Range requirements in radar cross-section measurements

The conditions for radar cross-section measurement are given in terms of permissible variations in the amplitude and phase of the incident field at the target. A number of minimum range conditions are listed and discussed. The minimum range associated with the uniform square aperture antenna is analyzed and its relationship to the sensitivity of measurement is determined. Measurements of small targets in the near zones of focused and unfocused apertures are examined. The substantial range reduction possible with the use of a suitable microwave lens, when measuring large and medium size targets, is indicated.     

Short distance attenuation measurements at 900 MHz and 1.8 GHzusing low antenna heights for microcells

Short-distance, low-antenna-height signal attenuation measurements are presented in connection with their use in the design of future microcell cellular radio networks. Measurements presented are based on the propagation along busy city streets in a direction radial to a fixed antenna site. Antenna heights between 5 m and 20 m were chosen for the fixed site, while 1.5 m was chosen for the mobile vehicle. The signal strength was then measured out to a distance of 1 km in a line-of-sight path from the fixed site at both 870.15 MHz and 1.8 GHz. The measurements show that extrapolation of the M. Hata (1980) and Y. Okumura et al. (1968) formulas into the low-antenna-height, short-distance area is not valid and can lead to inaccuracies. The results show that at short distances from the antenna the signal attenuation slope is very much less than that predicted by extrapolating the results of Hata and Okumura et al     

Multipath interference for in-flight antenna measurements

Formulations are presented to predict the pattern of interference (multipath) between direct rays and those reflected from the surface of the earth. These formulations can be used to assess the performance of ground-to-air, air-to-ground, ground-to-ground, and air-to-air communication and antenna measuring systems. Methods are also introduced to determine accurately the point of reflection and the path phase difference between the direct and reflected waves. Various divergence factors, used to account for energy spreading from a curved surface, are presented and compared. The formulations introduced can account for any polarization state (linear, circular, elliptical), sense of rotation (right hand, left hand), and tilt angle of the transmitting and receiving elements, and for polarization changes due to reflection.     

Gabor schemes for analyzing antenna measurements

The oversampled Gabor transform (OGT) is proposed as a powerful tool for analyzing antenna measurements in amplitude and phase. The OGT allows to perform a high resolution analysis of the measured signal leading to the identification and the separation of its wave components. The proposed technique is applied to measurements performed in a semi-anechoic chamber to remove the reflected components and to retrieve the results one would have measured in a perfect anechoic chamber. In the application considered, the superiority of the OGT over the matrix pencil method is illustrated     

Radiation patterns of a large UHF phased-array antenna: acomparison of measurements using satellite repeaters and patternsderived from measurements of antenna current distributions

The radiation patterns of a 404-MHz phased-array-radar antenna were measured relative to a reference dipole antenna, using 406-MHz repeaters on polar-orbiting satellites and associated ground-station equipment. More than 6000 measurements were collected from the three radar-antenna beams over a five-month period in 1995. The patterns were compared with those derived from measurements of the complex current distributions on the coaxial collinear radiating elements of the antenna. In addition to verifying the antenna specifications for gain, beamwidth, pointing direction, and sidelobe levels, the far-field satellite repeater measurements validated the convenient and inexpensive current measurement and model technique, developed for antenna testing and maintenance     

Swept-frequency antenna gain measurements

This paper presents a method of making a swept-frequency antenna gain measurement which yields a result based upon the known gain vs. frequency of a standard antenna. This antenna consists of a 295-mm length ofX-band waveguide with a cover flange on one end. It is an inexpensive and very satisfactory gain standard. The error analysis, which gives the accuracy of the resulting gain figure, consists of evaluating the conjugate mismatch losses at the antenna feed point or junction. This method of measuring gain is shown to yield useful gain data from an antenna test range that is not free of reflections. Gain vs. frequency curves are given for the standard gain antenna and a test antenna, having estimated accuracies of at least 0.27 dB and 1.00 dB, respectively.     

Microwave phase comparators for large antenna arrays

Three instruments are described which are useful in equalizing path lengths in large antenna arrays. With each instrument the difference in length between two paths is measured directly, thereby avoiding many of the difficulties in measuring very long paths. A gas discharge tube, used as a modulated reflector, is placed at each of two antenna elements of an array. A null in the detected modulation occurs when the paths leading to the reflectors are equalized in length. The accuracy, limited by instability of the discharge tubes, is about3degin phase at a wavelength of 10 cm. With the most sensitive instrument, this accuracy is maintained when there is a two-way loss of as much as 100 dB between each reflector and the receiver terminal of the array.     

Measurement distance requirements for both symmetrical andantisymmetrical aperture antennas

Universal curves giving the main-lobe reduction and the first sidelobe level change versus the measurement distance are furnished for various aperture distributions. The widely used Rayleigh 2D²/λ distance criterion (where λ is the radiated wavelength and D is the distance to the antenna) is shown to be unsatisfactory, since the minimum measurement distance depends both on the aperture distribution and on the specific far-field parameter under measurement. It is found that, in order to measure accurately the pattern of antennas with ultralow sidelobes, a measurement distance much greater than the Rayleigh one is needed. Such a distance is more than safe for measurement of the antenna gain and antenna patterns with moderate sidelobe level, but it is greater than necessary to measure the principal parameters of interest in a monopulse system. This latter conclusion is in full agreement with reported experimental results. Some experimental results concerning a monopulse antenna reported     

A novel antenna concept for double-directional channel measurements

In this paper, a novel concept for the characterization of the double-directional mobile radio channel is presented. It is based on a broad-band real-time channel-sounding device together with appropriate antenna arrays, thus forming a multiple-input-multiple-output system. For the propagation measurements within the 5.2-GHz wireless local area network band presented here, a uniform linear antenna array at the receiver site and a switched multibeam antenna at the transmitter site have been employed. By application of the multibeam antenna, the entire azimuthal range of 360/spl deg/ is covered without ambiguities. The measurement data obtained with this platform in a simple synthetic scenario, as well as in a typical multipath environment, is then analyzed in terms of the propagation path properties. Using the three-dimensional estimation of signal parameters via rotational invariance techniques algorithm together with a proper Fourier transform, the channel parameters of all relevant multipaths such as delay, direction of arrival, direction of departure, and path loss are extracted from the measurement data. For both scenarios, the obtained estimated paths are verified to the underlying physical propagation environment by means of geometrical considerations. This geometrical matching accounts for multipath components due to single-bounce as well as multiple-bounce reflections and demonstrates the high suitability and good performance of the proposed concept.     

Scan-plane reduction techniques for planar near-field antenna measurements

In this work, two planar near-field scan-plane reduction techniques are considered and results are presented. It is shown how truncation based on field-intensity contours, instead of simple geometric truncation, can in some cases improve the efficiency of the truncation process. Both techniques are applied to measured data sets, and it is shown how these methods can be used to reduce data-acquisition times, while also assessing the impact of the total acquisition surface reduction on the far-field radiation-pattern integrity.     

A broadband antenna array for sea scatter measurements

A nonuniformly spaced array of 25 side-terminated vertical half rhombics is synthesized for the entire high-frequency band. The array is to be used for both transmitting and receiving, as a sky-wave Doppler radar for sea scatter measurements. The array has the capability for providing a maximum transmitting-receiving product gain in the order of 60 dB at low take-off angles, a product azimuth pattern with a nominal beamwidth of2deg, a40degsectoral coverage, a final product sidelobe level of -38 dB, and a grating-lobe level of -30 dB.     

Complex antenna factor of a V-dipole antenna with two coaxialfeeders for field measurements

In transient field measurements, an antenna with wide band performance both in amplitude and in phase is desired. The complex antenna factor (CAF) is an appropriate characteristic of such an antenna. It is convenient that the CAP of the wide-band antenna is able to be calculated easily. For this purpose, a V-dipole antenna with a balun consisting of two coaxial feeders is investigated. The calculated complex antenna factors are compared with the measured values. The results show that the V dipole has the wide-band performance and the CAF is calculable     

Radiation pattern measurements of electrically large antennas usinga compact antenna test range at 180 GHz

A 3 m-width millimetre-wave compact antenna test range for antenna measurements up to 200 GHz has been constructed at Queen Mary College. Measurements of a 400 mm aperture width 90° offset test antenna operating at 180 GHz are presented, indicating that a 62 dB dynamic range is achievable     

A simple high-speed antenna rotator for millimeter-wave clutter measurements

A low-cost V-band antenna-rotator assembly, capable of continuous operation at up to five revolutions per second, and having an angular resolution better than 0.4/spl deg/, is described. The unit is intended for surface-clutter-map data collection out in the field, uses conventional car battery power, is lightweight, and allows simple speed adjustment. Rigid rectangular waveguide is used throughout, with no coax transitions or flexible sections. Commercial off-the-shelf components have been selected as much as feasible, e.g., optical encoders and geared motors. The measured cross-axis stability was better than 0.3 at full speed. Experiments indicated that the rotator can work unattended for one week with just one 60 Ah lead-acid pack.     

Ground-based measurements of inflight antenna patterns for imagingradar systems

An approach is presented for determining the inflight antenna pattern in the cross-track direction for air- and spaceborne synthetic aperture radar (SAR) systems. In the 1991 Oberpfaffenhofen DC-8/E-SAR calibration campaign, ground-based measurement, equipment comprising 18 precision calibration receivers and nine polarimetric active radar calibrators, all operating in C-band, were tested. These instruments are capable of handling various pulse lengths and repetition frequencies, and they have a very high dynamic range. Together with precise internal clocks, these instruments are suitable for recording the actual radar transmit pulse shape for the later evaluation of the desired inflight antenna pattern. Lining up these devices in the cross-track direction, each receiver yields an azimuth cut of the three-dimensional antenna pattern. The elevation pattern was then obtained by time correlation of these azimuth cuts. Further results concerning pulse shapes, squint angles, and H-V pattern misalignment are presented     

Real-time spherical near-field handset antenna measurements

A new spherical near-field measurement system, dedicated to personal-communications applications, has been installed at e-tenna Corporation. The system, sold by Satimo, uses an electronically scanned probe array, and offers a dramatic reduction of the measurement time (by a factor ranging from 60 to 1,000) with respect to conventional mechanical scanners. This test range has undergone extensive acceptance testing, which has verified the specified measurement accuracy. The speed of the system, combined with the availability of full-sphere measurement data, enables rapid measurement of antenna efficiency. The system also enables the measurement of other parameters, such as the percentage of radiated power dissipated into the body of a human test subject operating a cellular telephone     

An overview of near-field antenna measurements

After a brief history of near-field antenna measurements with and without probe correction, the theory of near-field antenna measurements is outlined beginning with ideal probes scanning on arbitrary surfaces and ending with arbitrary probes scanning on planar, cylindrical, and spherical surfaces. Probe correction is introduced for all three measurement geometries as a slight modification to the ideal probe expressions. Sampling theorems are applied to determine the required data-point spacing, and efficient computational methods along with their computer run times are discussed. The major sources of experimental error defining the accuracy of typical planar near-field measurement facilities are reviewed, and present limitations of planar, cylindrical, and spherical near-field scanning are identified.     

Near-field antenna measurements using nonideal measurementlocations

We introduce a near-field to far-field transformation algorithm that relaxes the usual restriction that data points be located on a plane rectangular grid. Computational complexity is O(Nlog N) where N is the number of data points. This algorithm allows efficient processing of near-field data with known probe position errors. Also, the algorithm is applicable to other measurement approaches such as plane-polar scanning, where data are collected intentionally on a nonrectangular grid