This paper presents a low profile dual polarized directional antenna composed of loop and dipole arrays mounted on a ground plane with each loops and dipoles being fed independently. Each loop antenna is paired with a reflector while each dipole antenna is paired with a director and a reflector. The proposed antenna is intended for an indoor base station (BS) with resonance frequency of 2.4 GHz and capable of producing four orthogonal directional pattern with downward elevation angle equals to 30 ° ; and half power bandwidth (HPBW) less than 80 ° ; in both vertical and horizontal polarization. The reflection characteristics of the loop and dipole arrays are less than -10 dB and the mutual coupling between the vertical and horizontal polarization elements is nearly less than -20 dB. In later progress, the dipole antenna was substituted with printed dipole antenna to achieve a better performance. Both the calculated and measured results demonstrated that the desired radiation patterns were achieved, and the measured results agreed well with the calculated ones. Consequently, a low profile antenna with a thickness of 0.16 λ (20 mm) having the expected radiation pattern is successfully designed.
The communication systems today is facing an ever-increasing demand on providing a wireless system that is the capable of enabling higher voice and data rates while at the same times supporting higher capacity users. Especially for indoor communication systems, various researches regarding channel capacity enhancement were done to oversee this situation [
One of an important design consideration aside from high performance characteristics is miniaturization. A low profile structure is desirable because of easy installation and low costing. However, a notable problem regarding miniaturization of antenna is mutual coupling. As the arrangement of the antenna elements especially the spacing between the elements will be narrow, the mutual coupling between the elements will increase instead. One of the methods to address this issue is by using a matching network [
In this paper, we present a novel low profile directional antenna intended for an indoor base station (BS). Although few researches have been reported regarding this topic [
Although dual polarization was achieved in [
Channel capacity can be enhanced efficiently by utilizing the radiation pattern and the polarization of designed antenna [
In the initial stage of the research, we designed a single loop with a reflector as shown in
pole, simply by adding a reflector with the same height to the loop antenna we managed to suppress and shape the radiation pattern into beam-like pattern and tilt it into an angle. From
Another three pairs of loop and reflector arrays were added and arranged symmetrically to each other as shown in
These output performances are able to be maintained simply by adjusting the length of the loop antenna and the distance between this feeding element and the edge of the ground plane to 60 mm, without altering the height of the element. As a result, an antenna of 15 mm in height representing vertical polarization element is achieved. By just adding a reflector, a directional radiation pattern is achieved while at the same time, a lower mutual coupling compared to without reflector is delivered as confirmed in
Successfully confirming the characteristics of vertical polarization element with desired results, the next following step was to add the horizontal element into the structure to produce dual polarization elements as targeted. This was done initially by extracting just one element of the loop arrays (port 1) from the four arrays shown in
cluded that to obtain a good reflection characteristic, we need to set the dipole at a height of between 18 and 20 mm.
Upon completing the analysis, we proceeded to add other 3 dipoles antennas to the structure in
The results also show that dipole array exhibits a back lobe problem as opposed to radiation pattern for loop array.
When only the feeding element of the dipole was mounted, it radiation pattern forms a half circle shape pointing to z-direction due to the reflection from the ground plane. To obtain obliquely upward beam tilted, we first consider mounting director, which location was optimized by analyzing a few parameters. We confirmed that a director operated well when the distance between a director and a feeding element was 30 mm. For the distance between a feeding element of the dipole and that of loop, more than 25 mm was found to be desirable, as indicated from the results in
After simulating the structure, we were able to decide the working position for the reflector. When the reflector of the dipole antenna was positioned outside the loop antenna, nearer to the edge of the ground plane than the loop antenna, impedance matching is difficult to achieve and unfortunately the tilted angle is larger than the desired value. However, when the reflector was moved and positioned closer to the feeding element of the loop antenna, horizontal polarization is generated in dipole array. Therefore, reflectors of the dipole antennas need to be located inside the feeding elements of the loop antennas, positioned nearer to the center of the ground than the feeding elements of the loop antennas.
The added reflector gave benefit to dipole where the reflection characteristic is improved and the back lobe problem is eliminated as shown in
are obtained. The HPBWs for loop and dipole arrays are 57.1˚ and 70.9˚ respectively, while the tilted angles of the beam and gains are 61˚ and 52˚, and 8.4 dBi and 9.1 dBi respectively. The radiation patterns for the other ports also display the same characteristics due to the symmetrical property of the structure.
On the other hand, the reflection characteristics result as illustrated in
In
HPBW [˚] | Tilted angle [˚] | Gain [dBi] | |
---|---|---|---|
Loop | 57.1 | 61.0 | 8.4 |
Dipole | 70.9 | 52.0 | 9.1 |
dimension of each parameter is shown in
After fabricating the proposed antenna, we measured it for both reflection characteristic and radiation pattern to compare it with calculated results.
Finally, we compared the calculated and measured radiation patterns in the xz plane for both loop and dipole arrays as shown in
As indicated by these results, the desired radiation patterns are achieve based on the design guidelines of 60˚ to 80˚ HPBW and a 60˚ tilted angle, and a low profile directional antenna with a thickness of 0.16 λ (20 mm) has been achieved.
We have proposed a low profile dual polarized directional antenna, consisting of loop and dipole arrays for an
indoor base station. The proposed antenna has four directional radiation patterns in four orthogonal directions in the horizontal plane. A narrow HPBW of approximately 60˚ to 80˚ and a tilted angle from the z-direction close to 60˚ were set as the target parameters of each radiation pattern. Both the calculated and measured results demonstrated that the desired radiation patterns were achieved, and the measured radiation pattern agreed well with the calculated ones. The reflection characteristics of the loop and dipole antennas are less than −10 dB and mutual coupling between vertical polarization elements, and between vertical and horizontal polarization elements is nearly less than −20 dB in the measured data. Consequently, we successfully developed a low-profile directional antenna with a thickness of only 0.16 λ (20 mm).
In this paper, the proposed antenna design is capable of transmitting up to eight beams, and up to four beams with vertical or horizontal polarization depending on the state of the mobile station. In addition, the proposed antenna can be used as a pattern diversity antenna, and vertical and horizontal polarizations can be radiated in the identical directions. Thus, the potential applications of the proposed antenna cover a wide range of possibilities.