Newsletter 5.4

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 in the full release notes.

New features

A number of small features have been added with this release. Here is a short overview of two of them.


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 where 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.

Scaling parameter values before exporting a 3D CEM model.

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.

An illustration of the decimation of data during import

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

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 the 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
Image of the 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=0.8), Tangential (p=2, A=0.9) and Polynomial (p=2). The various corrugated profiles – all with an equal number of slots and flare diameter – are shown in the Figure above.

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.

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.

E-plane (left) and H-plane (right) pattern comparison of the 20 dBi sinusoidal dielectrically–loaded, linear dielectrically–loaded and sinusoidal corrugated horn antennas
Profiled smooth conical horn with dielectric loading
Image of the 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.

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 20 dBi design – sinusoidal dielectrically–loaded (left), linear dielectrically–loaded (middle) and sinusoidal corrugated (right)
E-plane pattern comparison of the 20 dBi sinusoidal dielectrically–loaded, linear dielectrically–loaded horn and sinusoidal corrugated horn
H-plane pattern comparison of the 20 dBi sinusoidal dielectrically–loaded, linear dielectrically–loaded horn and sinusoidal corrugated horn
Profiled smooth conical horn
Image of the Profiled smooth conical horn.

The performance of smooth-wall horns may be improved through a number of techniques, one of which involves profiling the flare. The ease of manufacturing at the millimeter-wave range is a huge advantage, as the fabrication of corrugated horns can become extremely costly and difficult. Other advantages include controlling the mode conversion and improving the symmetry and side lobe level of the radiation pattern.

Compared to the linear conical horn, some of the profiled horns have improved pattern symmetry and reduced sidelobe levels, while others do not. It was found that the tangential and exponential profiles provide the best alternative to the linear profile. Available profiles for this antenna include 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=0.3), Tangential (p=2, A = 0.6) and Polynomial (p=4).

The introduction of the correct profile results can provide a more symmetrical beam, but at the expense of size.

E-plane pattern comparison of the 18 dBi profiled (tangential and exponential) vs. linear conical corrugated horns
H-plane pattern comparison of the 18 dBi profiled (tangential and exponential) vs. linear conical corrugated horns
Gaussian-profiled corrugated conical horn
Image of the 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. The phase centre also does not change with frequency. [Olver] For lower gain cases (< 20 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 -40 dB at the center frequency, with a reduced performance in the upper regions of the frequency band.

The advantages do carry a price, namely that of increased co-polar sidelobes due to the excitation of higher order modes, although this is generally only at frequencies in the upper portion of the band.

Typical total gain at the center frequency
E-plane pattern comparison of the 20 dBi profiled vs. linear conical corrugated horns
H-plane pattern comparison of the 20 dBi profiled vs. linear conical corrugated horns
Sinusoidal profiled (Bowl) corrugated conical horn
Image of the 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. According to [Granet], the power index should be smaller than one for low sidelobes. The optimised power index value was determined to be 0.8 giving the flare its characteristic ‘bowl’ shape.

This antenna may be designed for two different radiation performance criteria, namely, low sidelobes or a symmetrical radiation pattern. Depending on the selected performance, the slot-to-ridge width ratio is designed to be roughly 3:1 (for symmetrical beamwidth design) and 4:1 (for low sidelobe level design), whilst the pitch (slot width + ridge width) remains constant. The pitch is roughly equal to a tenth of a wavelength at the centre frequency [James].

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 20 dBi gain and it is clear that the profiled horns are shorter than the linear horn. The profiled horns are roughly 17% shorter.

Size comparison of 20 dBi design – low sidelobes (left), symmetrical beam (middle) and linear profile (right)
E-plane radiation pattern comparison of the 3 structures
H-plane radiation pattern comparison of the 3 structures
Piecewise linear (PWL) spline-profiled pyramidal horn
Image of the 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 approximately 50 %, but it is possible to achieve efficiencies close to 100 % 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 approximately 80 %.

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 the bandwidth 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.

Typical radiation pattern at the centre frequency
Normalised radiation pattern cuts at the centre frequency

Other antennas

Conical horn reflector (Cornucopia)
Image of the 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 at 90 degrees [Johnson].

The antenna has no frequency-sensitive elements, so performance bandwidth is limited only by the feed waveguide; and linear and circular polarisation is possible. Since the aperture is not partially obstructed, as is often the case with ordinary front-fed dishes, aperture efficiencies of 80%, as opposed to 55-60% 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.

Total gain pattern at the centre frequency
Typical gain versus angle performance
Circular polarised circular patch with trimming stubs
Image of the 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 90 degrees phase shift between the ports [Long et al]. The pin-fed circular polarised circular patch with trimming stubs antenna has the advantage of using a single coaxial feed, orientated at 45 degrees 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 a RHC polarised patch at the centre frequency
Typical broadside axial ratio versus frequency

S. A. Long, L. C. Shen, D. A. Schaubert and F. G. Farrar, “An experimental study of the circular-polarized elliptical printed-circuit antenna”, IEEE Transactions on Antennas and Propagation, vol. AP-29, no. 1, Jan 1981, pp. 95–99.

Axial-mode wire helix with linearly tapered ends
Image of the Axial-mode wire helix with linearly tapered ends.

The Axial-mode wire helix antenna with linearly tapered ends consists of a linearly tapered bottom section, a uniform middle section and a linearly tapered top section, mounted on a circular ground plane.

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 behavior of the uniform helix by reducing the generation of unwanted modes. [Angelakos and Kajfez]

Comparing the axial ratio and impedance response of this antenna to an ordinary axial-mode helix highlights the advantage of tapering the ends. One disadvantage is that the tapered ends reduces the gain when compared to an ordinary axial-mode helix.

Polarisation-specific gain pattern at the centre frequency
Axial ratio of antenna with tapered-ends compared to ordinary axial-mode helix
Input impedance of antenna with tapered-ends compared to ordinary axial-mode helix

D. J. Angelakos and D. Kajfez, “Modifications on the axial-mode helical antenna”, Proceedings of the IEEE, vol. 55, no. 4, pp. 558–559, Apr. 1967.

Axial-mode helix with tapered ends on a conical ground plane
Image of the Axial-mode helix with tapered ends on a conical ground plane.

This helix antenna consists of a linearly tapered bottom section, a uniform middle section and a linearly tapered top section, mounted on a conical ground plane.

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 behavior of the uniform helix by reducing the generation of unwanted modes. The conical ground plane further reduce sidelobe levels, while also allowing for impedance control. [Angelakos and Kajfez]

A comparison of the gain versus angle response highlights the advantage of using a conical groundplane to suppress sidelobes. The impedance versus frequency response illustrates that the input resistance of the antenna using the conical groundplane is reduced and essentially constant over the band. The conical groundplane also reduces the negative reactance of the antenna.

Polarisation-specific gain pattern at the centre frequency
Typical normalised circularly polarised gain versus angle pattern at the centre frequency for the antenna on a flat circular- and conical ground plane
Typical input impedance versus frequency for the helix on a conical ground plane and on a flat circular ground plane

D. J. Angelakos and D. Kajfez, “Modifications on the axial-mode helical antenna”, Proceedings of the IEEE, vol. 55, no. 4, pp. 558–559, Apr. 1967.

Pin-fed 2-by-2 patch array with underside corporate feed
Image of the 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 an input resistance between 50 Ohm and 150 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 30% in the E-plane when compared to the 2 by 2 microstrip patch array already present in Antenna Magus).

Example of the feed network designed by Antenna Magus

The antenna performance example which below is for a design of 50 Ohm input impedance on a 2.9% (in the medium) thickness substrate with a relative permittivity of 2.

Typical total gain pattern at the centre frequency
Typical reflection coefficient versus frequency
Typical normalised total gain patterns in dB at the centre frequency