transformer with air core

High-voltage transformers can be built with coaxial transmission lines. This paper describes a transformer design that uses the coaxial screen as primary winding and the inner conductor as secondary. An advantage of transmission line transformers is that the insulation problem is solved and the construction can be kept simple; the coupling between the primary and secondary coils is high even though the transformer uses an air core. The air core brings another advantage: the capacity to store large quantities of magnetic energy. The combination of a high coupling factor and large energy storage capacity makes this transformer ideal for charging high-voltage capacitors fast. The winding type for the transformer is alternating Archimedean spirals. Here, we present a magnetic field analysis of the transformer's primary, secondary, and mutual inductance using a finite-element solver. We compare the measured magnetic flux density versus the calculated value. The step-up winding ratio of the transformer influences the coupling factor marginally if the construction has an even number of spiral layers for each set of windings. However, the result from the finite-element solver predicts a drop in coupling factor if the step-up transformer construction has an odd number of spiral layers. The copper conductors used in the transformer resemble isotropic copper pipes.     

A Coaxial Calorimeter for Use as a Microwave Power Standard

A coaxial calorimeter has been built for use in the comparison of power at high frequencies with dc resistance and voltage standards. The temperature rise of the metal jacket of a 50-ohm coaxial termination is measured with a thermopile when a known level of dc power is fed into the calorimeter. The high-frequency sensitivity of the calorimeter is obtained by modifying the dc sensitivity for: 1) Input VSWR of the calorimeter, 2) Attenuation of a 6-inch length of thermally insulating input coaxial line, 3) The effect of current distribution in the load. The accuracy is estimated at better than ±1 per cent below 4 Gc/s with the largest errors due to the input VSWR of the calorimeter.     

Recent Advances in Broad-Band VHF and UHF Transmission Line Methods for Moisture Content and Dielectric Constant Measurement

Recent advances in VHF and UHF transmission line methods for determining either the complex permittivity or moisture content of bulky or film-like materials, or the thickness of film-like materials are discussed. These methods include TDR, CW phase, and |S21| transmission methods and employ a time-domain reflectometer (TDR), a vector voltmeter, and a network analyzer, respectively. Each technique involves monitoring the change of the transmission line parameters of a balanced open line, or of an unbalanced coaxial transmission line, when loaded with the material under test. Particular attention is given to the frequency range 10 to 1000 MHz and to broad-band methods. Experimental results are presented for several common materials, viz. concrete, asphalt, Plexiglas, moist paper, and moist soil.     

Permittivity of ice at radio frequencies: Part I. Coaxial transmission line cell

At radio-frequencies, measurements of the permittivity of ice are sparse and with unknown or large uncertainty. Coaxial transmission lines have been established for frequency-dependent permittivity determination for a broad variety of materials. Here we present a coaxial transmission line setup originally designed for soil samples, now adapted for measuring ice samples between 10 MHz and 1.5 GHz. Measured scattering parameters are assessed for artifacts against a forward calculation based on transmission line theory. A Debye-type relaxation function for the complex permittivity is assumed to obtain the permittivity of ice from the measured full set of four scattering parameters by means of a genetic optimization algorithm. The algorithm is successfully validated against quasi-analytical and iterative computation techniques with reference measurements of a low-loss Teflon standard. Based on the forward calculation and the Teflon standard, the total uncertainty for measuring the real part of the permittivity is estimated to be around 1%. Additional measurements of reference materials air, water, ethanol and methanol are used for validation. The real part of the permittivity of eight artificial pure ice samples is found frequency-independent between 10 MHz and 1.5 GHz at − 20 °C, with a mean value of 3.18 ± 0.01.     

利用改进的同轴线传输/反射法测量材料的射频电磁参数

介绍了传输/反射法测量射频介质参数(复介电常数(εr=ε′r-jε″r=ε′r-jσ/ω·ε0)和相对复数磁导率(μr=μ′r-jμ″r))的基本原理;对Nicolson算法中的厚度谐振、多值性和相角跳变等问题进行了深入分析,并提出了改进方案.利用网络分析仪的扫频技术解决了多值性的问题.文章利用矢量网络分析仪和同轴夹具组成的测试系统对聚氯乙烯、聚丙烯、聚甲醛、铁氧体及介电型吸波材料的电磁参数进行了测算,对空气间隙、试样长度和S参数等误差影响因素进行了分析.文章还利用两个系列的同轴夹具(Ф16mm/Ф7mm和Ф7mm/Ф3.05mm).对几种常用电介质材料进行了测算,对同轴传输线高次模对测试频率范围的影响规律进行了探索.本文为研究电工电子功能材料在高频应用场合下的储能特性和损耗特性奠定了基础.     

Metrological study for the optimal selection of the photoelastic model in transmission photoelasticity

In transmission photoelasticity, stresses and strains are not directly measured on the real piece, but on a photoelastic model. To improve accuracy, the photoelastic material type, size of the model, its thickness, and the applied load must be chosen properly. In this paper, the influence of selectable parameters in a photoelastic transmission analysis has been studied through the evaluation of measurement uncertainties. The experimental data and further study of a generic functional relationship, representative of a stress-separation technique, show that, for a given photoelastic material, the model of minimum uncertainty of measurement is the one whose ratio load/dimension is the maximum allowed by the data-acquisition technique used. The thickness affects only the amount of material used. Therefore, any size of the model can achieve maximum accuracy, provided that it is subjected to the greatest possible load within its elastic range.     

Two Methods for Measuring High Permittivity at Microwave Frequencies

Permittivity is measured by impedance measurement on thin cylindrical samples placed in an ordinary rectangular waveguide and in a cavity formed by an ordinary coaxial transmission line. The first method does not depend on resonance and is characterized by a rather high accuracy. The second, more conventional method, is used in conjunction with the first for study of dc bias dependence of the permittivity.     

Co-Ni@N-C/多孔碳复合材料的制备及电磁特性

以制备的多孔碳为基材,采用热还原法通过原位生长制备轻质化Co-Ni@N-C/多孔碳复合材料,研究了多孔碳的添加量对样品电磁波吸收性能的影响。采用X射线衍射仪(XRD)、扫描电镜(SEM)、X射线光电子能谱(XPS)、拉曼光谱(Raman)对样品的相结构、组分和微观形貌进行表征。通过矢量网络分析仪对样品在2～18 GHz频段内的电磁参数进行测试,并根据传输线理论分析了样品的电磁波吸收性能。结果表明：以多孔碳为基材制备的Co-Ni@N-C/多孔碳样品,当多孔碳含量占原材料总质量的2.7%时,样品具有良好的电磁波吸收性能,最大反射损耗值为-57.74 dB(15.12 GHz, 2.35 mm),最大有效吸收带宽为5.14 GHz(12.86～18 GHz, 2.25 mm),基本覆盖整个Ku波段。     

Investigation of a power divider using a coaxial probe array in a coaxial waveguide

A novel coaxial-waveguide power divider using coaxial probe array that achieved low-loss probe-to-waveguide transitions is presented. The small reflection theory of transverse electomagnetic (TEM) lines is extended to synthesise the gradual waveguide taper, and the equivalent-circuit approach is applied to analyse the coaxial probe array. The design and simulation of a single probe-to-waveguide transition have been developed. The detailed design, simulation, fabrication and measurements of a four-way coaxial power divider are discussed. The measured results agree well with the simulated results. The measured 15 dB return loss bandwidth of this waveguide power divider is demonstrated to be 8.2 GHz (fL = 7.8 GHz, fH = 16 GHz) and its 0.5 dB insertion loss bandwidth 11 GHz.     

Permittivity and permeability measurements using striplineresonator cavities-a comparison

The permittivity and permeability of five materials were measured during a comparison of the stripline resonator cavity technique. The National Institute of Standards and Technology (NIST) organized this intercomparison in which a total of seven organizations participated. Each participant measured two dielectric materials and three magnetic materials. Results for this comparison suggest that when the stripline resonator is used, dielectric property measurements are not as accurate as magnetic property measurements, provided that a correction for demagnetization is made. The results are compared to 7 mm coaxial transmission line measurements which have an uncertainty of less than 10% for the relative permittivity, ϵ_r'<15     

Numerical Solution of the Dielectric Equation for a Coaxial Line

Dielectric measurements using a coaxial transmission line terminated by a shielded open circuit give rise to the transcendental equation z tan z = c, where c = a + jb is a measured quantity. Since a ? 0, Newton's method will rapidly converge to one of the roots unless it gets trapped in an attractive cycle. Regions containing the lowest and second-lowest roots are sketched. Moreover, starting values are given in terms of the quantity c so that the method will converge to the desired root among the first four roots and also avoid attractive cycles. Using high-precision arithmetic, the error in z may be estimated numerically in terms of the error in c.     

A Four-Wire Line as a Standard for Electric and Magnetic Field Strengths

It is shown that an unmatched four-wire transmission line can be used as a combined standard for electric and magnetic field strengths as well as for the energy density and energy flux density in that field. The control parameters can be the electric field strength at the maximum, the short-circuit current in the line, and the power absorbed in the load. The main advantages are the simple formulas relating the control parameters to the quantities reproduced and the ease of building a device to reproduce them.     

Broadband millimetre-wave passive spatial combiner based on coaxial waveguide

A broadband millimetre-wave passive spatial combiner using a microstrip probe array and an oversized coaxial waveguide is successfully designed and tested. The equivalent circuit approach of transverse electromagnetic (TEM) lines is adopted to synthesise the coaxial stepped impedance transformer from a K connector with the oversized coaxial waveguide and to analyse the microstrip probe array. The equivalent susceptances of the step discontinuities for the power divider are calculated. A four-way coaxial waveguide passive power combining circuit operating at the entire Ka-band is designed, fabricated and measured. Experiments on the four-way passive combiner show that a minimum insertion loss of 0.7 dB has been achieved at about 29.5 GHz. The combiner has shown a bandwidth of 26.5?40 GHz with 10 dB return loss and less than 2.5 dB insertion loss.     

Shaping Fast Rise-Time Pulses with Tapered Transmission Lines

For the purpose of checking very fast time-domain reflectometer (TDR) systems it is desirable to have a fast rise-time pulse whose parameters are known. A means for obtaining pulses meeting these requirements has been achieved by suitably tapering the inner conductor of a rigid coaxial transmission line having air dielectric, commonly referred to as an air line. The air line is driven with a pulse that has a very fast rise time compared to the rise time of the expected output pulse. A reflected pulse will be generated whose shape is dependent upon the taper of the inner conductor. A procedure is outlined for determining the proper taper to shape the leading edge of the reflected pulse to match any continuous single-valued function. Air lines that were designed and built for producing a pulse whose leading edge is a linear ramp, a truncated Gaussian function, and the integral of a truncated Gaussian function are discussed and traces of these pulses are shown. The problem of re-reflections due to the multiple discontinuities along the air line was analyzed by means of a computer. This program is described. The effect of the driving-pulse parameters on the reflected pulse is analyzed on a theoretical basis. Rise time, pulse shape, and aberrations such as overshoot and ringing of the driving pulse are considered in this analysis.     

A New High-Frequency Current Standard

A short-circuited-ring electrodynamic ammeter is described. The short-circuited ring is supported by a fine quartz fiber and is suspended midway between the inner and outer conductors of a coaxial transmission line. The current on the line is found by measuring the torque exerted against the ring by the current on the coaxial line. A new technique is given for determining the relationship between the torque on the ring and the current on the transmission line, and is compared with the technique used in the past. The new method is an application of the resonator action theorem used by Cullen for calibrating a torque-operated microwave wattmeter. The major weakness in the previous evaluations of the torque-current relation is discussed. The ammeter is useful for measuring current from 1 to 100 amperes over the frequency range 1 MHz to 1 GHz, with an uncertainty of the order of 0.5 percent.