# A Compact Planar Wi-Fi Antenna with Optimized Radiation Patterns for Small UAV Applications
###### Abstract
In addition, both of them need an extra matching circuit to match their characteristic impedance to 50 \(\Omega\) for a better reflection coefficient at the operation frequency. The low-profile, quasi-omnidirectional substrate-integrated waveguide (SIW) multi-horn antenna was proposed [5] for small non-metallic UAV applications. It has good vertically polarized radiation and a low profile of 0.028 \(\lambda_{0}\) at 2.4 GHz. However, it does not have stronger radiation below the UAV, and its lateral dimensions of \(0.76\lambda_{0}\times 0.76\lambda_{0}\) are too large for small UAVs. Due to their low profile and ability to conform to planar and nonplanar surfaces, patch antennas were proposed [6; 7] for UAV applications. The high-gain dual-mode cylindrical conformal rectangular patch antenna [6] has a low profile of 0.508 mm (0.0175 \(\lambda_{0}\) at 10.323 GHz), stronger radiation below the quadcopter UAV, and good vertically polarized radiation. The required radius of the arm of the UAV is 25 mm at the lowest operating frequency of 10.288 GHz, and three patch antennas are needed on the UAV arm to cover the hemisphere below the UAV. The conformal patch antennas [7] have a low profile of 3 mm (0.058 \(\lambda_{0}\) at 5.8 GHz) and stronger radiation below the UAV. However, their maximum radiations are directly above each patch. Both models proposed multiple patch antennas on the UAV to ensure a stable link in the required directions. However, it takes up too much surface area, making them unsuitable for small UAVs. The design of a combined printed helical spiral antenna and helical inverted F antenna [8] was miniaturized by co-winding the radiation elements on the same ceramic rod with a height of 24.7 mm (0.129 \(\lambda_{0}\) at 1.57 GHz) and a radius of 6 mm. It was proposed for UAVs with a global positioning system (GPS) L1 band and telemetry communication frequency band (2.33 GHz) applications. However, it does not have stronger radiation below the UAV at 2.33 GHz. The low-profile broadband plasma antenna [9] has different radiation patterns at different operating frequencies and a low profile of 0.105 \(\lambda_{0}\) at 30 MHz. Its radiation pattern is omnidirectional at 30 MHz and stronger radiation below the UAV at 300 and 500 MHz. However, it must be matched with extra Butterworth low-pass and high-pass filters to isolate the 13.56 MHz radio frequency signal to activate plasma. Its complex structure and the need to use special materials such as plasma will result in a heavier weight and higher cost, making it unsuitable for small UAVs. The broadband slotted blade dipole antenna [10] radiation patterns vary at different operating frequencies. However, it must be matched with a special matching circuit and does not have stronger radiation below the UAV. In addition, its radiation patterns at all operating frequencies have nulls near the horizontal plane, causing the smallest gain when the UAV flies to the farthest position. Due to the advantages of simple structure and low manufacturing cost using modern printed circuit technology, printed planar antennas were proposed [11; 12; 13] for UAV applications. The planar dual-mode dipole antenna [11] has a short electrical length of 0.3297 \(\lambda_{0}\) at 0.86 GHz and good vertically polarized radiation. The omnidirectional vertically polarized antenna [12] also has good vertically polarized radiation and varying radiation patterns at different operating frequencies. However, neither has stronger radiation below the UAV, and their maximum radiations are near the horizontal plane. The wideband single-sided folded-off-center-fed dipole antenna [13] has good vertically polarized radiation and different radiation patterns at different operating frequencies. At 5 and 5.5 GHz, it has stronger radiation below the small UAV, and its maximum radiations are below the UAV and between 10\({}^{\circ}\) and 30\({}^{\circ}\) from the horizontal plane. At 3.5 GHz, it has quasi-omnidirectional radiation, and its maximum radiations are near the horizontal plane.