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Far-field patterns obtained from planar near-field measurements of a prototype of the AMSU-B radiometer antenna by phase retrieval at 94 GHz are presented in this paper. Comparison with results from a compact range facility show good agreement within the main beam A modified algorithm takes into account any misalignments of the two intensity data sets so that the RMS near-field error metric comparing retrieved and measured values converges to < -30 dB. Phase retrieval is revealing itself as a useful technique to be applied to electrically large antennas at frequencies extending into the millimetre and sub millimetre bands.
The feasibility of realizing a 500 GHz hologram type of compact antenna test range (CATR) for testing the 1.1 m antenna of the Odin satellite is studied. The quiet-zone field is analyzed theoretically by us ing an exact near-field aperture integration method. Due to fabrication errors the slots of the hologram are wider or narrower than in the ideal case. How ever, with reasonable value of fabrication errors the quality of the quiet-zone field is not degraded deci sively. The effect of the displacement of the different parts joined together to form a large hologram is the inclination of the amplitude and phase in the quiet zone. The analysis of the CATR feed scanning in the focal plane to avoid the need to rotate the AUT showed that at least a ±1° change of the direction of the plane wave in the quiet-zone is feasible.
Daimler-Benz Aerospace AG (Dasa) in Munich, Germany designed, developed, build and tested most of the INTELSAT VIII antennas. RF test requirements and results are presented for the Hemi/Zone antennas. These tests cover the Beam Forming Networks (BFN), the feed array in the cylindrical near field facility at ambient temperature and in a temperature range from -61 to +85 deg centigrade, and finally the complete antenna sub system, without and with satellite mock-up, in the large Compensated Compact Range. Dasa and TICRA software was used to calculate the far field results from the measured BFN coefficients and from the feed array results measured in the near field facility. Also alignment aspects are considered.
Modern low profile and conformal antennas are fre quently evaluated in the presence of a conducting surface. Antenna designers usually predict the an tenna scattering and radiation performance over an infinitely conducting ground plane. To bridge the gap between a (possibly curved) antenna host surface and the designer's infinite ground plane model, an antenna testbody is required. This testbody must possess a variety of demanding attributes, such as a very close approximation to an infinite ground plane, low testbody signature, ability to provide positional accuracy (in both azimuth and elevation), physical stability (for repeatability and background subtraction), just to name a few. The most widely regarded testbody has been the "almond" testbody [1, 2], which boasts a very low signature, and excellent fidelity when compared to an infinite ground plane. This paper addresses a new variation from the traditional "almond" testbody, in which a unique positioning design provides a phase-stationary antenna aperture center under rotation of both azimuth and elevation. This testbody will be used for a variety of antenna tests at Wright Laboratory's Radiation and Scattering Compact Antenna Laboratory (RASCAL).
Accurate mechanical-to-electrical axis alignment (boresighting), gain, and pattern testing of radar antennae requires specialized tooling/fixturing. This requirement is often taken for granted and seldom discussed in the EE community. Particularly in a production environment, where rapid change of test configurations to accommodate multiple radar platforms are required, a convenient mounting scheme is mandatory. This paper describes and illustrates a method implemented at the Warner Robins Air Logistics Center to satisfy this demand. Drawings and/or photos of a three-point Universal Adapter fixture and several UUT Specific radar mounting fixtures are discussed. The paper discusses tolerances, materials, manufacturing processes, alignment, and antenna boresight methodologies.
There is a need for finite ground planes to test an tennas which are normally mounted on large struc tures. These ground planes are used to simulate a large structure such as an aircraft fuselage but are limited in size based on the available target zone di mensions. For example, TCAS antennas are tested on a 4' circular ground plane based on FAA require ments. Since the conducting ground plane creates significant diffraction errors which are not present in the intended application, these ground plane tests become difficult to interpret because one can not easily separate ground plane diffraction errors from antenna characteristics. A solution to this dilemma is to attach an R-Card (resistive sheet) to a con ductor (PEC) and form an R-Card ground plane. With a properly designed resistive profile, an R-Card ground plane can greatly reduce the edge diffrac tion errors. As a result, the desired antenna charac teristics without significant ground plane corruption terms can be obtained. This paper demonstrates this new concept through calculated and measured results. Also, a Genetic Algorithm (GA) to optimize the resistive profile is presented.
Instrumentation grade, coherent, bistatic, radar cross section (RCS) measurement systems require a reliable low-noise method to link the reference, local oscillator (LO) and intermediate frequency (IF) coherent signals between the transmit and receive subsystems. One approach to this is the use of a fiber optic link (FOL). Phase noise measurements have been performed on a distributed feedback (DFB) type laser transmitter-photodiode receiver link with a delay of up to 2.26 kilometers, operating at 5 GHz, using a standard HP 3048A phase noise test measurement setup. System level tests have been performed, incorporating a FOL into a coherent bistatic instrumentation radar system local oscillator path, and performing image processing on an emulated target A first level analysis was conducted regarding the effects of the thermal noise on the radar perfonnance.
In some ways vector network analyzer technology has simplified antenna measurement tasks. However, its use with a conventional S parameter test set often requires the use of long extents of cables that must carry RF signals. Consequently, one must tolerate large cable losses, or resort to custom S21 test set designs. Unfortunately, due to limited mixer bandwidth, custom test sets typically require 2 or more separate diode mixers for measurements across the VNA's full bandwidth (typically 0.045 - 26.5 GHz). Associated mechanical and electrical switching circuits of the mixers can produce measurement glitches across the crossover frequency, and undesirable measurement artifacts. This paper presents a custom S21 test set circuit design that utilizes a unique, nine-octave virtual mixer circuit to eliminate the need for mechanical RF switches in the mixing state. Also described is a logarithmic amplifier circuit that increases the detection circuit's dynamic range by up to 40 dB.
Implementing communication applications such as Cellular, Personal Communication Service (PCS), Global Positioning System (GPS), and Intelligent Vehicle Highway System
This paper will describe theoretical and experimental techniques for the analysis of the performance of conformal automotive antennas. The theoretical techniques include the application of the method of moments (wire and plate models) and UTD. The experimental techniques include turntable range measurements and mobile on-road measurements. The antennas to be modeled and tested include an AM/FM annular slot windshield film antenna, a generic rear-window heater grid AM/FM antenna, and various configurations of cellular telephone antennas.
Automobile manufacturers have noticed the proliferation of after market antennas, primarily for cellular phones, defacing their otherwise stylish vehicle designs. Investigations are being made by the manufacturers to include antennas for communications requirements, such as cellular phone, personal communications service (PCS), global positioning system (GPS) and Intelligent Vehicle Highway System (IVHS), within their vehicle This paper presents the initial phase of an investigation undertaken within the Research Branch (ERB) of NASA Research Center (LaRC). The measurements, presented in this paper, were performed using a one-fourth scale model of a currently popular vehicle design. The bands of interest for this investigation include the cellular, GPS and FM broadcast frequencies. Comparisons of measured and computed patterns of commonly used antennas such as wire and microstrip patch antennas are presented.
Due to the revolution in communication technology very sophisticated communicative and navigational tools are becoming a part of automobile electronics. These different applications need antennas that operate at various frequencies and with different polarization requirements. One such antenna is a cavity-backed Circular Archimedean Spiral Microstrip Antenna (CASMA). This pa per will compare radiation pattern measurements of a CASMA with pattern predictions using a hybrid FEM /M oM/GT D technique. The measurements were done at NAS A-Langley Research Center's Low Frequency Antenna Chamber. The predicted and measured patterns are presented and are shown to exhibit a reasonable degree of agreement.
A frequency selective surface has been developed for use as a part of an automatic highway system. The FSS is attached as a stripe along the edge or center of the lane, and is designed to a strong retro-reflective echo for the design frequency, polarization, and elevation angle of the forward-looking radar installed on an automobile. The stripe provides directional information for automated steering, as well as other coded information such as lane number, and exit advance warning. This paper reports on initial development and testing of a prototype FSS highway stripe. The stripe was designed for an operating frequency of 10.5 GHz, and was built and tested using a prototype autonomous vehicle. Both FSS stripe performance, and performance of the vehicle will be reported.
Finite-Difference Time-Domain (FDTD) is prov ing to be a practical and accurate technique for an alyzing and predicting the performance of anten nas mounted on complex structures. As part of an effort to develop and validate an FDTD code, the impedance and radiation patterns of helicopter mounted loop antennas are predicted and compared to full-scale and 1:10 scale measurements. The input impedance and coupling of HF loop an tennas on the scale model helicopter are measured in the ElectroMagnetic Anechoic Chamber facility at Arizona State University. Although made difficult by the large mismatch between the highly reactive HF antennas and the instrumentation, the scaled impedance measurements agree well with the full scale measurements and predictions. In addition, ro tor blade modulation effects on the input impedance are examined.
Due to the limited size of modern helicopters, airborne antennas must be physically small and lightweight. Slot antennas have been widely used by the aerospace community to meet the size, weight, and aerodynamic requirements when flush-mounted to a platform surface. Having these characteristics, a ferrite-loaded cavity-backed slot (CBS) antenna is an excellent choice for as a tunable low-frequency antenna. Excitation of a magnetostatic mode in the ferrite results in resonances at frequencies below those of the dynamic modes of dielectric-loaded CBS antennas. Frequency agility is achieved by varying the applied DC magnetic bias. Two ferrite-loaded CBS antennas were built and their impedances and radiation patterns were measured. Reasonable (0-6 dBi) with dynamic 3 dB bandwidths in excess of 20% were measured in the UHF band. Air-filled versions of these antennas agree well with Method of Moments (MoM) predictions, but non-uniformity of the magnetic field in the ferrite violates assumptions made in the theoretical model, resulting in discrepancies.
NASA Langley Research Center built a compact range facility in 1990 with a dual rese,arch role. In addition to meeting measurement needs of the Electromagnetics Research Branch, the facility has been a test bed for compact range technologies. Initially, a Gregorian subreflector with untreated edges was used to feed the 16' x 16' blended rolled edge main reflector. The designed frequency range of the untreated subreflector was 6-18 GHz. In 1993 a new resistive card edge treated subreflector was built and installed enabling the frequency range to be extended on the low end down to 2 GHz. The subreflector performance was measured by probing the fields in the test measurement or quiet zone. A 14' linear composite prober was made to measure the fields out onto the rolled edge sections of the main reflector. Characteristics of the subreflector, main reflector, and the coupling aperture between the reflectors were identified. Of particular interest was the effect of the resistive card treatment on the nature of the caustic fields in the aperture. A surface distortion was also identified on the rolled edge portion of the main reflector.
Evaluation of serrated edge and blended rolled edge compact range reflectors is presented in this paper. An interactive approach is used to design the serrated reflectors. Several issues associated with the serrated reflectors are also discussed in this paper. Quiet zone fields for various serrated edge with an optimally designed blended rolled reflector are presented for comparison. In addition, simulations of a low sidelobe phased array measurement using serrated and blended rolled edge reflectors are shown to investigate their impact on the measurement accuracy.
A new technique for eliminating the desired planar wavefront (DPW) from the quiet zone fields of a compact range is described. In the technique, the probe data is modeled as a sum of a finite number of damped exponentials. A modified Prony's method is used to estimate the parameters of the damped exponentials. Next, the damped exponentials correspond ing to the DPW are identified and are subtracted from the probe data. Using simulated examples, it is demonstrated that at low frequencies the proposed technique performs much better than the other frequently used techniques for removing the DPW from the probe data. This, in turn, help in imaging the stray signals in a compact range.
Characterisation of the test zone field in a Compact Antenna Test Range (CATR) is traditionally done by scanning with a probe. The test zone field can thus be measured more or less directly at any position by the probe. This method, however, has some serious disadvantages. In this paper the scanning probe method is compared with a characterisation method using a reference target such as a flat plate, bar or cylinder. It will be shown that from an RCS measurement of the reference target, an accurate test zone field can be determined using Fourier transformation. An analysis of this method together with experimental verifications which validate the approach will be presented. A comparison between the probe and reference target method is also given.
Proper characterization of metal walled chambers or other non-anechoic facilities is normally difficult and time consuming. A novel technique for rapid charac terization is described that is available to high PRF, pulsed, chirp radar systems. A sphere is tethered to a crosspiece mounted on the axis of a motor using a fine cord. The system can be mounted on the ceiling or affixed to a variable height pole. adjusting the motor speed and length of the cord, a stable orbit is achieved having a fixed radius and height above the suspension point. Chirp data can be processed into range-time-intensity (RTI) plots that provide clear evidence of multipath and beam taper. By changing the orbit parameters it is possible to characterize a large volume and remedy problems in a very short period of time.