A new kind of Yagi array of quarter-wave patch antennas is presented. The Yagi array has a low profile, a wide bandwidth, and a high gain. A main beam close to endfire is produced, with a vertical polarization in the horizontal plane. 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. 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 mm2. An increase of the number of director elements would enhance the gain and have the main beam pointing closer to endfire.
YAGI-UDA arrays of classical electric dipole antennas are famous and widely used [
Recently, Yagi arrays of printed antennas [
In this paper, we propose a new type of microstrip Yagi array based on quarter-wave patch antennas. The microstrip Yagi array has advantages of low profile and simple structure and can easily be fabricated on a PCB with shorting vias. 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. An increase of the number of director elements would enhance the gain and have the main beam pointing closer to endfire. The front-to-back ratio of the presented Yagi array is higher than 10 dB. Compared with a half-wave patch antenna, a quarter-wave patch antenna has half the length of the half-wave patch, and therefore the Yagi array of quarter-wave patch antennas has a smaller length compared with the conventional Yagi array of half-wave patch antennas. The comparison between the Yagi array of half-wave patch antennas and the Yagi array of quarter-wave patch antennas will be discussed in the paper.
The structure of the Yagi array based four quarter-wave patch antennas is shown in Figure
Top view and cross-section of the microstrip Yagi array of quarter-wave patch antennas.
Radiation is mainly generated from the open apertures of the four quarter-wave patches (the open apertures opposite to the shorting vias). Since the tangential electric field at each open aperture can be considered as a magnetic current, the Yagi array can be considered as a Yagi array of magnetic elements. With respect to the principle of duality to the classical electric dipole Yagi array, the parasitic magnetic element would act as a reflector when it has an additional capacitive component and it would act as a director when it has an additional inductive component. The reactive component of each magnetic element can be controlled by adjusting the length of the quarter-wave patch (the distance from the open edge to the shorting vias of the quarter-wave patch). Therefore, a quarter-wave patch must have a smaller length in order to have an additional inductive component (in view at the open aperture), when it acts as a director. Otherwise, a quarter-wave patch would act as a reflector when it has a larger length. The lengths of the directors and reflector need to be tuned with simulation tools (such HFSS) to have optimum values and to have the array radiating in forward direction.
In order to make more power coupled into the first director from the driven element, three parallel microstrip lines with equal widths
The width
The length
Parameters for the Yagi arrays.
Yagi array of quarter-wave patch antennas (Figure |
Yagi array of half-wave patch antennas (Figure | ||
---|---|---|---|
Variable | Values | Variable | Values |
|
2.33 |
|
2.33 |
|
1.57 mm |
|
1.57 mm |
|
8.926 mm |
|
17.6 mm |
|
9.026 mm |
|
19 mm |
|
7.826 mm |
|
15 mm |
|
7.626 mm |
|
15 mm |
|
40 mm |
|
25 mm |
|
1.4 mm |
|
1.3 mm |
|
0.8 mm |
|
0.8 mm |
|
0.8 mm |
|
115 mm |
|
8.4 mm |
|
80 mm |
|
3.37 mm | ||
|
3.37 mm | ||
|
23.67 mm | ||
|
100 mm | ||
|
80 mm | ||
|
0.6 mm | ||
|
1.5 mm |
Simulated and measured results for the reflection coefficient are shown in Figure
Also shown in Figure
Figure
Figure
Figure
The photo of the designed Yagi array is shown in Figure
Photo for the 4-element Yagi array of quarter-wave patch antennas with the geometry shown in Figure
Reflection coefficients for the 4-element Yagi array of quarter-wave patch antennas without microstrip lines coupling, the 4-element Yagi array with microstrip lines coupling (Figure
Elevation (a) and azimuth (b) radiation patterns for the Yagi array (Figure
Measured elevation (a) and azimuth (b) radiation patterns for the Yagi array (Figure
Gains for the 4-element Yagi array (Figure
Photo of the 12-element Yagi array of quarter-wave patch antennas.
The reflection coefficient for the Yagi array with 12 radiators is shown in black dashed line in Figure
The gain of the antenna with 12 elements is shown in red line in Figure
Measured radiation patterns in the elevation (a) plane and in the azimuth (b) plane for the 12-element Yagi array (Figure
In this section, the Yagi array of quarter-wave patch antennas is compared with the conventional Yagi array of half-wave patch antennas. The geometry of the Yagi array of half-wave patch antennas is shown in Figure
Geometry of the Yagi array of half-wave patch antennas.
Simulated results for the reflection coefficient and gain for the 4-element Yagi array of half-wave patch antennas are shown in Figure
Comparisons between the Yagi array of quarter-wave patch Antennas and the Yagi array of half-wave patch antennas.
Character | Yagi array of quarter-wave patch antennas (Figure |
Yagi array of half-wave patch antennas (Figure |
---|---|---|
Band | 4.68–5.24 GHz | 5.0–5.43 GHz |
Fractional bandwidth | 11.3% | 8.25% |
Beamwidth (elevation plane) | 56–58° | 42–46° |
Beamwidth (azimuth plane) | 44–60° | 80–87° |
Gain | 8.65–9.9 dBi | 8.28–10.19 dBi |
Front-to-back ratio | >10 dB | >7.4 dB |
Radiation angle (from broadside) | 45–50° | 35–40° |
Radiation efficiency | 91–98% | 95–99% |
Size of driven patch | 8.926 × 40 mm2 | 17.6 × 25 mm2 |
Size of ground plane | 100 × 80 mm2 | 115 × 80 mm2 |
Reflection coefficient and gain for the 4-element Yagi array of half-wave patch antennas.
Elevation radiation patterns (a) in the
From Table
A new type of Yagi array of quarter-wave patch antennas has been presented and studied. The Yagi array has a wide bandwidth and a high gain and provides a vertical polarization in the horizontal plane. A Yagi array with 4 microstrip quarter-wave patch antennas is designed and measured. Measured results show that the Yagi array generates a gain of about 9.5 dBi and a bandwidth of 11.3%, with an overall size of
Compared with the classical Yagi array of half-wave patch antennas, the presented Yagi array of quarter-wave antennas has a smaller length, a slightly wider bandwidth, a beam closer to endfire, and a higher F/B ratio. However, the efficiency of the Yagi array of quarter-wave patch antennas is not as high as that of the Yagi array of half-wave patch antennas. Both types of Yagi arrays have almost the same gains.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported in part by the Research Project of Guangdong Province (2012B090600009).