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A new technique to implement wideband 90° hybrid coupling profiles on non‐optimal substrate configurations is presented. The technique uses an impedance taper over the whole length of the hybrid to remove the restriction of a substrate defined maximum coupling coefficient at crossover, thus resulting in much more flexibility for the implementation of hybrids on integrated printed circuit boards. The choice of board thicknesses and material type no longer needs to be dominated by the hybrid, but can be optimized for other components or priorities. The flexibility provided by the technique is shown in the implementation of a 2–18 GHz hybrid on a large range of substrate thicknesses as well as on a non‐optimal substrate material, for which acceptable measured performance is still obtained.

Broadband double‐ridged guide horn (DRGH) antennas are extensively used in antenna measurement and electromagnetic compatibility and interference testing, especially the 1–18 GHz DRGH antenna which is widely accepted as a standard for this band. Certain deficiencies in the radiation patterns have been identified and corrected by several authors, but the use was still limited to the 1–18 GHz frequency band. The incorporation of absorber materials and lenses has resulted in horn antennas with wider bandwidths; however, this complicates the manufacturing process and restricts these designs to lower power applications. Simulated and measured results for a new 0.5–18 GHz (36:1) DRGH antenna are presented here. The wider bandwidth is made possible by a new cavity design and optimising the design of the other antenna sections to allow wideband operation without using a lens or absorber. The new design has double the bandwidth ratio and is very compact, with an aperture size of 26.4 cm × 15.2 cm; the aperture sides are less than 12% larger than the sides of the aperture of the conventional 1–18 GHz DRGH antenna (24.2 cm × 13.6 cm).

This communication illustrates the ability to perform bistatic radar cross-section (RCS) measurements at a fixed bistatic angle in a compact range. Literature regarding bistatic RCS measurements in compact ranges is limited. The traditional setup of a compact range was adapted to perform bistatic RCS measurements. These bistatic measurements were conducted on canonical and complex realistic scale airframe models. The targets were illuminated with a plane wave created by an offset parabolic dish reflector. The bistatic scattering of the targets was measured by placing a receive antenna at a fixed bistatic angle and finite distance in the compact range. This communication also investigates the effect of the finite separation between the targets and the receiver on the bistatic scattering measurements of large complex targets. The accuracy of the bistatic RCS measurements is compared to full-wave simulations conducted with FEKO using the multilevel fast multipole method solver. Quantitative comparisons are drawn between the simulations and measurements using the feature selective validation method.

Many studies have suggested that a fully polarimetric radar or a bistatic radar can improve the classification and recognition of airborne targets due to enhanced target information. This paper makes use of the information theoretic concept of mutual information (MI) and investigates the comparison of the MI content between polarimetric monostatic and bistatic radar cross section (RCS) measurements of a 1:25 Boeing 707 scale model. It aims to quantify the increases in the MI content, and hence recognition performance, due to the additional reflected signals available in a fully polarimetric radar or in a bistatic scenario.

Grating lobe mitigation was achieved for planar, wideband dual-polarized Vivaldi arrays through the rotation and translation of equilateral pentagonally shaped subarrays that form an approximately circular array. The subarray outlines can be adjusted in order to included fewer elements per subarray, but this will result in less grating lobe reduction. By using dual-polarized elements optimized polarization in the main beam can be achieved. The array patterns were determined with both measured and simulated single element patterns of the dual-polarized Vivaldi element. The mutual coupling between the elements in the array was shown to be negligible.

A low profile dual-circularly polarized printed ring-slot antenna with a small footprint, which radiates above an open cavity loaded with an artificial magnetic conducting reflector, is presented. The feed network consists of two T-shaped capacitive feed structures connected to a miniaturized hybrid branch-line coupler. Experimental results for a final antenna design (with a size of $0.5\lambda _{\mathrm {\mathbf {0}}} \quad \times 0.5\lambda _{\mathrm {\mathbf {0}}} \quad \times 0.057\lambda _{\mathrm {\mathbf {0}}}$ ) show a 4% isolation bandwidth between the two ports of the dual-circularly polarized antenna and a maximum gain of approximately 6.8 dBic. Good front-to-back ratios and low cross-polarization were achieved. These very compact and low profile antennas are suitable for 2.4 GHz wireless local area network (WLAN) communication systems.

This letter presents a dual-band slot antenna with high gain for wireless local area network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX) communication covering the IEEE802.11a/b and IEEE802.16d/e standards. The antenna consists of a microstrip-fed slot with a complimentary stub on a single dielectric medium. A reflecting ground plane is employed to achieve a unidirectional radiation pattern with an average gain of 9.1 dBi. The feed of the antenna is optimized to achieve radiation efficiency above 95% in the WLAN and WiMAX frequency bands. The radiation properties of a prototype antenna were measured in a compact antenna range. Simulated and measured results (impedance bandwidth, gain, and radiation patterns) are presented for the proposed antenna.

Many studies have suggested that bistatic radar can improve the classification and recognition of airborne targets due to enhanced target information. This paper makes use of the information theoretic concept of mutual information (MI) and investigates the comparison of the MI content between monostatic and bistatic radar cross section measurements of a 1:25 Boeing 707 scale model. It aims to quantify the increases in the MI content, and hence recognition performance, due to the additional reflected signals available in a bistatic scenario.

The traditional Double Ridged Guide Horn (DRGH) Antenna consists of three sections: coaxial-to-ridged waveguide transition, cavity and a ridged waveguide flared section. Different variations of the coaxial-to-ridged waveguide and cavity sections of DGRH antennas are available to improve the VSWR to a 2:1 performance. In this paper three different profiles, exponential, Bezier and elliptical, are investigated for the ridges in the flared section, to obtain improved gain and radiation pattern performance for DRGH antennas. The DRGH antenna with an elliptical profile has a mark able increase in gain values, at the lower frequencies of the band.

We present the design of a dual-band slot antenna with high gain for wireless local area network (WLAN) covering the IEEE802.11a/b and IEEE802.116d/e standards. The antenna consists of a feed line with a complementary strip-slot pair and a parasitic slot-element and has a size of $57\times 40\ mm^{2}$ . The antenna is combined with a single-band artificial magnetic conductor (AMC) surface to achieve a unidirectional radiation pattern with an average gain of 9.6 dBi. The size of the antenna with the AMC reflector is $68\times 53\ mm^{2}$ . The proposed antenna was designed and simulated. The simulation results show that the antenna operates from 2.39 – 2.50 GHz and 4.90 – 5.97 GHz, which covers the 2.4, 5.2 and 5.8 GHz WLAN bands. It has a directional radiation pattern with peak gains of 7.5, 10.3 and 10.9 dBi at 2.4, 5.2 and 5.8 GHz, respectively.

Either passive calibration targets are expensive and complex to manufacture or their bistatic radar cross section (RCS) levels are significantly lower than the monostatic RCS levels of targets such as spheres, dihedral, and trihedral corner reflectors. In this paper the performance of an active calibration target with relative high bistatic RCS values is illustrated as a reference target for bistatic RCS measurements. The reference target is simple to manufacture, operates over a wide frequency range, and can be configured to calibrate all four polarizations (VV, HH, HV, and VH). Bistatic RCS measurements of canonical targets, performed in a controlled environment, are calibrated with the reference target and the results are compared to simulated results using FEKO.

The radar cross section (RCS) of passive calibration targets is relatively low, which leads to a commensurate increase in the uncertainty of the radar's calibration. To address this deficiency, an active radar calibration (ARC) target was developed, which was mounted on a small remote controlled tri-copter and used for radar calibration. Results are presented for this relatively small, light weight and cost effective airborne ARC, which is suitable for the calibration of a ground to air RCS measurement system. The static RCS characteristics of the airborne ARC target were measured in a compact range and compared to outdoor measurements with the ARC target mounted on the tri-copter. The airborne capability of the calibration target was used to reduce the effects of multi-path and clutter. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1041–1044, 2016

A design approach for printed planar slot array antennas is presented. The antenna array consists of half-wavelength slot radiators positioned on a rectangular grid, and a slotline feed network. Three planar slot array antennas for use in IEEE 802.11a applications are designed, a 2 × 2, a 2 × 4, and a 4 × 2 array, all radiating above an electric conductor ground plane placed a quarter-wavelength below the printed slots. These slot arrays have higher aperture efficiencies and occupy less space than typical microstrip patch arrays. The measured impedance bandwidths of the designed unidirectional slot arrays were 19.8, 15.3, and 16.7%, respectively, with peak gains of 11.7, 13.9, and 14.4 dBi. Measured results show very good agreement with the simulated results, which serves as validation of the array design procedure and the accuracy of the simulated results.

Conventional rectangular waveguides are commonly used for high power and other microwave and millimeter-wave applications. Their use at lower frequencies has been limited by their bulky nature. A new type of compact waveguide called a partial $H$ -plane waveguide has previously been proposed that has only one quarter of the cross-sectional area of a conventional waveguide. However, only limited information relating to the feeding of such waveguides is available. This paper presents two types of transitions to partial $H$ -plane waveguides from the coaxial and microstrip transmission mediums. Additionally, in the coaxial case, the transition is end-launched to offer collinear properties. The development of both transitions is discussed and optimized designs presented with simulated and measured results over $H$ -band (3.95–5.85 GHz).

An approach where polar curves are used to miniaturize hybrid couplers is presented. The design of such couplers can be done with the optimization of a few parameters using full-wave analysis software. The results show similar or better electrical performance for a rat-race and branch-line coupler with, respectively, a 75 and 50% size reduction. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2637–2640, 2015

Ultra-wideband (UWB) communications technology increasingly plays an important role in modern communications systems. The complex antenna transfer function (CATF) of a UWB antenna provides valuable information that can be used for better UWB channel and communication system designs. Currently, only the two-antenna and three-antenna measurement techniques are used to measure the transfer functions and impulse responses of UWB antennas. In this paper, a modified version of the gain-transfer method is presented to enable the characterization of UWB antennas using a compact antenna test range facility (CATR). A double-ridged guide horn (DRGH) antenna was used as a UWB transfer-standard, and the complex antenna transfer function of the transfer-standard was determined a priori using full-wave simulations. The gain-transfer method to measure the complex antenna transfer function of UWB antennas in a compact antenna test range was illustrated with two test cases. In the first test case, the measured complex antenna transfer function of an ETS-Lindgren double-ridged guide horn antenna was verified with numerical data from a FEKO model of the antenna. The second case compared the complex antenna transfer function of a UWB circular disc monopole antenna measured in a compact antenna test range to measured data using two identical antennas in a two-antenna measurement setup.

This paper illustrates the feasibility to perform bistatic RCS measurements in a conventional monostatic compact range that has been reconfigured to perform bistatic measurements at relatively large bistatic angles. The bistatic RCS measurements are conducted by illuminating the target with an incident plane wave generated by an offset parabolic reflector. The bistatic scattering of the target is measured with a wideband double-ridged horn receive antenna that is positioned at the side of the chamber at various fixed bistatic angles. In this study bistatic RCS measurements were conducted on canonical structures as well as a realistic complex scale aircraft model. The bistatic RCS measurements are compared to the full-wave multi-level fast multipole method (MLFMM) in FEKO.

The accuracy with which various full-wave and asymptotic CEM techniques can calculate the RCS of lossy dielectric objects is investigated. This is conducted through comparison to measured RCS data. The investigated methods include MLFMM, FEM, PO and RL-GO. Different lossy dielectric materials are used to construct canonical targets. The RCS of these targets are measured in a compact range at the University of Pretoria, South Africa. The material properties of these lossy dielectrics are accurately characterised using a waveguide technique. Accurate CAD models of the dielectric objects are constructed and used for simulations. The calculated RCS results are compared to the measurements to determine the accuracy of the various methods. The performance and computational efficiency of these methods are also investigated.

When calculating the radar cross section (RCS) of electrically large complex objects with computational electromagnetics tools, computer aided design models of the objects are required. Guidelines regarding the level of fidelity with which the models need to be constructed to achieve specific levels of accuracy, however, remain unclear. In this paper the effects of microgeometry features on the RCS of two electrically large canonical objects are investigated in order to help clarify when such features become important to include.

The wideband gain and voltage standing wave ratio (VSWR) performance of quad-ridged horn antennas in general are restricted due to the excitation of higher order modes in the coaxial to quad-ridged waveguide transition. This results in large gain variations vs. frequency and break-up of the radiation patterns at higher frequencies. In this paper, a quad-ridged horn antenna is presented with optimized radiation characteristics and VSWR performance. The performance of the antenna was optimized by separating the antenna into two subsections, viz. a coaxial to quad-ridged waveguide transition section and a flared quad-ridged horn section. The performance of each section was optimized individually using a numerical solver – CST Microwave Studio. Three possible configuration of the coaxial to quad-ridged waveguide transition section were considered and optimized. Four configurations for the flared quad-ridged horn section were also investigated. The best performing configuration from each one of the two section...

We present a quad-ridge horn antenna as a feed for a reflector antenna for use in radio astronomy applications. The antenna uses elliptically shaped sidewalls to limit the variation of beamwidth over a wide frequency range and to obtain greater radiation pattern rotational symmetry. The antenna is dual-polarized and matched over more than a 4:1 bandwidth. A design procedure is presented and a prototype designed according to this procedure is shown. Measured and simulated results of the prototype agree with each other well. The antenna is analyzed with a prime focus reflector to determine the range of efficiencies that can be expected.

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.

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.

The analysis and design of an omnidirectional antenna that functions over a very broad frequency band is presented. Design guidelines for this antenna have been derived using electromagnetic analysis software. Measured results of a prototype developed according to these guidelines are presented. These results are compared to simulation results to illustrate the validity of the model used to derive the empirical design guidelines. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1352–1356, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25990

Quad-ridge horn antennas of various geometries are considered as possible candidates for a wideband reflector antenna feed. It is demonstrated that shaping the sidewalls of the ridge horns significantly improves the radiation characteristics of the horns. The conical quad-ridge horn with an elliptically shaped sidewall shows the most promise as a wideband reflector antenna feed. The radiation patterns are rotationally symmetric, and the 10-dB beamwidth is reasonably constant over a wide frequency range.

Experimental measurements that accurately simulate the effect of a retrodirective cross-eye jammer on a monopulse radar are described. The accuracy of a recently published extended analysis of retrodirective cross-eye jamming and the limitations of the conventional phase-front analysis of cross-eye jamming are illustrated by the experiments.

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 classic gain-transfer method assumes a uniform plane wave incident over the apertures of the gain-transfer standard (typically a standard-gain horn) as well as the antenna under test. Variations in the incident field over the quiet zone of an antenna test range can produce large errors in the estimated gain of the antenna under test. These errors can be significant, especially when there is a large difference in aperture dimensions between the antenna under test and the standard-gain horn. In this letter, a modified gain-transfer method is described to reduce errors in gain measurements when antennas significantly larger than the standard-gain horn are measured in a test range using the gain-transfer method. The aperture of the antenna under test is usually much larger than that of the standard-gain horn, and therefore these antennas will typically be exposed to different variations in the incident field. Measured results are presented to illustrate accurate gain measurements for antennas significantly larger than the standard-gain horn.

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. ...

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.

A double-dipole antenna design is proposed for wireless local area network (WLAN) applications. Two parallel dipoles are used to facilitate operation in both the standard WLAN frequency bands (IEEE 802.11b and IEEE 802.11a) simultaneously. The lower band dipole is rhombus shaped and radiates above a planar ground plane, whereas radiation from the rectangular higher band dipole is directed through the use of a passive director. The antenna is characterized in terms of impedance bandwidth, gain and radiation patterns through simulations and measurements. The design addresses the need for dual-band operation, whereas delivering directional radiation patterns with adequate gain, low cross-polarization, and good front-to-back ratio. The 2.4/5 GHz WLAN bands are covered with an impedance bandwidth of 17.8 and 26.4%, respectively, for a VSWR better than 2:1. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2034–2038, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24540

This paper presents a dual-band dual-polarized antenna array design for WLAN applications. Four double-dipole elements are orthogonally interleaved to facilitate operation in both the standard WLAN frequency bands (IEEE 802.11b and IEEE 802.11a) simultaneously. The two linear polarizations have separate ports. The presented design is characterized by dual-band operation, reasonably good front-to-back ratios, average gains of 5.2dBi and 6.2dBi over the 2.4 and 5.2GHz bands respectively, stable end-flre radiation patterns and very low cross-polarization levels.