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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).

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.

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.

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

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.

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.

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.

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.

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.

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.

Design results of a small, low profile printed ring-slot antenna with uni-directional radiation properties will be presented. A single linearly polarized, a dual linearly polarized, and a dual circularly polarized version of the antenna will be shown. The ring-slot antenna radiates above an open cavity loaded with an artificial magnetic conducting (AMC) reflector (see Figure 1). This work is an extension of a previously proposed cavity-backed linear slot antenna (J. Joubert, J.C. Vardaxoglou, W.G. Whittow and J.W. Odendaal, IEEE Trans AP, vol. 60, no. 2, Feb. 2012, pp. 735–742). The newly proposed ring-slot radiator is significantly smaller than the linear slot radiator, and has the ability to be fed in such a way that dual linear and dual circularly polarized radiation can be achieved.

The edge wave diffraction model for pyramidal horn antennas is extended to include the finite radii of curvature of the primary incident wave approximated by an astigmatic ray tube in determining the edge diffraction fields contributing to the radiated fields. The model is adapted to calculate the effective near-field directivity of a pyramidal horn antenna for generating standard electromagnetic field strengths for the calibration of field probes. The predicted directivity results for a range of standard gain antennas are verified with method of moments data and measured values obtained in a near-field antenna range.

Improvements proposed over the years in the edge-diffraction model for a pyramidal-horn antenna have been extended to include the three-dimensional nature of the pyramidal horn in evaluating the edge-diffraction fields. The theoretical model was compared with measured results, measured over a number of years at an international standards laboratory for frequencies ranging from 2.45 GHz to 18 GHz, with measurement uncertainties between /spl plusmn/0.04 dB and /spl plusmn/0.1 dB.

A synthesis method is modified to synthesize patterns with multiple nulls in an otherwise omnidirectional pattern for cylindrical microstrip patch arrays. The directive radiation pattern of a cylindrical microstrip patch is incorporated into the synthesis procedure. The effect of the number of array elements and interelement spacing is investigated. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 32: 442–446, 2002.

A number of papers have been published on the prediction of backscatter from engine-inlet-type structures. Measured data used to verify these predictions mostly consists of backscattered data from canonical structures, e.g., cylindrical waveguide cavities. In this paper, measured backscattered data for an unmanned aerial vehicle-type engine-inlet system is presented. The effects of the application of radar-absorbing materials to the engine-inlet system on the contribution to the RCS is discussed, and data measured in a compact-range facility are presented.

An important condition that has to be satisfied when implementing the gain-transfer method is that the fields in the test zone of the measurement facility should be as close to the plane wave as possible. In this paper, the effect of amplitude and phase deviation from a perfect plane wave, on gain measurements for microwave aperture antennas, conducted via the gain-transfer method, is determined and quantified. The pyramidal horn antenna is used as a basis for all calculations as it is the universal standard for microwave antenna gain measurements. Coupling, between the antenna being illuminated and the test zone fields, is evaluated by means of the reciprocity theorem. Test zone field variations are simulated and the effect thereof, on the predicted measured gain, is illustrated.

A few widely used primary incident wave models for pyramidal horn antenna analysis are compared on the basis of gain calculations. The analysis is based on edge-wave diffraction theory, as presented in a previous publication, with some modifications made to the underlying theory. It is shown that these models agree well for high-gain horns with small flare angles, but that the differences are more profound for lower gain horns, where the flare angles are larger. The accuracy of the various models are studied by comparing the results to actual measurements. The respective effects of amplitude and phase variation in the primary incident wave are also illustrated.

The effect of back-plane distance on the mutual admittance between twin coplanar waveguide (CPW)-fed slots on conductor-backed two-layer substrates with an air bottom layer was investigated. Distances of λo/6 and λo/4 yielded significantly increased mutual coupling over the case of no back plane. Measurements supporting the numerical results are presented. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 47: 407–409, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21184

The design and radiation characteristics of a wideband quarter-wave patch with a single-layer capacitive feed are presented. The E-plane radiation pattern of the antenna is shown to be very dependent on the size of the ground plane, and the results for two extreme cases are obtained in order to illustrate this effect. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 45: 225–227, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20777

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.

A technique to estimate the frequency dependence of electromagnetic scattering centers is proposed. The technique employs a superresolution algorithm to estimate poles corresponding to the various scattering centers contributing to the total backscattered field. A relationship between the amplitudes of the estimated poles and the frequency dependence of the scatterers is derived. The technique is applied to estimate the frequency dependence of simulated and measured backscattered field data. © 1995 John Wiley & Sons, Inc.

The high computational load associated with multidimensional high-resolution algorithms is overcome by using only 1D LP techniques. The 3D backscattered field is approximated by three 1D AR models that are solved in succession for the down-range, horizontal cross-range, and vertical cross-range positions of the scattering centers of a target.

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.

A compact quad-ridged horn antenna is presented. This antenna was designed by separating the antenna into the co-axial to ridged waveguide transition and the flared horn sections. The two sections were simulated and optimized separately and afterwards combined to form a complete unit. It is shown that the complete antenna's performance is a combination of the performance of the two sections. The antenna is dual-polarized and a return loss that is greater than 10dB is achieved across a 15.7∶1 bandwidth.

A compact quad-ridged horn antenna is presented. This antenna is smaller, in terms of wavelength, than previously published designs. The antenna is dual-polarized and a return loss of greater than 10 dB is achieved across a 15.7∶1 bandwidth.

It is shown that the far-out sidelobes of a horn antenna can be suppressed by curving the edges in the aperture. A three-dimensional uniform geometrical theory of diffraction (UTD) analysis of the E-plane radiation pattern of the modified horn antenna was performed and compared with straight edges. This analysis indicates that the curvature of the edges does indeed influence the sidelobes to the effect that the modified horn antenna has significantly lower far-out sidelobes. The radiation patterns for modified horns were measured and compared with the calculated patterns and were found to be in good agreement. >

Different attachment modes for use within the spectral-domain moment-method analysis of probe-fed microstrip patch antennas are investigated. These modes are used for the accurate analysis of probe-to-patch junctions and, in this paper, two of the most widely used approaches are compared in terms of their ability to model patches of various sizes on substrate materials ranging from quite thin to relatively thick. It finally turns out that the subdomain approach is somewhat more versatile than the entire-domain approach.

A new free-space characterization method is proposed to evaluate the performance of absorber material in terms of both its specular reflection and creeping wave performance. A conducting cylinder is chosen as the reference object to generate a creeping wave contribution in the backscattered field. The specular reflection and creeping wave are separated in time due to a path-length difference. This time difference allows the use of time-domain techniques to characterize the absorber material. The individual attenuations of the two components are obtained instead of the attenuation of the total backscattered field. The new measurement technique will be implemented in a compact range to evaluate the performance of commercial radar-absorbing material (RAM).

When implementing the gain-transfer method to measure the gain of an antenna, it is tempting to assume that there should be no measurement errors due to non-plane wave effects in the test zone fields, given that both the test antenna and the standard gain antenna have equal aperture dimensions. The reasoning behind this is that both antennas are exposed to the same amplitude and phase variations in the test zone fields, and therefore the measurement errors should cancel out. However, in this paper it is shown through simulated gain measurements, that equal aperture dimensions can indeed result in significant measurement errors if the incident wave is not plane.

A method whereby the aperture field distribution of a horn antenna can be measured is proposed. The method is based on the measurement of the scattered field of a small conducting sphere in the aperture. The position of the phase center of the aperture field is calculated using the measured phase distribution of the field in the aperture via a simple type of convolution. In principle, the method is suited to determine the field distribution of other types of apertures illuminated by an electromagnetic field. >

Measured Radar Cross Section results obtained in an indoor static measurement facility are presented for various wing configurations. The study serves to characterize the internal scattering from electromagnetic transparent wings. The effect of various parameters e.g. spars, ribs, etc. on the scattering contribution from the wing will be demonstrated using spheres with flat plates to simulate the internal contributors to the backscattered field.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The radar cross section (RCS) of microstrip patch antennas is studied. Various RCS reduction techniques are described. These are investigated using finite element method codes. The results of a series of compact range RCS measurements are discussed. Good agreement between measured and computed RCS is observed.

A high-gain dual-polarised planar etched slot array for use in wireless local area networks (WLAN) is presented. Two sets of four collinear slots are used as radiators for each polarisation. Each set of four collinear slots is constructed as a centre-fed long slotline, with etched crossovers to split the slotline into four resonant and in-phase half-wavelength slots. A reflector etched on a separate substrate spaced a quarter-wavelength away from the slot array ensures unidirectional radiation. The final antenna achieved a gain of 14.5 dBi, a return loss better than 10 dB, and port isolation in excess of 30 dB over the 2.4-2.484 GHz WLAN frequency band.

Measured and simulated results for a planar 2×2 slot array corporately fed by a slotline feed network are presented. The antenna array has a coplanar-waveguide input port and the slots are etched on the finite ground plane of a single substrate. The coplanar-waveguide transmission line is transitioned into a four-way slotline feed network, which directly feeds the four half-wavelength slots from the narrow edges of the slots. An experimental array achieved an impedance bandwidth of more than 12%, stable bidirectional radiation patterns with low cross-polarisation, and a maximum gain of more than 8 dBi.

The radiation efficiency and impedance bandwidth of matched broadside twin slot dipoles, fed by a coplanar waveguide on a conductor-backed two-layer substrate, are investigated as a function of distance between the slots and height of the bottom substrate layer. It is confirmed that the low radiation efficiency associated with a single slot due to the excitation of parallel-plate modes may be greatly increased by using two slots spaced half a wavelength of the dominant two-layer parallel-plate mode apart. It is also demonstrated that the radiation efficiency of optimally spaced twin slots may be further improved by increasing the height of the bottom substrate layer, in general at the cost of decreased bandwidth. The effect of bottom substrate height on gain and directivity is investigated, and radiation patterns are provided for the case of optimally spaced twin slots. Twin slots on a conductor-backed two-layer substrate can yield high radiation efficiencies similar to those of previously reported twin slot antennas with a back plane, with the advantage of reduced overall height.

A new feeding mechanism is presented that overcomes the usual problems associated with matching an annular-ring antenna. It consists of a probe-fed rectangular strip that is capacitively coupled to the annular ring. Numerical and experimental results for gain, bandwidth and radiation patterns are presented.

Results are presented for antenna gain measurements in a compact antenna range at a frequency below the minimum specified operating frequency of the range. Hardware gating is used to reduce clutter or unwanted scattering from the backlobe of the feed and the walls of the chamber. Measured time domain data are presented which prove that these stray signals can be eliminated from the incident field in the test zone, and frequency data are presented to show the improvement in measured antenna gain.

The results are presented of a synthesis procedure for omnidirectional radiation patterns with multiple nulls for circular arrays of vertical dipoles. The gain ripple in the omnidirectional region can be controlled by the use of window functions in the design procedure. The results are verified using the numerical electromagnetics code.

Radiation efficiency of twin slots fed by coplanar waveguide on a conductor-backed two-layer substrate with high-low dielectric permittivities is investigated. It is shown that radiation efficiency can be significantly increased compared to the radiation efficiency of a single slot when the slots are appropriately spaced. Radiating slot feed lines are explicitly accounted for.