I. Introduction
The GROWING interest in millimeter-wave and microwave applications has motivated research in manufacturing microwave components and devices such as waveguides, antennas, and mixers. Microwave waveguide assemblies are constructed by connecting various components with waveguides, which often results in numerous joints that increase the size and weight, and degrades the performance. On the other hand, machining high-precision waveguides imposes high manufacturing cost and difficulty especially for millimeter-wave and terahertz frequencies. Thus, a solution to manufacture light-weight waveguides and antennas at a low cost is needed. A more efficient way to manufacture microwave devices is using additive manufacturing methods such as three-dimensional (3-D) printing. Waveguide assemblies can be integrated into a single part without the need for additional interfaces, eliminating adapters [1], [2]. Furthermore, 3-D printing is able to create complicated structures with high precision without adding difficulty to the manufacturing process, thus reducing the cost for highly complicated components such as electromagnetic crystal waveguide [3], corrugated horn antennas [4], and structural electronics [5], [6]. Another advantage is the weight reduction. Traditional waveguides and antennas are made by machining highly conductive metal pieces such as copper. However, the amount of copper needed to achieve good performance is only a layer as thin as the skin depth from the surface. With the development of metalization techniques such as plating-on-plastic [7]–[10], plastic waveguides can be metalized to guide electromagnetic waves [11], [12]. Thus, 3-D printing combined with the plating-on-plastic technique offers a solution to make light-weight devices that are metalized only on the necessary surface. Though the cost and weight have been reduced, the performance of 3-D printed waveguides and antennas are comparable to the ones manufactured with the traditional methods [4], [13], [14].