# Smart Antenna for Application in UAVs
## 2 State of the Art
### Communication Systems  
Currently, UAVs have incorporated a communication system capable of providing data to the Base Station (BS), such as location, image, and information of the conditions of the device. The communication between the UAV and the BS must be bidirectional, since it is also necessary to send control information from the Base Station to the UAV [6].  
Antennas are passive devices that, in a wireless system, are the forefront of emitters and receivers, presenting three fundamental properties: gain, directivity, and polarization. Given their passive characteristic, antennas redirect the energy they receive from the emitter to a certain direction (directional antennas) or radiate uniformly in all directions (omnidirectional antennas) [7].  
In the case of UAVs, antennas are dimensioned according to the needs of the aircraft, and the project is still limited by aerodynamic characteristics. Weight and robustness are fundamental characteristics, mainly in UAVs for military applications.  
The antennas normally used for this purpose are planar antennas, whip antennas, and loose-wire antennas. Planar antennas are the most frequently used since they can be used as omnidirectional  
Figure 1: Unmanned vehicle (UV) classification by field of performance.  
or directional antennas, which makes them very flexible. At the aerodynamic level, they do not significantly interfere with the structure.  
Whip antennas, despite their low cost, have aerodynamic problems and a low gain; these are classified as omnidirectional antennas. Finally, the loose-wire antenna, which, despite its high gain allowing greater range, sees its use limited by the aerodynamic slope and risk of damage to the UAV [4].  
The antenna to be studied is the planar antenna (Figure 2) because it allows for a change in the main propagation lobe of electromagnetic energy and improves the aerodynamic behavior of the aircraft, this being a crucial factor.  
Planar antennas are antennas used when antenna design constraints require reduced size and weight, low cost, and high efficiency.  
The main parameters that should be considered in this antenna project are: the relative dielectric constant of the substrate (\(\varepsilon_{r}\)); thickness \(h\) of the substrate and its size; patch configuration; and patch dimensions. These characteristics determine the radiation pattern of the antenna. Figure 3 shows a planar antenna with the above-mentioned characteristics.  
The feeding method used in the antenna design was the line-feed method, given the simplicity and ease of input impedance matching.  
One of several types of planar antennas are electrically steerable passive array radiator (ESPAR) antennas. These antennas allow to adapt the direction of radiation, constituted by \(N\) elements, where one element is active and the others passive elements coupled to the active. Mutual coupling is created by changing the spacing between the elements of the ESPAR antenna array and using reactance coupled with the passive elements [9].  
The ESPAR antenna uses the mutual coupling to excite the parasitic elements. These elements are charged according to a variable reactance originating from varicap diodes. The variation of the value of the reactance is reflected in the alteration of the main lobe of the radiation diagram [10]. The process of changing the main lobe is called beamforming.  
Unlike conventional antennas, the ESPAR antenna does not have individual lines for receiving/ transmitting because only one antenna element is connected to the circuit. ESPAR antennas are an  
Figure 3: Specification of a planar antenna [8].  
Figure 2: (a) Planar antenna [4]; (b) whip-monopole antenna [4]; (c) dragged-wire Antenna [4].  
increasingly used solution because it allows beamforming, i.e., switching the main lobe, maximizing the received signal, and minimizing interference [11].  
This article presents the construction of an intelligent antenna for a UAV using this technology.