Newsletter 5.4 May 215 Antenna Magus Version 5.4 released! Version 5.4 sees the release of eleven new antennas (taking the total number of antennas to 277) as well as a number of new features, improvements and bug fixes. In this newsletter we will briefly look at some of the new features and antennas that have been made available. More detailed information on these antennas as well as feature extensions can be found on the Antenna Magus website and in the full release notes. New Features Export models which support the CST STUDIO SUITE Student Edition have been added for 2 antennas (the pyramidal horn antenna and pin-fed circular patch antenna). This support is available in the Evaluation, Classroom and Professional versions of Antenna Magus. Scale by frequency before export It is now possible to scale designed values to a new frequency before exporting a model of the antenna to a 3D CEM tool. Where the frequency range of a specific antenna is too limiting and the antenna structure does not contain dielectrics, this scaling can be very effective. In cases where dielectrics are included in the antenna, some additional modification of the parameter values may be needed, but the scaled design does provide an excellent starting point. Decimation of imported trace data When importing data points from a file or from the clipboard into Antenna Magus, the number of imported points may be reduced by choosing to decimate the data. In the decimation process, the minimum number of points required to accurately reflect the underlying trace is determined and only these are stored. This makes data storage and processing a lot more efficient. The application of a scaling factor to a design in export mode to adjust the structure from 1GHz to 2GHz The decimation of data during import from a file or clipboard New Antennas In addition to a number of profiled horn antennas, the following new antennas have been added: Conical horn reflector (Cornucopia) Circular polarised circular patch with trimming stubs Axial-mode wire helix with linearly tapered ends Axial-mode helix with tapered ends on a conical ground plane Pin-fed 2-by-2 patch array with underside corporate feed The profiled horns The majority of the antennas released in this version are part of the profiled horn antenna family. These are: Profiled corrugated conical horn Profiled smooth conical horn with dielectric loading Profiled smooth conical horn Gaussian-profiled corrugated conical horn Sinusoidal-profiled (Bowl) corrugated conical horn Piecewise linear (PWL) spline-profiled pyramidal horn A conical corrugated horn antenna with various profiles: (a) sinusoidal, (b) tangential, (c) exponential, (d) hyperbolic and (e) polynomial By profiling the flare, the mode content in the aperture is tailored to arrive at a structure that radiates a symmetrical beam with a low level of crosspolarisation and the highest possible gain and efficiency. Various profile shapes, including cubic splines, as well as various analytical formulations (e.g. hyperbolic or sine-squared) may be used in smooth-walled, corrugated and dielectrically-loaded horns. Although a single profile may be applied to the entire length of the horn flare, different profiles may also be used in different sections of the flare to form more intricate structures with specific aperture distributions. If properly designed, profiled horn implementations should be more compact (in terms of axial length) than un-profiled horns, while maintaining acceptble performance characteristics. Profiled horns do, however, present certain disadvantages. An increase in co-polarised sidelobes due to the excitation of higher order modes generated by the variation in flare angle is typical (although this is generally only at frequencies in the upper portion of the band). Let us take a closer look at a few of these horns.
Profiled corrugated conical horn The profiled corrugated conical horn antenna may be designed for a number of profiles, including sinusoidal, tangential, exponential, hyperbolic and polynomial. Variables controlling the shape of the profiles include a profile power index (p) and a profile addition index (A), which are chosen (where applicable) as Sinusoidal (p=2, A=.8), Tangential (p=2, A=.9) and Polynomial (p=2). The various corrugated profiles all with an equal number of slots and flare diameter are shown in the Figure on the right. Although the introduction of a profile may result in a shorter structure, it does come at the expense of raised side lobe levels and low cross-polarisation, mainly due to the HE12 mode excited by the varying flare angle along the profile. The various profiles perform quite differently some better than the standard linear profile, and some worse. Typically, the sinusoidal and polynomial profiles provide the best compact alternative to the linear profile. Length comparison of 2 dbi design linear (left), hyperbolic (middle) and sinusoidal (right) Comparing structures designed for equal gain, it is clear how a correct profile results in a more compact structure, albeit at the expense of increased side lobes. Also, in cases where the profiled structure is comparatively sized to the linear one (see hyperbolic vs linear) the overall performance of the profiled version seems slightly better, with lower shoulders than the linear version. Normalised Gain [db] -1-2 -3-4 -5-6 Sinusoidal Linear Hyperbolic Normalised Gain [db] -1-2 -3-4 -5-6 Sinusoidal Linear Hyperbolic -7-15 -1-5 5 1 15-15 -1-5 5 1 15 E-plane (left) and H-plane (right) pattern comparison of 2 dbi profiled (sinusoidal and hyperbolic) vs. linear conical corrugated horns -7 Profiled smooth conical horn with dielectric loading The profiled dielectrically-loaded conical horn antenna uses two dielectric core materials to achieve low cross-polarisation and low sidelobes over a wide frequency range. Compared to corrugated horns, the dielectrically loaded horn has a simpler design and is easier to analyse; however, its drawbacks include the effects of dielectric losses. Overall, the structure is a suitable alternative to corrugated horn antennas. Profiling the flare may further improve the performance of the standard linear hybrid horn, already available in Antenna Magus. While this may result in a more compact structure at higher gains for certain profiles, it may also achieve worse performance than its linear counterpart for others. Available profiles for this antenna also include sinusoidal, tangential, exponential, hyperbolic and polynomial. Size comparison of an 18 dbi design without dielectric loading (a) linear, (b) tangential and (c) exponential A comparison of versions of the profiled dielectrically loaded vs linear dielectrically loaded vs profiled corrugated is shown. The introduction of the correct profile results in a more compact dielectrically-loaded structure, albeit at the expense of increased side lobes. Although potentially more difficult to machine, the corrugated horn equivalent still wins in terms of size. Size comparison of a 2 dbi design sinusoidal dielectrically loaded (left), linear dielectrically loaded (middle) and sinusoidal corrugated (right)
Normalised Gain [db] -1-2 -3-4 -5-6 Sinusoidal dielectric Linear dielectric Sinusoidal corrugated Normalised Gain [db] -1-2 -3-4 -5-6 Sinusoidal dielectric Linear dielectric Sinusoidal corrugated -7-15 -1-5 5 1 15-15 -1-5 5 1 15 E-plane (left) and H-plane (right) pattern comparison of the 2 dbi sinusoidal dielectrically loaded, linear dielectrically loaded and sinusoidal corrugated horn antennas -7 Gaussian-profiled corrugated conical horn The Gaussian-profiled corrugated conical horn antenna uses a combination of flare sections to improve the performance of the standard linear corrugated conical horn. The horn comprises of a circular waveguide section that transitions through a smooth linear section into a linear corrugated section, acting as a mode converter, followed by a straight phasing section, which in turn feeds into the Gaussian profiled section. At higher gain levels it is more compact and has better performance than its linear counterpart. Since the flare angle at the aperture is zero, the horn radiates with the highest possible gain and efficiency, while achieving a low level of cross-polarisation. For lower gain cases (< 2 dbi), the profiled horn described here is larger than the standard linear case, due to the overheads of the various sections that make up the overall flare. The sidelobe level is generally below -4 db at the center frequency, with a reduced performance in the upper regions of the frequency band. Typical total gain at the center frequency E-plane (left) and H-plane (right) pattern comparison of the 2 dbi Gaussian-profiled vs. linear conical corrugated horns
Sinusoidal-profiled (Bowl) corrugated conical horn The sinusoidal profiled corrugated conical horn antenna consists of a number of sections, namely, circular waveguide, mode converter, phasing section and a profiled Sinusoidal flare. The shape of the sinusoidal profiled flare is controlled by a profile power index, which should be smaller than one (1) for low sidelobes. The optimised power index value was determined to be.8 giving the flare its characteristic bowl shape. Below is a comparison between designs for the two performance criteria and the linear corrugated conical horn already present in Antenna Magus. The structures compared are designed for 2 dbi gain and it is clear that the profiled horns are shorter than the linear horn, by roughly 17%! Size comparison of 2 dbi design low sidelobes (left), symmetrical beam (middle) and linear profile (right) E-plane (left) and H-plane (right) radiation pattern comparison of the 3 structures Piecewise linear (PWL) spline-profiled pyramidal horn The geometry of a rectangular horn is more suitable for array applications, as the geometrical efficiency within the array cell is higher compared to a circular horn. Conventional pyramidal horns have an aperture efficiency of 5 %, but it is possible to achieve efficiencies close to 1 % by optimising the flare profile and, in turn, improve overall array efficiency. The piecewise linear spline profiled horn described here achieves an aperture efficiency of 8%. The horn consists of a rectangular waveguide section, a mode converter and a pyramidal flare. The mode converter consists of five PWL (piecewise linear) sections followed by the pyramidal flare to reach a specified aperture size. The bandwidth for the PWL spline profiled horn is less than a conventional pyramidal horn as it is limited by the pattern performance. For gains higher than 18 dbi the overall length is less than a conventional horn of the same gain. Normalised gain [dbi] -1-2 -3-4 -5-6 -15-1 -5 5 1 15 Typical radiation pattern at the centre frequency E-plane H-plane Other antennas Axial-mode wire helix with linearly tapered ends on flat and conical groundplanes The helical element of both antennas consists of a linearly tapered bottom section, a uniform middle section and a linearly tapered top section. The end-tapers serve to match the feed and the termination of the uniform helix to the feed line and free-space, respectively, while the central uniform helix operates as an ordinary axial-mode helical antenna. These matching techniques improve the axial ratio and impedance behaviour of the uniform helix by reducing the generation of unwanted modes. The conical ground plane reduces sidelobe levels and provides some control of the input impedance. A comparison of the gain versus angle response highlights the advantage of using a conical groundplane to suppress sidelobes.
Normalised polarisation-specific gain pattern comparison at the centre frequency Comparing the axial ratio of a standard vs. an end-tapered helix highlights the advantage of tapering the ends, albeit at the expense of gain. The impedance of the standard, end-tapered and end-tapered with conical groundplane shows how the latter has a reduced real impedance level which is essentially constant over the band, as well as a reduced reactance. Axial Ratio 1.9.8.7.6.5.4.3.2 Helix with Tapered Ends.1 Uniform Helix with Same Gain Uniform Helix with Same Total Number of Turns.8.85.9.95 1 1.5 1.1 1.15 1.2 Normalised frequency Impedance [Ω] 35 3 25 2 15 1 5-5 Real (Conical Ground Plane) Imaginary (Conical Ground Plane) Real (Flat Circular Ground Plane) Imaginary (Flat Circular Ground Plane) Real (Uniform axial-mode helix) Imaginary (Uniform-axial mode helix) -1.8.85.9.95 1 1.5 1.1 1.15 1.2 Normalised frequency Axial ratio (left) and input impedance (right) of various helix implementations Conical horn reflector (Cornucopia) The Conical horn reflector antenna, a.k.a conical cornucopia, is a modified version of the Pyramidal horn-reflector antenna already present in Antenna Magus. The Conical cornucopia is a combination of a conical electromagnetic horn and a parabolic reflector hence horn reflector. The conical cornucopia has found preference over his pyramidal counterpart due to structural advantages and the absence of the high diffraction lobes which the pyramidal cornucopia produces. The antenna has no frequency-sensitive elements, so performance bandwidth is limited only by the feed waveguide. Linear or circular polarisation is possible. Since the aperture is not partially obstructed, as is often the case with ordinary front-fed dishes, aperture efficiencies of 8%, as opposed to 55-6% for front-fed dishes, may be achieved. The disadvantage is that it is far larger and heavier for a given aperture area than a parabolic dish. Typical gain pattern at the centre frequency
Circular polarised circular patch with trimming stubs Dual-fed patches may be used to produce circularly polarised radiation but this requires the use of a feed network to provide equal excitations and a 9 phase shift between the ports. The pin-fed circularly polarised circular patch antenna with trimming stubs has the advantage of using a single coaxial feed, orientated at 45 with respect to the stubs to produce circular polarisation. Manufacturing tolerances can also be tuned out by trimming the stubs. Similar to the elliptical patch, the basic resonant structure is perturbed such that two, spatially orthogonal resonant modes are induced by a single feed pin. The axial ratio of the antenna has a narrow bandwidth, but approaches perfect circular polarisation at the centre frequency. Typical total gain pattern of RHC polarised patch at the centre frequency (left) and broadside axial ratio versus frequency (right) Axial Ratio 1.9.8.7.6.5.4.3.2.1.9.95 1 1.5 1.1 Normalised frequency Pin-fed 2-by-2 patch array with underside corporate feed This 2-by-2 patch array design in Antenna Magus combines the design of the individual rectangular pin-fed patch element with the design of a corporate microstrip feed network. Not only does Antenna Magus allow to design for a specific substrate, one can also design for input resistance between 5 Ohm and 15 Ohm. Two advantages of an underside corporate microstrip feed is a reduction in spurious feed network radiation and a reduction in antenna size (up to 3% in the E-plane when compared to the 2 by 2 microstrip patch array already present in Antenna Magus). The antenna performance example which follows for a design of 5 Ohm input impedance on a 2.9% (in the medium) thickness substrate with a relative permittivity of 2. Example of the feed network designed by Antenna Magus and the typical total gain pattern at the centre frequency S 11 [db] -5-1 -15-2 6 3-1 -2-3 33 3 9 27 E-plane H-plane -25 12 24-3.9.95 1 1.5 1.1 Normalised Frequency 15 18 21 Typical reflection coefficient versus frequency (left) and normalised total gain patterns in db at the centre frequency (right)