Johann W. Odendaal

Two-dimensional superresolution radar imaging using the MUSIC algorithm

The multiple signal classification (MUSIC) algorithm developed by Schmidt [1986] is applied for two-dimensional radar imaging. The performance of the MUSIC algorithm using spatial smoothing for decorrelation is demonstrated. Two-dimensional radar images are generated for a simulated target as well as a target measured in the compact range at the University of Pretoria, South Africa. >

Single-layer capacitive feed for wideband probe-fed microstrip antenna elements

In this paper, it is shown how a single-layer capacitive feeding mechanism, consisting of a small rectangular probe-fed patch, which is capacitively coupled to the radiating element, can be used to obtain wideband operation for probe-fed microstrip antennas on thick substrates. The main advantages of this feeding mechanism are that all the elements reside on a single layer and that it is very easy to fine-tune the input impedance. Calculated as well as measured results for rectangular, circular and annular-ring geometries are included.

CPW-Fed Cavity-Backed Slot Radiator Loaded With an AMC Reflector

A low profile coplanar waveguide (CPW) fed printed slot antenna is presented with uni-directional radiation properties. The slot antenna radiates above a closely spaced artificial magnetic conducting (AMC) reflector consisting of an array of rectangular patches, a substrate and an electric ground plane. The electromagnetic bandgap (EBG) performance of the cavity structure between the upper conducting surface in which the slot is etched, and the ground plane at the bottom of the reflector, is investigated using an equivalent waveguide feed in the place of a half-wavelength section of the slot antenna. From the reflection coefficient of the equivalent waveguide feed one can determine the frequency band where minimum energy will be lost due to unwanted radiation from the cavity sides. The dimensions of the cavity were found to be very important for minimum energy loss. Experimental results for the final antenna design (with a size of 1.02λ0×0.82λ0×0.063λ0), mounted on a 1.5λ0×1.5λ0 back plate, exhibit a 5% impedance bandwidth, maximum gain in excess of 10 dBi, low cross-polarization, and a front-to-back ratio of approximately 25 dB. This low-profile antenna with relatively high gain could be a good candidate for a 2.4 GHz WLAN application.

Synthesis of Conformal Arrays With Optimized Polarization

Adaptive array theory is applied to a conformal antenna array to synthesize a main beam with optimized polarization employing dual polarized patch antennas as radiators. The required polarization in the main beam region of the array is realized by combining the weighted individual feeding ports of the patch antennas before the beam forming optimization is performed. This significantly reduces the number of unknowns in the beam forming optimization problem compared to previous optimization schemes for conformal arrays with optimized polarization properties. A numerical example for an array of dual fed linear patch antennas conformal to a spherical surface is presented. Measured pattern data for the patches are used to synthesize a main beam with specific linear polarization.

Extended Analysis of Retrodirective Cross-Eye Jamming

An extended and rigorous analysis of retrodirective cross-eye jamming in a radar system scenario is presented. This analysis removes the approximations that limit the validity of other analyses of cross-eye jamming. These results imply that under certain conditions, a monopulse radar system can be more easily deceived than suggested by conventional cross-eye analyses. Furthermore, the cross-eye jammer antenna patterns do not affect the induced monopulse error.

An Improved Design for a 1–18 GHz Double-Ridged Guide Horn Antenna

It is a well known fact that the traditional 1-18 GHz double ridge guide horn (DRGH) antenna suffers from pattern deterioration above 12 GHz. At these frequencies, instead of maintaining a single main lobe radiation pattern, the pattern splits up into four lobes. It was shown in the literature that higher order modes are causing the pattern breakup. A benchmark study is performed to establish the performance of typical current and historic 1-18 GHz DRGH antennas. The performance of the antennas are evaluated in terms of gain, VSWR and radiation patterns. An improved 1-18 GHz DRGH antenna is presented. The new design has better gain and VSWR performance without any pattern deterioration. It also consists of significantly fewer parts, reducing the possibility of performance deterioration due to gaps between parts. Two prototypes of the new design were manufactured and tested with excellent agreement between measured and simulated results. The aperture dimensions of the new design are identical to that of the traditional DRGH, making it the only 1-18 GHz DRGH without pattern breakup whose aperture dimensions comply with the requirements specified in MIL-STD-461F - 24.2 by 13.6 cm.

Radar cross section measurements using near-field radar imaging

In this paper a technique to obtain the far-field scattering signature of bodies, using near-field measurements, is proposed. The method is based on near-field radar imaging techniques. The backscattered field data are collected in a controlled environment over a large frequency band and aspect angle using a near-field antenna. A focused radar image of the body is generated. Probe correction to compensate for the radiation pattern of the interrogating antenna is conducted during the two-dimensional imaging of the object. The contribution from each scattering center to the total backscattered far-field is obtained from the radar image. The proposed technique is applied to obtain the far-field radar cross section (RCS) for an object from near-field measurements conducted in an anechoic chamber at the University of Pretoria, South Africa.

Tolerance Analysis of Cross-Eye Jamming Systems

The matching required between the two directions through a retrodirective cross-eye jammer is considered using both the traditional phase-front analysis and an extended analysis. The design parameters to achieve a specified tracking error are derived and an optimal design is proposed. The results for the extended analysis show that the tolerances required to induce large angular errors in a monopulse radar are not as strict as the traditional analysis suggests.

The Effect of Manufacturing and Assembling Tolerances on the Performance of Double-Ridged Horn Antennas

Double Ridged Guide Horn antennas are highly sensitive to tolerances in machining and assembly. This study shows that most of the sensitivity problems can be attributed to gaps formed between various subsections in the waveguide launcher assembly. The coaxial feeding section is especially sensitive to manufacturing tolerances. It was found that gaps between the ridges and feeding section causes deep resonances in the on axis gain. Due to the small tolerances involved, a highly detailed broadband simulation model of the antenna that includes a coaxial feeding section as well as an N-type connector had to be used in the investigation. The model is implemented in the electromagnetic simulation software package FEKO, which uses the method of moments (MoM) technique to calculate the electromagnetic fields. Gaps between the subsections of the waveguide launcher were implemented in the simulation model and compared to typical measured results. Excellent agreement between measured and simulated data is obtained. Corrective measures as well as improved designs for the waveguide launcher section are presented.

Quad-Ridge Horn Antenna With Elliptically Shaped Sidewalls

A quad ridge horn antenna is presented as a feed for a reflector antenna for use in radio astronomy applications. The antenna uses shaped sidewalls to limit the variation of beamwidth over the frequency range and to obtain greater radiation pattern rotational symmetry. The antenna is dual-polarized and matched over more than a 4∶1 bandwidth.