Newsletter 2016.2

Antenna Magus Version 2016.2 released!

Version 2016.2 sees a number of improvements as well as the addition of new antennas. This newsletter highlights some of these additions. More information can be found in the full release notes..

New Antennas

The new antennas are:

  • Centre-fed linear resonant waveguide slot array with longitudinal broad-wall slots
  • 4 Waveguide-fed dual polarized pyramidal horn antenna
  • Waveguide-fed Smooth Spline Profiled Pyramidal Horn
  • Waveguide-fed pyramidal Horn array with reduced sidelobe levels
  • Six Resonant Mode Loop Antenna

Centre-fed linear resonant waveguide slot array with longitudinal broad-wall slots
Image of the Centre-fed linear resonant waveguide slot array with longitudinal broad-wall slots.

The centre-fed resonant waveguide slot array is a variation of the end-fed resonant waveguide slot array, already contained in Antenna Magus. The centre-feed is a popular feeding approach for planar arrays, using an underside feed guide below the radiating guides. The feed guide is orientated perpendicular relative to the linear arrays that make up the planar array, feeding each of them at their centre via an inclined slot in the (common) broad-wall.

The advantages of the centre-fed option include simpler implementation of complex power divider networks in the feed, as well as better pattern stability (related to beam peak squint) just off resonance.

The antenna produces a fan beam in the plane of the waveguide cross-section, with a maximum at broadside. The bandwidth is rather narrow, in terms of impedance and pattern quality.

Total 3D gain at the centre frequency for different numbers of slots using a Villeneuve distribution.
4 Waveguide-fed dual polarized pyramidal horn antenna
Image of the 4 Waveguide-fed dual polarized pyramidal horn antenna.

Many applications require an antenna which can transmit/receive two orthogonal polarisations. A common technique to achieve either this, is the addition of ridges to the horn antenna. Machining costs are expensive due to the high precision needed in the antenna. The 4 waveguide-fed pyramidal horn achieves this specification and is relatively inexpensive to manufacture.

The structure consists of four individual rectangular waveguide sections which are connected and separated by a central metallic block which extends the length of the waveguide. Each waveguide is flared in both directions to create an aperture which forms a cross. The waveguides are excited in parallel pairs (ports 1&3 or ports 2&4).

Due to the in phase excitation of the two waveguides in the dominant mode and the cross shaped design of the flare, a more symmetrical aperture field distribution in the central ‘square’ of the aperture may be achieved. This results in a symmetrical radiation pattern with good cross polarisation characteristics.

The reflection bandwidth is limited by the frequency range of the waveguide section. Good cross polarisation is achieved over this bandwidth, with beam symmetry limited to a region centered around the design frequency.

Typical radiation pattern at the centre frequency; Typical normalized radiation patterns at the centre frequency.
Waveguide-fed Smooth Spline Profiled Pyramidal Horn
Image of the Waveguide-fed Smooth 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 smooth 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 control points for the profile of both the width and height. The aperture’s width and height itself, forms the sixth control point for the horn’s width and height profile. A spline curve is then fitted through these control points to form the geometry of the horn.

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; Typical normalized radiation patterns at the centre frequency.
Waveguide-fed pyramidal Horn array with reduced sidelobe levels
Image of the Waveguide-fed pyramidal Horn array with reduced sidelobe levels.

Low sidelobes are beneficial for many antenna applications. Reduction in sidelobe level for a single horn can be achieved by increasing the flare length and adjusting the the aperture dimensions accordingly, however, this leads to a larger structure. In arrays the sidelobe levels are normally reduced through specific excitation distributions.

The waveguide-fed horn array consists of three horns with a simple uniform feed. The reduction in sidelobes are achieved through a flatter phase distribution across the aperture due to the combined effect of three smaller apertures.

The bandwith of a conventional horn is limited by the waveguide’s bandwidth, but this horn array’s bandwidth may be limited even more by its pattern performane.

Typical radiation pattern at the centre frequency; Typical normalized radiation patterns at the centre frequency.
Six Resonant Mode Loop Antenna
Image of the Six Resonant Mode Loop Antenna.

The need for multiband antennas to cover various bands like GSM/LTE, WiFi, GPS etc. are becoming increasingly higher. While PIFA/IFA antennas are very popular their unbalanced modes have high ground plane surface current which make them sensitive to user interaction. The balanced modes of loop antennas on the other hand have less surface current which make them more suitable for user interaction. However, these balanced modes have smaller bandwidths than those of conventional PIFAs/IFAs.

The six mode loop antenna operates at six modes. Four of these modes are due to the loop antenna itself and the other two modes are from the addition of two parasitic elements. The lowest loop mode’s bandwidth needs to be extended using a matching circuit, while the others are increased by widened sections in the loop. The modes from the parasitic elements extends the upper operating frequency range of the antenna.

Typical total gain radiation pattern in dB at fmin, 3.76fmin and 6.52fmin
Reflection coefficient in dB versus frequency overlaid with popular LTE bands