CONDICIONES DE USO, MANTENIMIENTO Y SEGURIDAD Condiciones Generales:

The double-ridge waveguide consists of a pair of ridges symmetrically placed at the centre of the rectangular waveguide, parallel to the sidewall. In double-ridged horn antennas, the waveguide section interconnects the horn section with the feed section. The excitation method for double-ridged horn antennas is commonly achieved by using a coaxial connector. Therefore, it is necessary to use a transition between the coaxial probe and the double-ridged rectangular waveguide. The transition between the coaxial probe and the double-ridged waveguide is important to the performance of the antenna. The principal goal is to obtain low levels of VSWR throughout the transformation of the TEM-mode in the coaxial section to the TE-mode in the waveguide. Figure 5.14 (a) shows the general structure of the waveguide section with the coaxial line inserted into one of the ridges. The excitation source as port 1 in the previous design step is replaced by a shortening wall also known as the cavity back. The cavity back on the waveguide helps maintain a low S11 at the lower frequencies of operation. To achieve a low S11, the effect of the cavity back length 𝐶_𝑙, the initial distance between ridges in the rectangular waveguide, probe spacing from the ridged edge 𝑃_𝑠 , the tapering and chamfering the back end of the ridges, and the coaxial probe offset were all studied and optimised. A standard rectangular cavity back design is shown in Fig.

5.14 (b). A model SMA connector and a coaxial line is introduced in the waveguide (Fig. 5.14 c and d) and regarded as the source of the excitation signal (port 1). The input impedance of 50 ohms is matched from the source of the excitation signal to the coaxial connector.

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Figure 5-14: Coaxial line to double ridge transition

From the optimisation process, it was found that the probe spacing from the ridged edge, 𝑃_𝑠 affects the gain of the waveguide (from 𝑆₂₁) and shaping of the main lobe at high frequencies when incorporated with the horn section. Numerous simulations have been done to optimise the transitional performance using parameter optimiser approach to cover the 0.5 to 5.5 GHz frequency band. It is very common to use a cavity back to obtain a much lower return loss in coaxial to double-ridged waveguide transitions. It was found that the VSWR of the waveguide is critically dependent on the shape and dimensions of the cavity back.

Figure 5.15 illustrates the E-field at 2.5 GHz (a) & 5 GHz (b) and surface current distribution at 2.5 GHz (c) inside the waveguide structure. It can be seen there is a backward radiation from the double ridges towards the shortening wall. The reflection back towards the double ridges introduces phase error and distorts the propagation of the signal between the double ridges. This is more severe at higher frequencies.

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Figure 5-15: E-field distribution inside a cavity backed double-ridged rectangular waveguide at 2.5 GHz (a), 5 GHz (b), and surface current distribution at 2.5 GHz (c).

To improve the performance of the waveguide, a new design for the cavity back is proposed and realised. As shown in Fig. 5.15 (c), the backward surface current travels along the edge of the ridges and the shortening walls. To stop this, a serration technique is employed where the walls surrounding the ridges are loaded with slots to minimise and absorb the travelling surface current.

The serrated space in the walls is then filled with high quality Radar Absorbent Material (RAM).

This design is integrated into a pyramidal shaped cavity with an extended shortening wall to furfur reduce the unwanted reflection. The free space RAM (ECCOSORB FGM 125) is considered and simulated according to the specification provided by the Laird Technology. Figure 5.16 shows the basic principles of the RAM and the design that has been employed. When a wave is illuminated onto RAM, part of the incident wave is reflected off the medium, and part is transmitted through the medium. Both reflected and transmitted waves are modified in amplitude and phase based on the material properties. Attenuation through the material is achieved by the dielectric, conductive losses and magnetic losses. If the surface of one side of the RAM is covered by a ground plane, then the transmitted wave will undergo a second reflection through the material back towards front face. When primary reflection and secondary reflection are identical in magnitude and 180 degrees out of phase, reflection minimum occurs. Magnitude cancellation occurs when the thickness of the material is around a quarter of a wavelength.

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Figure 5-16: Radar Absorbent Material (RAM) principle

The quarter of a wavelength for the frequency of 500 MHz yields to thick RAM, of the order of a few centimetres. This increases the cost of the RAM as well as occupying larger space in the structure. To overcome this issue, the angle of the RAM inside the serrated walls in the direction of the incident wave is modified so the E-fields experience multiple reflections inside the walls.

Figure 5.17 (a) shows the overall design of the cavity back with integrated RAM and serrated walls. Figure 5.17 (b) illustrates the propagation paths of the backward radiation and how it is influenced by the angle of the serrated walls.

Figure 5-17: Design of the serrated walls in the cavity back

The cavity dimensions which are obtained by optimising its parameters such as the flare angle, the size of the inner and outer height of the cavity, the width and the length of the cavity. The parameters of the serration and its overall design have also been optimised towards a good trade-off between performance and the complexity of the structure (e.g. fabrication cost). The dimensions are shown in Fig. 5.18.

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Figure 5-18: Dimensions of the cavity back geometry

Figure 5.19, shows the effect of the modified design to reduce the backward radiation. As it can be seen, the E-field distributions towards the end of the cavity inside the modified waveguide structure (b) are considerably less compared to the standard cavity back design (a). The tapered conical structure with integrated RAM and serrated walls, show a good performance in absorbing the waves and reduction of the unwanted radiation towards the coaxial probes and the horn antenna. Therefore, this improves the impedance matching of the whole antenna structure.

Figure 5-19: Standard cavity back design (a), proposed cavity back design (b) in order to reduce unwanted radiations

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