A set of microstrip lines are introduced between the driven element and the first director element to enhance the coupling between them, and therefore the bandwidth could be increased and the back lobes could be suppressed. The microstrip Yagi array has advantages of low profile and simple structure and can easily be fabricated on a PCB with shorting vias.
In this paper, we propose a new type of microstrip Yagi array based on quarter-wave patch antennas. The quasi Yagi array based on classical dipole antennas has a high gain, a wide bandwidth, and a main beam pointing exactly at endfire, but this type of Yagi arrays could only generate horizontal polarization. This type of antenna can generate a vertical polarization in the horizontal plane. The microstrip Yagi array of half-wave patch antennas provides a high gain and has its main beam tilted away from the broadside. Recently, Yagi arrays of printed antennas have been studied, since printed antennas have a low profile, light weight, and easy fabrication. In mobile communications, vertical polarization is usually preferred, since transmitter and receiver can keep the same vertical polarization for good connection no matter how transmitter or receiver rotates on a horizontal platform.
Nonetheless, the Yagi array of monopole antennas has a high profile of (where is the wavelength in free space). Yagi array of monopole antennas is also developed that produces a beam close to endfire and with a vertical polarization in the horizontal plane. However, the classical Yagi array of dipole antennas has a high profile (about half wavelength) if it is set for generating vertical polarization. YAGI-UDA arrays of classical electric dipole antennas are famous and widely used, because they provide a high gain and have a simple structure with only one driven element. An increase of the number of director elements would enhance the gain and have the main beam pointing closer to endfire. Measured results show that the Yagi array with 4 elements generates a peak gain of about 9.7 dBi, a front-to-back ratio higher than 10 dB, and a 10 dB return loss band from 4.68 GHz to 5.24 GHz, with a profile of 1.5 mm and an overall size of 80 × 100 mm 2.
A main beam close to endfire is produced, with a vertical polarization in the horizontal plane. The Yagi array has a low profile, a wide bandwidth, and a high gain. A new kind of Yagi array of quarter-wave patch antennas is presented.