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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.
In the past few years there have been new application of transient, ultra-wide band microwaves include cooperating aircraft identification and ground penetration Radar's, high power microwave weapons and others. These applications typically require the use of ultra-wideband antennas with characteristics suitable to radiate transient pulses. This paper describes the capabilities of the USAF Phillips Laboratory's new Transient Antenna Range. The antenna range can measure the radiated characteristics of sources/antennas wave forms with risetimes in the 75 ps regime, and with greater than 50 ns pulse width. The antenna range incorporates a hardware suite controlled by a powerful software data acquisition system that runs on a PC. Automatic data reduction can yield values of wave form peak electric field, risetime and waveform spectrum at a single point, or across an azimuthal scan. This paper will also describe a unique wave form splicing technique used in the data processing algorithms of the Transient Antenna Range. This splicing technique allows test personnel to record the (typically) very fast early time history of the radiated waveform with an SCD-5000 scan converter (operating at a maximum bandwidth, 5 ns of record available), and the long time history of the waveform with a DSA-602 transient digitizer.
High performance antennas require very accurate measurements which are difficult to meet in the conventional compact antenna test ranges. This measurement errors are produced by the non perfect plane wave synthesized by the compact range system. By the application of the reaction between the antenna under test true pattern and the compact range incident field, a closed form relation is found for the measured radiation pattern. Under certain conditions, this measured pattern can be approximated by the convolution of the two diagrams. In this paper it is presented the inverse procedure: the deconvolution to numerically calculate either the true radiation pattern of the antenna under test or the plane wave spectrum of the compact range incident field . The effectiveness and limitations of the method are discussed by numerical simulations and tested by measurements.
Accurate multiwavelength radiometric remote sensing of the ocean and the atmosphere from an aircraft requires antennas with the same beamwidth at the various frequencies of operation. Scientists at the National Oceanic and Atmospheric Administration designed an offset antenna with a pressure-compensating corrugated feed horn to meet this criterion. A specially designed fairing was incorporated into the antenna to optimize the aerodynamics and minimize the liquid buildup on the antenna surfaces. The antenna has two positions: the zenith (up) position and the nadir (down) position. The planar near-field facility at the National Institute of Standards and Technology was used to determine the far-field pattern of the antenna. The results show that the antenna beamwidths at 23.87 and 31.65 GHz are nearly the same as expected from the design criterion. This antenna was recently used in an ocean remote-sending experiment and performed according to expectations.
Ultra low sidelobe antenna measurements are very difficult to perform even in the best of ranges. This problem results from the fact that small stray signal errors within the range can be amplified by the antenna main beam gain and result in a error term that is larger than the desired ultra low sidelobe level. With this in mind, one can attempt to reduce the range stray signals, but it is only practical to reduce them so far. However, one can always desire to measure a lower sidelobe level than is feasible for the range. To correct this problem, a new measurement method has been developed that can significantly reduce these. It involves taking two measurements and properly processing the results. It has been shown that one can reduce complex range errors by as much as 35 dB in a real range environment.
The RMS surface error predicted by holographic measurements is often smaller that the one predicted by radiometric measurements. At the FCRAO, a difference of 40% was observed for the 14-millimeter radio telescope. To find the explanation for this discrepancy, simulations and additional near-field measurements were performed. The near-field measurements were carried out at 91.9 GHz on a part of the aperture of the telescope. This paper describes the near-field measurements and presents a careful comparison between the results from holographic, radiometric and near-field surface error measurements. This comparison and simulations revealed that the main reason for the discrepancy between the radiometric and h9olographic results was the smoothing of the holographic data. The smoothing has been used for reducing the effects of the truncated far-field data in the FFT process.
The MSAT satellite payload [1] included the large L-Band Tx and Rx deployable reflector antenna subsystems, the Ku-Band downlink antenna, and the telemetry and command omni. The technical challenges associated with these antennas required a considerable amount of advanced testing for concept development and design, as well as for customer acceptance. The L-Band feed array breadboards were measured to quantify effects of mutual coupling. The L-Band antenna performance was verified by near-field measurement techniques and computer modelling. The Ku-Band shaped reflector antenna was tested in SPAR's compact range. A short summary is given of the omni antenna tests, and the PIM tests carried out on the L and Ku antennas.
This paper will describe a new type of automotive AM/FM conformal antenna. The slot formed between the body of the automobile and a metal solar heat reduction film imbedded in the front windshield of the automobile is used to form an annular slot. Such partially conducting (4 to 12 ohms per square) metal films represent an opportunity to incorporate an antenna in the overall design at only marginal costs. The characteristic impedance and gain patterns will be described and techniques for improving the impedance match will be shown. A mobile measurement system will be described along with an on-road system to characterize the performance of a number of difference vehicle antenna systems in urban and suburban environments. The application of this system to the measurement of calibrated fain patters will be demonstrated.