AXIAL AXIAL AXIAL can be used to analyse combined circular and multiple coaxial structures with partial dielectric loading(optional). The dominant TE11 mode excitation of either a circular or coaxial waveguide is used as input. In principle, any circularly symmetric structure which can be decomposed into circular and coaxial regions can be modelled (Figure 1). The limitation to gain is the size of the aperture of the antenna in wavelengths and a peak gain of 40.0 dBi is probably the upper limit.
Typical structures (Figure 2) are
 Figure 1 Sample Circular Cross-Sections which can be implemented in AXIAL Yellow and Orange indicate loading with two sorts of dielectric |
 FIGURE 2 Sample Geometries for AXIAL. Yellow indicates dielectric loading of the structure |
The mathematical basis of the analysis program exploits a modal matching technique. The radiating structure to be analysed must be modelled as a number of discrete cylindrical segments supporting propagating as well as evanescent TE and TM circular or coaxial waveguide modes. For waveguides partially loaded with dielectrics intrinsic hybrid modes of the cross-section are utilised. The modal matching employs a scattering matrix approach to determine the complex coefficients of the modes at the aperture of the structure. The far-field radiation patterns are then obtained by summing individual radiation patterns of TE, TM and hybrid modes at the aperture, weighted by the complex mode coefficients.
The dominant TE11 mode excitation of either a circular or coaxial input waveguide is assumed.
The program suite AXIAL consists of a number of pre and post processor programs and a mode-matching analysis program AXIAL.
Input data files for AXIAL can be prepared interactively using the pre/post processor which incorporates many screen editing facilities. A waveguide/horn profile generator is also built into the pre-processor to enable repetitive generation of complex structures such as corrugated horns by simply altering a few key parameters. The profile of the resultant structure can be viewed and modified interactively prior to analysis. The computed radiation patterns of the horn can also be displayed on the computer screen and each radiation point can be examined.
A number of small utility programs to generate ring-loaded horn geometries and dielectrically loaded horn profiles are also included in the AXIAL package.
AXIAL was developed by Dr.Soe Min Tun of S.M.T. Consultancies Ltd. and is marketed exclusively by Antenna Software Ltd.
Two versions of AXIAL are available as executables. The full version incorporates dielectrics where multiple layers of dielectrics can be placed inside any of the coaxial and circular waveguide regions. The reduced version assumes uniform air-filled waveguide regions. AXIAL is available on these platforms.
A typical example is an axisymmetric Cassegrainian reflector with a splashplate feed [1], [2], [3]. The exciting waveguide with its dielectric is modelled as a circular waveguide while the exterior of the waveguide and the reflector are modelled as a coaxial section (Figure 3). The great advantage of this type of analysis is that is allows an optimisation to be carried out which is much more accurate than the usual ray tracing. The result is an ability to design a high efficiency (65%) reflector with sidelobes of -22 dB and low spillover (<-40 dB) when the aperture is relatively small in wavelengths. Examples up to 12 wavelengths have been designed. An example of aperture diameter 5.5 wavelengths is described in [3].
A very low sidelobe Potter horn has been designed and optimised using AXIAL (Figure 4 and Figure 5). AXIAL predicted the peak gain to be 22.55 dBi at 5.255 GHz and this has recently (2004) been measured by NPL, Teddington, UK for QinetiQ, Farnborough, UK to be 22.454 +/-0.05 dBi at 5.255 GHz.
Waveguide components can also be modelled. Figure 6 shows an Ortho Mode Junction to separate RX/TX at C-band.
Here are a splash-plate feed and a co-axial diplexer, designed using AXIAL.
Here is a brief presentation on two systems designed using AXIAL (originally given at ESTEC in October 1997 by Pat Foster)
AXIAL is available on these platforms.
FIGURE 3 Cross-section of a Cassegrainian Reflector with splashplate feed
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FIGURE 5 Comparison of measured and predicted patterns of Low sidelobe Potter Horn together with the template. Left is E-plane. Right is H-plane
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