Mechanical Design and Construction

Various fabrication processes were considered to realise the complex structure of the antenna.

Additive Manufacturing (AM) is now a new trend to synthesize a 3D object. AM technology can vary from one technique to another, and the most common ones include Stereolithographic (SLA), Selective Laser Sintering (SLS), Fused Deposition Modelling (FDM), Direct Metal Laser Sintering (DMLS) and 3D printing. In 3D printing, successive 2D layers of material of choice are formed upon one another under computer control to create an object. DMLS, metal laser cutting and milling, 3D printing and waterjet cutting techniques were used to prepare various parts of the antenna.

As shown in Fig. 6.1 before the fabrication of the antenna, the design has to be revised. In order to fabricate the design practically, mechanical considerations have to be taken into account. This involves dividing the design into pieces that can be assembled after fabrication. One of the concerns with fabricating horn antennas is the quality of welding the pieces together. Any discontinuity on the surface of the pieces will cause reflections of propagating signal. To minimise this, large parts of the design were fabricated in one piece; this includes the double ridges and the conducting walls. Figure 6.2 shows the aperture matched conducting walls and how it is slotted inside the double ridges. The double ridge is then secured and maintains a good connection to the conducting walls by consecutive screws placed along the ridge. To join the pieces together, an interlocking mechanism is used where two pieces form a right angle with a minimum gap from the inside and are screwed and held together from the outside. Figure 6.3 shows how the rectangular waveguide is formed. The top and bottom pieces of the rectangular waveguide is cut together with the conducting walls and it is bent to form the correct profile. The side pieces of the rectangular waveguide and conducting walls are cut separately and are joined.

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Figure 6-2: Fabricated conducting walls and Double Ridges of the Horn Antenna

Figure 6-3: The interlocking mechanism to join the pieces together

The next step is to construct the feed for the double ridges. Since the height of the ridges is 7.1 cm, no off-the-shelf SMA coaxial connector was available to feed the ridges. Instead, a semi-rigid coaxial cable was used to make a 50 Ω connection line. The dimensions of the dielectric and inner conducting pin were re-entered in the simulation design to ensure the integrity of the performance is maintained. The holes inside the ridges were made by special drilling machines in the workshop to obtain a relatively smooth surface. Figure 6.4 shows how the feeding mechanism is implemented inside the double ridges.

Figure 6-4: Feeding mechanism of the double ridges

121 The inner pin of the feed was particularly decided to be inserted deeper into the opposite arm of the double ridges to keep a stable connection and preserve the symmetry of the structure. The third step in the fabrication process is to prepare and attach the Band 2 antenna (thin dielectric ridges) since the structure has to be open to access and place them inside. The thin ridges are fabricated on a Rogers substrate and cut in a way that can be placed inside the horn structure perpendicular to the double ridges. Figure 6.5 shows the PCBs and how they are mounted inside the structure.

The side walls of the rectangular waveguide have been modified with a dielectric support to hold the PCBs in place. The SMD resistors and standard 50 Ω SMA connectors are added on, the substrate and the copper surface cleaned and dried before being fixed inside the structure.

Figure 6-5:

The design of the back cavity structure is realised by laser cutting and milling techniques. Before closing the cavity back structure, the RAMs are introduced and kept in place with an adhesive material. The RAMs are cut according to the dimensions of each serrated walls inside the cavity.

Figure 6.6 illustrates the cavity back and how it is connected to the rest of the horn structure.

Figure 6-6: The back cavity structure

122 The smaller rectangular waveguide is connected to complete the back of the horn structure. The waveguide is formed by the same mechanism as the larger waveguide, and the coaxial SMA connector is screwed on. Figure 6.7 shows the cavity back the smaller rectangular waveguide assembled and how it is fed from an extended dielectric coaxial SMA connector.

Figure 6-7: Smaller rectangular waveguide

To complete the horn structure, the dielectric filling of the double ridges are prepared. The profile of the dielectric shape resembles the inner mouth of the ridges and is cut accordingly. The sheet is 8 mm thick TEFLON (PTFE) material and is cut with a drill bit of 3 mm in diameter. Fig. 6.8 shows the profile of the dielectric filling sheet and how it is secured in place with two clamps that were designed and cut to keep the sheet in place from both sides. This is to ensure the gap between the sheet and the ridges is minimised.

Figure 6-8: Dielectric filling of the double ridges

123 To reduce signal leakage and increase the conductivity of the structure, all intersections, the surface of the conducting walls and double ridges were covered with CuPro-Cote Paint. This is a copper particle conductivity in a water based paint and has a surface resistivity of less than 1 Ω/sq at 1 mm dry film thickness. Copper paint offers a good trade-off between cost and performance as opposed to silver paint and gold plating (Willis, 2012). It is reported by Willis (2012) that increase in a number of layers of copper paint will increase the gain of the horn structure at certain frequencies. It is suggested that the structure is painted 1-2 skin depths thick to minimise the losses in the structure. The skin depth is calculated at several frequencies across all three bands, and the antenna was covered in 3 layers of paint. All the intersections were also covered by copper tape from the outside as well as paint from the inside. Figure 6.9 shows the complete structure. The aperture matched walls were supported and secured by 3D printed blocks (from ABS plastic material) that are screwed on the structure outside.

Figure 6-9: The complete fabricated horn structure

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