AMTA Paper Archive
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Antenna test facility at ISAC-Bangalore
The paper describes a simple but unique antenna test facility suitable for aerospace antenna developments. The total idea can be easily adopted by organizations who wish to carry out antenna measurements with minimum required instrumentation. The facility majorly caters for omni and wide beam antenna measurements, has been set up at ISRO Satellite Centre, Bangalore, India. It has been extensively used for omnidirectional antenna developments in VHF, UHF, L, S, and X-bands for India’s various space programs. Radiation pattern, gain, polarization and impedance measurements can be carried out both in near free space conditions as well as the ground reflection modes. The main feature of the facility is the use of large fiber-glass mounting structures for avoiding reflections and perturbations in radiation patterns due to impressed surface currents, specially in VHF ranges. Field probing is done by the use of a fiber-glass X-Y probe positioner. The facility used Scientific Atlanta 1752 Receiver and 1540 Recorder. Suitable software has been added to the facility for contour plotting of radiation levels, calculation of efficiency isotropy, and polarization properties.
A Partial loop source of E and H fields for antenna factor calibration (a loop cell)
The loop cell is fabricated using two intersecting metal sheets joined at the intersection and forming a 36 deg angle. A section of a loop is mounted between two coaxial panel jacks, one on each sheet at a distance equal to the loop radius from the intersection. A known current through this section of electrically small loop produced calculable E and H fields between the sheets in the plane of the loop. These known fields may be used to determine the antenna factor of small E and H antennas placed in the field if the mutual impedance due to the antenna images in the sheets is negligible and the antenna is not close to the open edges of the cell. Measured and calculated antenna factors agree within ±2 dB between 0.25 MHz and 1000 MHz.
Design and Calibration of Standard Gain Horns in the 200-400 MHz Range
Paper not available for presentation.
Indoor impedance measurements using a time-domain filter
This paper examines the development tests performed at Rockwell International in Anaheim, CA on VHF meandering monopole and dipole antennas which are part of the Global Positioning System satellite. The development tests included numerous impedance measurements of individual antennas configured first in their operational positions on a full-scale mockup of the GPS satellite spacecraft and second while mounted on an indoor ground plane. The initial measurements of antennas positioned on the mockup required the mockup to be located in an exceptionally large, obstruction-free environment because of the low operating frequencies (large wavelengths) of the antennas under test, and in our case a suitable environment was an empty parking lot approximately one-half mile away from the necessary test equipment. This situation necessitated frequent transportation of fragile test equipment to and from the test site which was both impractical and time-consuming. To avoid this situation when production units are to be tested later this year, a ten-foot diameter ground plane was constructed in order to perform the antenna parameter measurements indoors, which presents a very reflective environment. To minimize and theoretically eliminate the effects of these reflections on our measurements, the time-domain gating (time filter) feature of the HP 8510 Network Analyzer was utilized at the indoor test site. The gating function removes any time-domain responses outside of the gate span, the span in this case being the radius of the ground plane. When the time-domain response is Fourier-transformed back to the frequency domain, the effect of the unwanted (gated) responses is eliminated in impedance measurements. While the gated, ground plane parameter measurements will not yield the same values as those measured on the mockup, they can be used to establish an impedance, VSWR, or return loss standard from a known “good” antenna against which production antennas can be compared to determine electrical failures.
ANA antenna impedance measurements using finite-length non-precision transmission lines
A method for calibrating an automated network analyzer for antenna impedance measurements through a long interconnecting transmission line is developed. The transmission line is non-precision and of nominal characteristic impedance, loss, and dispersion
The Measurement of both complex permittivity and permeability of absorptive materials
Measurement of complex permittivity (er) and permeability (µr), both vector quantities of absorptive materials, has gained increasing importance with expanding use of the RF and microwave spectrum, particularly in communications and electromagnetic countermeasure applications. In addition, the network analyzer has seen increasing use in non-destructive measurements to determine the chemical composition of a sample dielectric material. The method described here is suited for the measurement of complex permittivity and permeability of ansorptive materials. These measurements have been made for years using numerous methods. A conventional technique involves a two-step process using a slotted line or network analyzer. First, the sample is backed up by a short circuit and the input impedance is measured. Next, the short circuit is moved ¼ ? from the sample to simulate an open circuit termination (where ? is the incident signal wavelength), and a second measurement is made. The results of these two measurements are used to solve simultaneous equations for er and µr. This procedure is repeated for each frequency of interest. Uncertainties in the measurement include test set-up frequency response, mismatch, and directivity errors, as well as the uncertainty in the physical position of the short circuit.
On site measurement of antenna impedance, current and voltage at H
It is often desirable to measure antenna impedance during an operational state. In applications such as experimental studies of ionospheric properties it is desirable to determine the instantaneous impedance. Most applications will involve adaptive tuning such as in antenna filter or multicoupler systems to maintain resonance despite other operations being conducted on the system. Adaptive tuning of HF whip antennas will provide compensation for environmental conditions such as ice load, or proximity to various objects. In cases where the antenna, or its surrounding, is affected by the power level, it is also desirable to measure the impedance over the power range as well as frequency. Two methods of determining impedances will be discussed in this paper. The first method is that of voltage and current probes and the second that of directional couplers for measuring forward and reflected waves.
Out-of-band response of array antennas
At the National Bureau of Standards (NBS) we have examined the out-of-band response of array antennas from both a theoretical and experimental point of view. Theory shows that the out-of-band response of an antenna depends primarily on two factors: the antenna's input impedance, and its directivity. Experiment shows that, for most practical purposes, the out-of-band response of an antenna can be estimated from a measurement of the antenna's input reflection coefficient alone. If the reflection coefficient is low, the antenna response will be good; if the antenna coefficient is high, the antenna response will be poor.
Fast electrical functional testing of the ERS-1 synthetic aperture radar antenna
The Synthetic Aperture Radar Antenna for the European Remote Sensing Satellite ERS-1 is a 10 by 1 metre deployable slotted waveguide array operating at 5.3 GHz. Electrical performance of the complete antenna is derived at the end of the environmental test programme from near field measurements on a planar NF scanner. In order to obtain very early information on electrical integrity of the flight model antenna, suitable for pre- and post-environmental comparison, a fast electrical functional test was implemented in the total test sequence. It basically consists of a 2D slot probing of a well distributed number of slots in combination with complex input impedance measurements. The paper describes the method and presents results of different test steps. The data of pre-/post-environmental measurements are compared.
The Design of wedge absorber using the periodic moment method
To improve measurements at lower signal levels and/or reduce the size of the compact range chamber, absorber with much better scattering performance is required. This high performance absorber can be realized by introducing multiple layers to obtain a better impedance transition from air to the absorber. The inhomogeneity leads to the use of the Moment Method. However, the truncated ends of a finite absorber panel produce a scattering so strong that the edge and valley diffractions from a typical wall of absorber cannot be recovered. Thus, an approach to solve and infinite wall of identical wedges has been developed for the TM case using the Periodic Moment Method (PMM). In this paper, PMM will be briefly discussed. Then, some interesting designs will be presented, including ordinary wedge absorber with different dopings, wedge widths and wedge heights, wedges with curves surfaces, and multi-layer wedge absorber designs.
Wideband polarimetric determination of antenna radiation and scattering characteristics by RCS-measurements
A novel and very powerful measurement technique is presented which allows the determination of antenna radiation and scattering by radar-cross-section (RCS-_ measurements. The antenna under test is treated as a loaded scatterer using a polarization dependent network model that allows a complete antenna description in terms of scattered, radiated and absorbed waves. A load variation principle is used to determine the network model parameters and all commonly used antenna parameters like gain, antenna polarization, axial ratio, polarization decoupling, input impedance and also structural scattering can be derived from the backscatter measurement without using any additional standard antenna. With the antenna network description it is furthermore possible to examine the antenna behavior for arbitrary excitation or loading on their waveguide or radiation port.
High performance 2-18 GHz power amplifier provides increased power and reduced ring down time
This paper describes new developments in broadband Microwave power amplifiers for compact RADAR Cross Section (RCS) Ranges. The RF Power level of transmitters used in compact RCS ranges for the most part has been limited to a watt or two. This is due to the limitations of the power available from solid state RF amplifiers and the power handling capabilities of PIN diode switches, used to pulse modulate the RF amplifier output. Inherent impedance mismatches of the PIN diode switch, RF amplifier and RF output circuits produce reflections of RF energy. The reflected RF energy reverberates between the output circuits of the RF amplifier and the antenna. Reverberation of RF energy between mismatches continues until circuit losses reduce the energy to zero. These reverberations manifest as deterioration of the RF output pulse fall time waveshape. The radiated pulse fall time is extended and damped rather than abrupt. This deterioration of pulse waveshape, due to reverberations, is ring down time. RF pulse ring down deteriorates the resulting RCS measurements. New broadband microwave Traveling Wave Tube (TWT) technology, combined with extremely quiet power supplies and modulator, provide increased power, low noise floor and reduced ring down time resulting in improved RCS measurements.
Implementation considerations for a compact range array feed
Implementation of a two-ring array for feeing a compact range reflector is investigated. The array is designed to produce a shaped beam with a null at the angle corresponding to the rim of a circular-aperture offset paraboloid. Therefore fields diffracted from the reflector rim are reduced and no reflector edge treatment is necessary. The advantages and disadvantages associated with various feed systems are discussed. A dielectric-filled radial transmission line is proposed as a simple, cost effective implementation of the array beam-forming network. Curves for determining the required dielectric constant for null placement at a desired angle are presented. System bandwidth is examined. Methods for impedance matching and suppression of higher order modes in the beam-forming network are proposed.
Modern facility for test and evaluation of full scale aircraft antenna systems, A
A modern outdoor test facility has been designed for comprehensive evaluation of antenna systems on full scale aircraft. The aircraft are mounted to a positioner/tower assembly in an underground handling facility, and are raised to a height of 25 meters by a hydraulically activated lift. A source site 1000 meters downrange provides illumination of a 7 meter radius test zone over a 0.1-18 GHz band. All source site functionality is remotely controlled from the operations center located near the aircraft support tower. The range is designed to provide the capability not only for conventional automated antenna pattern measurements, but also for the support of ECCM testing. This is accomplished by activating both fixed and mobile jamming transmitters available to illuminate the test zone using either CW or modulated waveforms. The FR Model 959 Automated Antenna Measurement Workstation is being enhanced to allow for control of the jammer sites as well as the primary range sited. The system design and operation is described.
RF marking principle and its application in making antenna measurements
The antenna analyzer is specifically designed to make use of measurement techniques that have been difficult to use until now The analyzer is an original vectorial receiver design, based upon the analysis of one of the sidebands of the marked RF measurement signal. Thanks to the RF marking process, the antenna analyzer is not the only equipment that allows characterization (in amplitude, phase or return loss) of all devices in a transmitting chain, including the high power elements, without cutting off the transmission. Originally introduced for the analysis of wired antennas in UHF-VHF bands, its use is now extended to microwave antenna measurements, especially printed circuit antennas. A special characteristic of the new analyzer, ESTAR 2110 is its capacity to measure the phase of RF signal with power levels as low as -120dBm. The analyzer is ideal for elaborate analysis of fundamental antenna parameters such as RF current distribution, close field, antenna pattern, impedance and phase balance of antenna network. The paper describes the marking principle and its use in making antenna parameter measurements.
Characterization and modelling of conducting polymer composites and their exploitation in microwave absorbing materials
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
Annular slot windshield antenna
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.
Analysis of amplitude dispersion in radar scattering using the MUSIC algorithm
At high frequencies, the scattered fields from a radar target can be modeled as a sum of contri butions from a finite number of scattering centers. We use a parametric model based on the Geometric Theory of Diffraction (GTD) to estimate the location and type of scattering centers present in a frequency domain data set. The parameters of the model are estimated using a modified MUSIC algorithm that incorporates the GTD model. A new spatial smoothing algorithm is also introduced.
Influence of noise and calibration errors on HRR and ISAR
Several approaches are known for the identification of noncooperative air-borne targets with radar. Assuming that the tar get can be tracked during a certain flight path, observations from different aspect angles will be obtained. High-resolution radar (HRR) systems use these observations to create one-dimensional range profiles. With Inverse Synthetic Aperture Radar (ISAR) the data from all observed aspect angles are combined to obtain two-dimensional images. In recent years, techniques for resolution enhancement have been developed for both techniques. The choice for one of the two approaches should depend on the applicability of the target representation for identification. ISAR is the most suitable for reproduction on a display and identification by human observers. In case of identification by a machine, for example an algorithm on a computer, the choice is not straight forward. In this paper an overview of the influence of several errors on the performance of HRR and ISAR will be given. The error sources that will be evaluated are: • uncertainty of the absolute distance of the target; • errors in the mutual alignment of observations; • additive noise. The errors are generated numerically and applied to data from simulations and low-noise measurements. The influence of the bandwidth and angular span on the quality of the target reconstruction will be regarded as well as the performance of some high-resolution techniques. Finally, conclusions are drawn concerning the applicability of ISAR and HRR.
Techniques for the measurement of the impedance of wideband balanced antennas
In this paper, we present a new technique for measuring the input impedance of balanced antenna systems. The process uses standard two-port scattering parameters for balanced antennas, feeding each of the balanced input ports as the port of a two-port. The scattering-parameters will be related to the designed input impedance which may be obtained by post-processing the data. In addition, the scattering-parameters may be used to check for the assumed balance of the system. Both experimental and simulated results will be presented to validate the technique.
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