# Waveguide Manufacturing Technologies for Next-Generation Millimeter-Wave Antennas
## 2 High-Precision Subtractive Machining  
Machining using high-accuracy subtractive means has been the classical approach for the manufacture of waveguide devices. Starting from the block of an electric conductor (typically aluminum or brass, although it is possible to use other metals with good electrical conductivity), the internal parts of the waveguides are carved out (typically by a drill, although, as will be discussed, other options are possible). These techniques normally provide a smooth finish for the internal walls of the waveguide, resulting in a low level of power losses. Additionally, as the waveguide device is built from a block of metal, it will demonstrate great mechanical robustness [20]. Moreover, subtractive manufacturing techniques provide the clear advantage of having been established for a long time; therefore, they present a high degree of reliability [30]. Although they typically require dividing the device into several parts and, in comparison to additive manufacturing, they are less flexible with respect to the geometries that can be built, these kinds of techniques should be chosen whenever possible. The main disadvantage of subtractive manufacturing is that it normally requires the division of the device into several parts that are manufactured independently, for later assembling to create the desired structure. This assembling process has two critical points: the alignment and the fastening of the different parts. A misalignment will alter the internal faces of the waveguides, which will affect the response of the device. To avoid this, it is necessary to introduce alignment pins into the design to guarantee proper positioning. Moreover, it is not only important that the number of pins is great enough, but also that they are placed at the appropriate positions in the structure. The parts that comprise the device are joined by means of assembling screws, conveniently located. It is very important that these screws tighten the different parts with enough strength to avoid any power leakage, which would increase the power losses of the device [31]. A recent work by the authors that illustrates the different possibilities offered by high-precision subtractive manufacturing can be found in [22], showing a novel monopulse horn antenna with a triple-radiation pattern. The antenna is comprised of three main components: the comparator network, the mode converter, and the horn itself [32]. A representation of the internal parts of this antenna is presented in Figure 1.  
Figure 1: Monopulse horn antenna [22]. The three components that form the antenna are highlighted by different colors: solid red for the comparator network; dashed blue for the mode converter; and dashed-dotted green for the horn. Each component of the antenna was built using a different subtractive machining technique. The horn was based on a splined profile, requiring higher accuracy than horns based on a simpler conical profile. It was manufactured using a high-precision lathe. Additionally, given the length of the horn (215 mm), it was of extreme importance to minimize any possible degradation of the symmetry of the component in its manufacturing. The comparator network was divided into two separated parts (i.e., the body with the routing and cover), which were built by means of using an H-milling plane along the direction of the width of the waveguides. Electrical discharge machining (EDM) was used for the waveguide sections corresponding to the ports at the external interfaces, as they were too long for the drill. A representation of this component, along with the main dimensions and the placements of the holes for the screws and alignment pins, is presented in Figure 2.  
It must be noted that this milling process does not produce perfect right angles at the internal corners, but rather a certain rounding. Therefore, it was important to study the effect of this characteristic before manufacturing the device.