Dimensions of the coupled antenna Array.
Abstract
This chapter introduces a novel design concept to reduce mutual coupling among closely-spaced antenna elements of a MIMO array. This design concept significantly reduces the complexity of traditional/existing design approaches such as metamaterials, defected ground plane structures, soft electromagnetic surfaces, parasitic elements, matching and decoupling networks using a simple, yet a novel design alternative. The approach is based on a planar single decoupling element, consisting of a rectangular metallic ring resonator printed on one face of an ungrounded substrate. The decoupling structure surrounds a two-element vertical monopole antenna array fed by a coplanar waveguide structure. The design is shown both by simulations and measurements to reduce the mutual coupling by at least 20 dB, maintain the impedance bandwidth over which S11, is less than −10 dB, and reduce the envelope correlation coefficient to below 0.001. The boresight of the far-field radiation patterns of the two vertical monopole wire antennas operating at 2.4 GHz and separated by 8 mm (λo/16), where λo is the free-space wavelength at 2.45 GHz, is shown to be orthogonal and inclined by 45° with respect to the horizontal (azimuthal) plane while maintaining the shape of the isolated single antenna element.
Keywords
- 4G/5G LTE/LTE-A
- cellular communications
- coplanar waveguide fed vertical monopole
- decoupled antenna array
- descattered antenna array
- GPS
- microstrip fed vertical monopole
- MIMO
- radar
- RFID
- Wi-Fi
- WiMAX
1. Introduction
Contemporary wireless systems including, but not limited to, 4G/5G LTE/LTE-A, radar, RFID, Wi-Fi, WiMAX, GPS, geolocation, biomedical imaging, and remote sensing dictate the use of miniaturized MIMO antenna arrays on mobile terminals. They can also be permanently installed on fixed structures for increased gain, which will improve link reliability and quality of service, increase communication range, and increase battery life through a variety of diversity schemes [1, 2], and/or increase data rate/throughput through MIMO spatial multiplexing schemes [3]. Moreover, these structures can provide esthetic, miniaturized wireless consumer devices. Recent trends toward miniaturized, esthetically appealing, battery efficient handheld wireless devices, and green wireless systems require antenna arrays that should be implemented within a restricted physical space.
In this chapter, we report a new approach of how to reduce the complexity of prior existing designs for mutual coupling reduction such as periodic metamaterial/metasurface constructs [4, 5, 6, 7, 8], defected ground plane structures [9, 10], soft/hard electromagnetic surfaces [11, 12], parasitic elements [13], matching and decoupling networks [14, 15, 16], and neutralization lines [17]. Our proposed approach can still maintain, and in certain circumstances exceed the performance metrics of traditional approaches well-known in previous art, [18, 19, 20] using much simpler, cost-effective, and novel design alternatives.
Numerous techniques were proposed for the mutual coupling reduction among the elements of MIMO antenna arrays, the most notable and highly embraced is the use of metamaterials, which subsequently turned into an unique topic of its own used in various disciplines including acoustics, RF, optics, laser, and nanotechnology [21]. A defected ground plane consisting of a slitted pattern is disclosed in [9] to suppress mutual coupling between two monopole antennas separated by 0.093
A complicated approach for mutual coupling suppression between two monopole antennas operating at 2.49 GHz and separated by 0.09
It should be noted, however, that these artificial magnetic inclusions obviously suffer from extremely narrow resonance bandwidth. The design methodology is based on creating artificial negative permeability, which consequently presents extremely high attenuation to the near-field that exists in the region between the two monopoles for both propagating and evanescent fields. The authors claim a reduction of at least 20 dB in
Several defected ground planes [9, 10, 11, 12], soft artificial electromagnetic surfaces, matching and decoupling networks [14, 15, 16], and neutralization lines [17] have received considerable recent research interest in both academia and industry. However, they were not applied specifically to vertical monopole antennas, the subject of this chapter. It should be noted that these traditional approaches suffer from:
Extremely narrow bandwidths since metamaterials, defected ground structures, and soft electromagnetic surfaces embrace inherently high-resonant structures;
Repeated periodic structures are employed periodically in 1-D, 2-D, or in 3-D using vias, which further complicates the design and increases the fabrication cost;
Insertion of many 2D cells (arranged in multiple rows and/or columns) and sometimes 3D unit cells [7, 8] between the antenna elements limits the empty space between the antenna elements and hence complicates the task of miniaturizing the MIMO antenna array; and
Distortion of the far-field radiation patterns and/or reduction the operating bandwidth [18, 19, 20] from these artificial structures.
To conclude, most of the related research utilized meta-surfaces inclusions inserted in the space
2. MIMO array structure
We will elaborate on one embodiment involving a miniaturized two-element antenna array system composed of two vertical monopole antennas separated by 8 mm, corresponding to
To obtain a boresight oriented by 45° relative to that of a conventional vertical monopole, the design uses a partial reference ground plane instead of a traditional full ground as shown in Figure 1. The monopole is mounted on a dielectric substrate and is fed by a coplanar waveguide (CPW) circuit. The ground plane of the coplanar feeding circuit serves as a partial ground for the antenna as well. The height of the monopole
This behavior is exploited to construct an array consisting of two vertical monopoles, the radiation patterns of which are identical in shape but orthogonally oriented in space. The antenna array is formed by placing monopole 2 and its feeding circuit as a mirror image of monopole 1 around the
When the two antennas are positioned in a close proximity,
Monopoles separation ( | 8 |
Monopole height ( | 25 |
Monopole diameter ( | 1.6 |
Feeding probe length( | 11 |
Feeding probe width ( | 2 |
Ground width ( | 10.85 |
Ground length ( | 5 |
Center strip width ( | 3 |
CPW gap width ( | 0.5 |
Substrate ( | (40 × 40 × 1.27) |
Dielectric constant ( | 10.2 |
Despite the reduction in transmission level between the feeding ports, the distribution of surface currents depicted in Figure 6, resulted from exciting port 1 while terminating port 2 with 50 ohms, shows the existence of significant interfering currents distributed on the CPW feeding of monopole 2. The current induced on monopole 2 and its feeding circuitry is due to the mutual coupling between the two antennas.
In Figure 7, we show the reflection coefficient,
The proposed MIMO antenna array consists of two monopole antennas positioned on a printed circuit board. The antennas are surrounded by a decoupling circuit printed on the top side of the substrate and a feeding circuit on the bottom side. For short, the design will be called the decoupled coplanar waveguide fed monopole antenna array (CPW-MAA). The substrate has a length and width of 40 × 40 mm, and a thickness of 1.27 mm.
In order to further enhance the performance of pattern diversity for the two-element antenna array and minimize the contribution of mutual coupling among the elements of the antenna array to the channel correlation, the mutual coupling effect needs to be reduced with minimal distortion to the radiation patterns of the individual elements, preferably such that the 3-D far-field patterns of the individual elements being identical in shape and orthogonal in space. This applies to the signals at the output of the antennas when transmitting and at the input of the feeding circuit when operating in the receive mode.
The decoupling structure, shown in Figure 9, consists of a simple 2D planar rectangular metallic ring and two tuning metallic strips printed on the surface of the substrate.
First, the performance of the CPW-MAA is compared against the coupled monopole antenna array shown in Figure 5 using the same substrate. Furthermore, as a design alternative and to further validate the disclosed design concept, another substrate has been used with a dielectric constant
The CPW consists of a central strip having a length of
Dimension defined name | Substrate RO3210 ( | Substrate RO3006 ( |
---|---|---|
Monopoles separation ( | 8 | 8 |
Monopole height ( | 28 | 27 |
Monopole diameter ( | 1.6 | 1.6 |
Feeding probe length ( | 11 | 11 |
Feeding probe width ( | 2 | 2 |
Ground width ( | 10.5 | 10.8 |
Ground length ( | 5.0 | 5.0 |
CPW trace width ( | 3.0 | 3.0 |
CPW gap width ( | 0.5 | 0.2 |
Rectangular ring outer length ( | 22 | 22 |
Rectangular ring outer width ( | 24 | 30 |
Rectangular ring trace width ( | 2.5 | 2.5 |
Tuning strip length ( | 16 | 16 |
Tuning strip width ( | 1.5 | 4 |
Tuning strips separation ( | 6.5 | 8 |
Substrate ( | (40 × 40 × 1.27) | (40 × 40 × 1.27) |
Prototypes are fabricated and measured at the University of Arkansas at Little Rock’s Antennas and Wireless Systems Research Laboratory (AWSRL). The prototype which is shown in Figure 11 used Roger RO3210 substrate. The frequency response has been measured using E5071B and N5242A VNAs. The radiation patterns are measured inside an anechoic chamber. Using Roger RO3210 (dielectric constant
The simulated and measured results of the scattering parameter,
Introducing the decoupling network resulted in a reduction of −19 dB in mutual coupling level, from −4 to −23 dB. The antenna array system offers a bandwidth of 10%, in which the coupling coefficient,
Table 3 summarizes the performance metrics of the simulated models and the measured prototype. In this table,
Performance metric | Conventional array | HFSS | CST | Measured |
---|---|---|---|---|
−8 | −24 | −40 | −20 | |
−8 | −23 | −40 | −33 | |
−4 | −29 | −45 | −23 | |
— | 2.25 | 2.22 | 2.12 | |
— | 2.58 | 2.56 | 2.74 | |
— | 2.28 | 2.26 | 2.32 | |
— | 2.51 | 2.51 | 2.58 | |
BW ( | — | 0.33 | 0.34 | 0.62 |
BW ( | — | 0.23 | 0.25 | 0.26 |
% BW ( | — | 13.6% | 14.2% | 25.5% |
% BW ( | — | 9.6% | 10.4% | 10.6% |
The maximum value of the total realized gain is plotted versus frequency in Figure 15. Another advantage of the proposed antenna array is the enhancement achieved in the realized gain and hence the efficiency. The total realized gain of a single element varies over the range from 1.6 to 1.69 dB over the frequency range 2.25–2.52 GHz over which
The 3-D radiation patterns in terms of the realized gain are shown in Figure 16.
3. Conclusions
A compact MIMO antenna array with identical yet orthogonal radiation patterns and high isolation level is reported in this chapter. The shape and size of the partial ground plane and its distance from the monopole allowed the boresight of the radiation pattern of the isolated monopole to be oriented at an elevation angle of 45o. The 45o elevation achieved for the single monopole made it possible to place two vertical monopoles back to back to achieve an array with orthogonal radiation patterns. However, the mutual coupling between the two elements when placed in close proximity resulted in a distortion of the radiation patterns. To mitigate the effects of mutual coupling and increase the isolation between the two ports of the array, a decoupling element that surrounds the array elements which provides more flexibility on how close the elements need to be from each other. The dimensions of the rectangular ring and the tuning strips have been tuned to achieve the desired isolation level and restore the radiation pattern of the isolated element. The CPW-MAA with orthogonal far-field radiation patterns opens new opportunities for modern handheld devices and/or fixed wireless access points implementing MIMO techniques. Unlike existing design approaches that are based on inserting artificial resonant structures between the radiating elements, the design presented in this chapter encloses the radiating elements by simple and versatile planar conducting structures. The proposed design is demonstrated by simulations and measurements to significantly reduce the mutual coupling when the monopoles are spaced by 8 mm (
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