# Novel Beam Scan Method of Fabry-Perot Cavity (FPC) Antennas
## 2 Materials and Methods  
### Operation Principle of the Proposed Antenna  
Overviews of the FPC antenna based on the proposed design method and its beam-scanning scheme are presented in Figure 1. The proposed antenna consists of a tapered PRS superstrate and an AMC ground plane, forming an FP cavity. By satisfying an FP resonance condition in the cavity and adequately adjusting the reflection phase of each AMC cell, the antenna can successfully produce a scanned beam with fairly high gain in the desired target direction of \(\theta=30^{\circ}\) on the \(xz\)-plane (\(\phi=0^{\circ}\)). For a feeding antenna, a \(y\)-polarized microstrip patch antenna is employed instead of a slot antenna or others for easy fabrication [19]. Figure 1: Overviews of (**a**) the proposed FPC antenna that can scan the main beam on the \(xz\)-plane (\(\phi=0^{\circ}\)) and (**b**) its beam-scanning scenarios. Figure 2 describes the conventional and the proposed operation principles of an FPC antenna for a scanned beam. For the FPC antenna, there are two kinds of fields distinguished by the PRS; one is being transmitted through, the other is being reflected. By carefully utilizing the reflected fields to generate an FP resonance in the cavity, the antenna can obtain high gain in a target direction [6]. Now we are going to discuss how to scan a main beam in the FPC antenna. To obtain the scanned beam in the target direction, all fields transmitted through the PRS should be in-phase on the tilted target wavefront. For that, we can consider the following two methods:  
1. A phase-controllable PRS superstrate (see Figure 2a);
2. A phase-controllable AMC ground plane (see Figure 2b). First, let us consider method 1 which is used in previous works. In [11; 12; 13; 14; 15], a phase-controllable PRS is devised to make transmitted fields constructively interfere on the inclined target wavefront. However, there is a critical problem that a beam-scan direction is unpredictable as there is no precise design formula. Moreover, method 1 is not suitable for commercial applications, requiring wide beam-scan capability due to the limitations of a PRS: the narrow reflection and transmission phase ranges. In fact, in method 1, the PRS should have wide reflection and transmission phase ranges with a high reflection magnitude to provide various beam scan-directions as well as high gain [6]. However, in general, it is extremely difficult for an ordinary PRS to realize broad phase ranges while maintaining high reflection magnitudes [12; 13; 14; 15]. Though we can use a stacked PRS consisting of two or three layers for the wide ranges, it is obviously unfavorable in obtaining high gain (or high efficiency) due to high material losses [20]. To overcome these problems, we propose method 2 which introduces a phase-controllable AMC ground plane. The phase-controllable AMC is far more desirable than the phase-controllable PRS for various beam-scan directions because the AMC generally provides a much wider reflection phase range with very low loss compared with the PRS [21]. Moreover, another strong point of the AMC is its biasing configuration for electrical beam-scanning. To understand this, we need to revisit the configuration of electrical beam-scanning antennas. As we know well, they employ active RF chips such as varactor- or PIN-diodes that require DC bias lines. Varactor-diodes, for example, are usually selected for continuous beam-scanning as their electrical properties are continuously varied according to the applied reverse DC voltage. When installing the varactor-diodes on the PRS, bias lines will be in the cavity, as described in Figure 3a. In this case, the fatal problem is that the FP resonance becomes extremely difficult  
Figure 2: Beam-scanning principles of (**a**), a conventional FPC antenna, and (**b**) the proposed FPC antenna. To obtain a directional beam in the target direction, transmitted fields should be in-phase on the target wavefront.