**3.6. Inserting stubs**

222 Ultra Wideband – Current Status and Future Trends

and operates over the range of 2-6 GHz.

in [50].

**3.5. Using spatial and angular variations** 

coupling, isolation and impedance matching are presented.

**UWB-MIMO Systems** - In [46], a diversity antenna operating at a frequency range of 3.1-5.8 GHz is designed consisting of two orthogonal half circles with the radiators placed symmetrically with respect to a protruded T-shaped ground plane, which has a slot at the upper center portion of the ground plane. This slot helps in enhancing the isolation and matching the impedance. In [47], this technique is used in real sense where circular slot antenna with a stepped ground plane is proposed. A stepped ground plane generates nonplanar connections and discontinuous interfaces between the elements and the system ground planes. This strategy has effectively decreased the mutual coupling and provided 10 dB enhancements in isolation characteristics. The antenna consists of four radiating elements

**Narrowband MIMO Systems** - The technique of using spatial and angular variations relative to the antenna elements of array is very commonly used to reduce mutual coupling. It is well demonstrated that by increasing the space between the radiating elements decorrelates them and even the spacing greater than or equal to ߣȀʹ gives mutual coupling less than -20 dB, where ߣ is free space wavelength at the center frequency [48]. However, the spacing becomes less than ߣȀʹ in the case of compact MIMO antennas for portable devices, thus it requires considering the mutual coupling effects to be compensated [49]. Therefore, in addition to separating the radiating elements by some distance, positioning of the radiating elements at different angles with respect to each other helps to reduce mutual coupling by exploiting the diversity in polarization. Chae et al. [27] has presented the detailed study using this technique. Further, the same technique is described and employed

**UWB-MIMO Systems** - Being very simple technique, it has not some specific constraints relating to the bandwidth but with size of the antenna. First of all, this technique is used for UWB diversity antenna by Wong et al. in [51] where the antenna consists of two truncated square monopoles orthogonally and symmetrically printed on the two sides of a T-shaped protruded ground plane as shown in Table 2. This antenna operates over 2.3-7.7 GHz giving isolation more than 20 dB. Using T-shaped ground plane also indicates that the modification of ground plane is an additional technique used together with polarization diversity to enhance the isolation. Recently, Chen et al. have used the similar technique [52]. It presents very compact UWB diversity antenna exploiting polarization diversity. The antenna elements are fed orthogonally and are designed for the lower band of UWB, i.e., 3.1-4.8 GHz. The isolation between two antennas is greater than 20 dB across the bandwidth. Also, the same research group has presented a detailed analysis of two suspended UWB plate antennas operating over 3.0-6.0 GHz in [53] for UWB-MIMO systems. They tested two configurations; (i) when shorting walls are vertically positioned (ii) when shorting walls are horizontally positioned. The effects of the variation of distance between antennas on mutual **Narrowband MIMO Systems** - The technique of using stubs to get better isolation also deals with the ground plane instead of the radiating elements. One or more stubs are inserted to enhance the isolation. To the best of our knowledge, there is no scientific publication presenting the use of this technique for narrowband MIMO systems.

**UWB-MIMO Systems** - The method of inserting stubs is mainly found in the literature for UWB-MIMO antennas. For instance, in [54], two elements diversity planar antenna, with three stubs on the ground plane to improve the isolation, has been proposed particularly for PDA phone. The 10 dB return loss bandwidth is achieved from 2.27 GHz to 10.2 GHz and isolation is always more than 15 dB. Similarly, another printed UWB diversity antenna consisting of two square radiators and a cross stub placed between them on the ground is presented in [55]. The 10 dB return loss bandwidth of the antenna ranges from 3.1 GHz to 10.6 GHz and the isolation between the two ports is higher than 18 dB within 3.3 GHz to 10.5 GHz.
