A Theoretical Feasibility Study of a Source Stirring Reverberation Chamber

A novel theoretical feasibility study to reverberate a large metallic chamber is proposed. The method is based on the displacement of a radiating antenna inside the chamber, and the reverberating effect is obtained by coupling the antenna current in different ways with the cavity modes. This technique eliminates mechanical rotating paddles inside the chamber and offers the advantage of a continuous-wave operation compared with the frequency stirring method. The reverberating characteristics of the proposed technique are evaluated by calculating the field statistics. The analyzed situation concerns a real antenna, and its radiation into the reverberation chamber is modeled using Green's function of cavity. The numerical problem is solved by the method of moment. The model has been tested and is able to rigorously characterize the reverberating environment for the design and optimization of antenna configurations to be addressed in future studies.      

Electrical Mode Stirring in Reverberating Chambers by Reactively Loaded Antennas

Electrical mode stirring in reverberating chambers for electromagnetic compatibility is accomplished by antennas that are loaded by variable reactances. The principle is derived from a network model of the chamber that employs measurable coupling factors. The concept is experimentally confirmed in the gigahertz-range for a 1.92 m x 1.84 m x 2 .91 m chamber with four conical antennas placed on one sidewall. One antenna is connected to a generator; the other three antennas are reactively loaded by variable short circuit plungers. The resonances are considerably shifted by varying the reactive loads.     

Numerical Study of Position Stirring and Frequency Stirring in a Loaded Reverberation Chamber

A numerical model of a loaded reverberation chamber is used to study the effectiveness of 3-D position stirring and frequency stirring. The numerical model is based on thin wires, the moment method, and a cavity Green's function. The average power transfer level between two dipole antennas is compared to the average power transfer level between a dipole antenna and a loop antenna. The two transmission levels should ideally be the same, as both the dipole and the loop are treated as being lossless and impedance matched. The standard deviation of the level differences is a measure of the accuracy of the chamber, and this is used to estimate the stirring effectiveness. It is shown that frequency stirring must be done over a larger bandwidth than the average mode bandwidth to be effective. The 3-D position stirring is also shown to be much more efficient than expected.      

Measurement of electrically large array antennas in small anechoic chambers [Measurements Corner]

A post-processing procedure that yields accurate array-antenna radiation-pattern estimations is described. By resorting to this strategy, measurements of large array antennas inside small anechoic chambers become feasible. The method employs an adequate phase-correction derived from an optical ray approach, under the assumption that the elementary radiators themselves are electrically small and can be measured under far-field conditions inside the relevant anechoic chamber. The effectiveness of the proposed technique is demonstrated by examining the case of a nonuniform linear array antenna.     

A microwave anechoic chamber for radar-cross section measurement

A microwave anechoic chamber has been developed at the Department of Electrical Engineering, University of Malaya, for monostatic and bistatic radar-cross-section measurements. The structure of the chamber is a quarter-section geodesic dome, with a 12 foot radius, and raised three feet above the floor. An antenna railing system is installed inside the chamber. The antennas can be moved along the rails in the elevation direction, with the microwave beam pointing at the center of the dome where the target is located. This design enables a very large combination of incidence and scattering angles in bistatic measurements. Four transmitting antennas are fixed at different elevation angles next to one of the antenna rails. By using an azimuth-over-elevation positioner as the pedestal for the target, and by positioning the movable antenna along that rail beside those fixed transmitting antennas, monostatic measurements with incidence angles ranging from 0 to 90 degrees can be accomplished. A vector network analyzer is utilized to measure the amplitude and phase of the radar returns. An IEEE-488.2 interface bus is used to control various hardware components, as well as to perform data acquisition. A computer program was written to automate the measurement system. Data are stored in raw format, and processed later with dedicated software, so that different processing methods and parameters can be applied. This paper highlights the design and construction of the microwave anechoic chamber, as well as the measurement-system configuration for scattering-cross-section measurements     

Study of antennas in reverberation chamber using method of moments with cavity Green's function calculated by Ewald summation

A numerical model of wires in rectangular metal cavities is introduced for computing antennas in a simplified theoretical reverberation chamber. The code is based on the method of moments, and it uses the Ewald summation for efficient calculation of the cavity Green's function. The Q-value of the chamber is accounted for in the model by a homogeneous material filling the chamber. The S-parameters of two dipoles placed in many random positions in the cavity are computed and averaged to provide the average transfer function of the chamber. This represents mode stirring by moving the antennas, referred to as position stirring. The computed results are compared with theoretical statistical values as well as experimental results. The discrepancies are discussed.     

双发射天线对混响室场性能影响的数值分析

提出了双天线激励混响室的方法,研究了双天线激励对混响室场性能的影响。分析了双天线改善混响室场均匀性的原理,对单天线激励下的混响室模型以及处于不同位置的双天线激励下的混响室模型进行了仿真计算,从场均匀性和场强两个方面研究了双天线对混响室场性能的影响,对双天线影响场强的机理进行了分析。研究表明:采用双天线激励改善了混响室工作区域的场均匀性;采用双天线可以降低对功率放大器的要求,降低大型混响室测试成本。最后对双发射天线在大型混响室中的应用进行了探讨和展望。     

Experimental study on direction dependency of reflectioncoefficients of microwave electromagnetic anechoic chamber

For measurements of reflection coefficients in an anechoic chamber by the free-space voltage standing wave ratio (VSWR) method, the composite antenna technique, using a combination of two antennas, is proposed. Reliable data on reflection coefficients can be obtained by this technique, with respect to reflected waves from arbitrary directions. Using the measurement results, the direction dependency of reflection coefficients is studied in relation to specular reflection and to the directivity of a transmitting antenna     

An antenna pattern measurement technique for eliminating the fieldsscattered from the edges of a finite ground plane

A novel antenna pattern measurement technique is presented to eliminate the effects of the finite ground plane and anechoic chamber wall reflections which significantly perturb antenna pattern measurements. This technique consists of the measurement of the edge-diffracted fields and their subsequent subtraction from the original pattern. A simple theoretical model is developed to introduce the subtraction technique, and comparisons are made which show the excellent agreement between theoretical (assuming an infinite ground plane) and `corrected' experimental antenna patterns. Experimental results are given for open-ended waveguide, microstrip patch, and monopole antennas mounted on circular and square ground planes of various sizes. Time gating techniques are illustrated as well     

Efficiency measurement of UWB antennas using time reversal in reverberation chambers

Using the reverberation chamber to obtain antenna radiation efficiency is considered. The reverberation chamber, which has for many years been used for electromagnetic compatibility measurements, can also be used with great advantage for antenna measurements since it simulates effectively a uniform multipath propagation environment. How a reverberation chamber can be used to measure the ultra-wideband (UWB) antenna efficiency in a very short time is described. The procedure based on time domain is described and some experimental results are given and compared to measurements obtained by the Wheeler cap method extended by Schantz to UWB antennas.     

Compact antenna test range without reflector edge treatment and RF anechoic chamber

There are several types of CATRs (compact antenna test ranges) used in antenna-pattern measurements. An offset reflector is generally used to generate the quiet zone of a CATR. Serrated edges, rolled edges, or R-cards are generally chosen along the reflector's edge to reduce the edge-diffraction field inside the quiet zone of the CATR. In order to reduce stray signals from the environment, a high-quality RF anechoic chamber is required for a CATR. In this paper, a new type of CATR, without either a reflector edge treatment or an RF anechoic chamber, is developed. A commercially available DBS (direct-broadcast satellite) reflector antenna, without edge treatment, is used as the reflector antenna of the CATR to generate the quiet zone of the antenna test range. In order to improve the quiet zone's performance, the fields due to feed spillover, edge diffractions, and other stray signals are gated out by the ITDAMS (impulse time-domain antenna measurement system). The RF interference in the environment can also be reduced by time synchronization and pulse integration of the impulse time-domain antenna measurement system. In order to verify the capabilities of the proposed CATR, three kinds of antennas (a low-directivity horn antenna, a high-directivity 60 cm direct-broadcast satellite reflector antenna, and a 25 cm Ka-band Cassegrain LMDS - local microwave distribution system - antenna) were measured by the proposed CATR. The antenna-pattern results agreed quite well with those of a near-field range and a far-field range.     

The sinuous antenna-A dual polarized feed for reflector-based searching systems

The sinuous antennas have a numerous applications in military and civil systems such as direction finding systems and reflector feeds due to their superior broadband characteristics and simultaneous polarization capability. In this paper, design, construction and measurements of planar sinuous antennas are investigated for 1–5 GHz frequency range. Feeding sections of this antenna are realized by using microstrip tapered baluns. We have determined the microstrip tapered balun dimensions using Ansoft HFSS-simulation program. The performances of the manufactured antenna are measured using HP vector network analyzer. Polarization patterns and return loss characteristics of the designed antenna that have been measured in anechoic chamber, are presented.     

A novel mode content analysis technique for antennas in multimode waveguides

This paper presents a novel technique for analyzing the mode content excited by antennas placed in multimode waveguides. The technique is based on measuring the frequency response between the two antennas coupled into a waveguide and using that information to extract the mode content generated by the transmitting antenna. The technique is applicable to cases in which the mode amplitudes are approximately constant over the frequency range of interest. This method is valuable for determining the mode mix generated by arbitrary transmitting antennas in a multimode waveguide propagation environment. An example of such an environment is heating, ventilation, and air-conditioning (HVAC) ducts used for indoor communications, where an important antenna characteristic is the mode sensitivity (analogous to the antenna directive gain in free space). We validate our technique with the example of a monopole probe antenna coupled into a multimode cylindrical HVAC duct.     

一种新型微带分形贴片天线的设计

采用两点格式法构造了一种新型的分形结构,并利用其设计了微带分形贴片天线,采用AnsoftHFSS软件对天线进行仿真优化,然后制作了实物并完成实验测量。仿真结果和测量数据表明:一阶和二阶微带分形贴片天线的面积尺寸较传统微带贴片天线分别能够缩减46.85%和60.01%,这与传统的Koch微带分形贴片天线和Minkowski微带分形贴片天线比较,具有良好的尺寸缩减性。因此,该结构在天线小型化领域具有研究价值。     

新型圆环嵌套多频印刷天线

提出了一种具有多频特点的新型圆环嵌套印刷天线,该天线由不同直径的圆环嵌套得到。通过电磁仿真软件CST Microwave Studip进行仿真研究,结果表明嵌套圆环的外径、宽度和嵌套数目可以对其通频带中心频率、频带宽度和通频带数目进行控制。设计出了具有2．4GHz,3．5GHz,5．2GHz,5．8GHz四个通频带的共面波导馈电的单极印刷天线和以平衡微带线馈电的对称振子天线。制作了天线实物并在微波暗室中进行测试,测试结果与仿真吻合较好,该天线具有近似独立的频率可控性和较小的尺寸,可用于WLAN和WiMAX等领域。     

Effects of External PIM Sources on Antenna PIM Measurements

Antenna Passive Intermodulation (PIM) level measurement results are rarely credited to be due to external signal receiving characteristics of the antennas or serious effects from external PIM sources, such as the anechoic chamber absorber and antenna tower. This paper presents an antenna PIM model for a reflected PIM measurement method. Based on the findings of null point generation and the behavior of the third order PIM values obtained by theoretical predictions and experimental results, we concluded that the results of the antenna PIM level test were influenced by the external PIM sources generated by the anechoic chamber absorber and the path differences of PIM signals coming into the antenna.     

Dielectric-loaded horn antenna

A wide-band dual-mode horn, which has a rotationally symmetric beam and extremely low sidelobe levels, can be obtained by loading a dielectric band inside the horn antenna. Measured radiation characteristics of such antennas, including the so-called shaped-beam antenna, are shown.     

The measurement of solar array discharges in a simulated spaceenvironment

A spacecraft in a plasma builds up charge on all the dielectric surfaces and interfaces. Once the net charge exceeds the dielectric breakdown of the material, a discharge occurs. One of the more susceptible pieces of equipment is the antenna/receiver system. The radiated E-field may be strong enough to create an ambiguous signal which may be misinterpreted by the system electronics and cause a system malfunction. A technique is developed to monitor the radiated E-field of materials discharging in an electron environment, using vacuum chambers for measuring the material discharges which are made of highly reflective materials. These chambers affect the radiated E-field due to multiple reflections from the walls. The technique developed defines a method for correcting the effects caused by the measurement facilities. The methodology is: monitor the radiated E-field with a broadband dipole antenna, and digitize the radiated signal as a function of time. Determine the frequency response of the radiated E-field using an FFT algorithm. Measure the transmission and reflection characteristics of the two-port network inside the measurement chamber, and determine the impedance network from the measured E-parameters across the frequency band of interest. Transform the measured E-field frequency response through the impedance network to obtain the frequency response of the actual radiated discharge current. Find the inverse FFT of this response to obtain the actual radiated discharge current response. This technique aids in the prediction of the E-field coupling into receive antennas on-board actual satellites     

Using Free-Space Transmission Loss for Evaluating Anechoic Chamber Performance

Performance of a rectangular RF anechoic chamber is evaluated by measuring the relative insertion loss versus separation distance between the sources of chamber illumination and small dipole or open waveguide antennas on a particular measurement axis. Measured data are compared to free-space transmission loss calculated using finite-range source antenna gains. A lack of fit between measured insertion loss and calculated transmission loss is a measure of reflections from chamber surfaces assuming the finiterange gain calculations are exact for the separation distances considered. Experimental results show excellent agreement between a smooth-curve fit to the measured data and the calculated free-space data. Standard fields in this chamber are used for calibrating hazard meters and dipole antennas used for field-strength measurements.     

Back-to-back measurement for characterization of phased-arrayantennas

A back-to-back measurement method for characterizing phased-array antennas is described. The method yields the complex active impedance of an antenna in a large phased array at any desired frequency and scan angle without the need of a feed network to excite the antenna under test. This avoids the cost and de-embedding procedure associated with the feed network. Measurements are performed by using two different transmission networks to connect identical arrays in a back-to-back configuration. The new method is particularly well suited to printed antennas and is illustrated by using tapered-slot antennas. Back-to-back measurements in waveguide simulators compare well to traditional waveguide simulator measurements and measurements in an anechoic chamber compare well to results from computer codes based on the full-wave method of moments