3.3 Dipole Antenna Modules and 20K Interface
The ALPACA dipole was carefully designed by taking into consideration the wide bandwidth requirement along with careful electromagnetic simulations to consider mutual coupling for a given array geometry. These efforts ensure that the focal plane phased array feed would have an optimum sensitivity and access to a large instantaneous field of view. The dipole array’s geometry, performance and the survey speed metric are discussed in Section 3.1. This section focuses on the mechanical design and manufacturing of the dipole antenna, its interface with the ALPACA cryostat and, design features implemented to exploit symmetry and minimize number of unique parts to deliver an assembly that is hot-swappable, ensures excellent thermal connection to cool the LNAs and is cost-effective.
The dipole assembly has several critical dimensions to ensure optimal performance. Each of these have been carefully accounted for in the individual parts and the total stack-up tolerance. Some of the most critical dimensions are identified in Figure 1 and are required to be maintained within +/-0.1 mm (0.004”).
The absolute height of the top disk from the ground plane (52.8 mm) is achieved by using off-the-shelf stand-offs measuring 2-5/64” (52.78 mm). We have rigorously measured, cataloged and binned them using a micrometer for use in the assembly. The wedge arm (shown in silver in Figure 1) is the most critical part in the dipole assembly and two pairs of wedge arms provide sensitivity to two orthogonal, linear polarizations. The relative spacing between it and the top disk (0.8 mm) is essential to guarantee the performance of the dipole. The signal carrier connecting the wedge arm to the cryo-LNA is implemented as an air coax; a solid core placed in a hollow tube. The top end of this hollow coax tube also acts as a support for the wedge arm. The diameter of the through hole in the coax tube is tightly controlled, 0.109 +/- 0.001” to ensure that the air coax has a nominal impedance of 50 Ohms. Potential errors in concentricity of the coax and the hole in the coax tube have negligible effect on the impedance. The center of the top coax needs to be 3.14 mm from the top disk and the two coax wires are separated by 1.7 mm (center to center distance). Conforming to these critical dimensions ensures that the mechanical assemblies would be in compliance with the model used in the rigorous electromagnetic simulations conducted to predict the performance of the ALPACA array using CST.
20K Base Plate Interface
Figure 2: Left: The ALPACA dipole array as seen from top runs across the three sections. The 20K interface is built with a 120 degree symmetry so that the dipoles can be plugged in while keeping relative alignment across the three sections. Right: Shows the details of the individual dipole assembly.
The dipole arms needs to maintain the same relative orientation across the three sections of the array. The rest of the cryostat parts are all identical and rotated by 120 degrees exploiting the symmetry.
The cold bus allows a sliding fit with the base of the dipole module (also aluminum) at room temperature. Slits are cut into the cold bus to provide radial compliance, similar to a collet on a machine tool. As the system cools down, the Nylon ring radially contracts on the joint since the integrated thermal contraction, from room temperature to 20 K, of Nylon is much greater than that of aluminum (over a factor of 3). The ring strongly clamps the aluminum components together ensuring good thermal conduction across the joint and thereby keeping the LNAs well thermally sunk. Combined with the modular design of the antenna/LNA units, the cryo-clamp simplifies field repairs and facilitates future upgrades.
Prototype ALPACA Dipole Assembly
To ensure manufacturability and deliver the strict tolerances required, we worked with a CNC vendor to prototype the entire assembly. We critically analyzed all the parts and sub-assemblies with particular attention to machining, assembly and alignment and used that feedback to revise the parts before initiating the production run (which is underway, with all parts expected to be delivered in Oct 2021).
Figure 3: Left: Shows the dipole assembly/LNAs and the 20K interface into which it plugs. Right: Shows a fully assembled prototype dipole plugged into the 20K interface assembly. The Nylon ring contracts as the temperature lowers and clamps the cold fingers to the dipole assembly. Most of the dipole assembly components and the LNA package will be gold plated.