The purpose of this paper is to introduce a compact Ultra-Wideband (UWB) diversity antenna with a very low Envelope Correlation Coefficient (ECC). The design employs a hybrid isolation enhancing and miniaturization technique. The antenna consists of two counter facing monopoles, and is miniaturized by using not only inverted-L stubs but also a Complementary Split Ring Resonator (CSRR) on the ground plane. The added components enhance isolation and enable tighter packing of the antennas. The result is a very compact MIMO array with an overall size of 23 x 29 mm2, that covers the entire UWB spectrum from 3 GHz to 12 GHz, with mutual coupling lower than-15 dB. Moreover, the CSRR unit that acts as a resonator is applied for the first time to suppress the interference of RF currents flowing through the ground plane of this UWB MIMO/diversity antenna. The performance of the fabricated prototype in terms of scattering parameters, broadside (peak) gain, radiation patterns, efficiency and envelope correlation coefficient is presented and discussed

A compact planar UWB-MIMO antenna array with WLAN band rejection is presented. The array consists of four monopole radiators and a common ground plane. These monopoles are placed in such a way that the polarisation diversity of nearly placed radiators is exploited, resulting in high isolation. The proposed MIMO antenna array is electrically small (50 × 39.8 mm 2), printed on a low loss 1.524 mm thick Rogers TMM4 laminate with a dielectric constant of 4.5 and a loss tangent of 0.002. A band-stop design was inserted on the ground plane to behave similar to a LC band-stop filter and reject the WLAN band. Simulation and measurement results satisfy the return loss requirement of better than 10 dB and isolation better than 17 dB over the entire 2.7-5.1 and 5.9-12 GHz bandwidths.

S-Shaped Conformal ArrayAs far as the S-shaped conformal array is concerned, once again the projection method proves to be an effective mean for pattern recovery. Figure 10 shows the absolute value of the co-polar component of the electric field jE / j of the S-shaped array without and with phase-compensation, normalized with respect to the peak value obtained through simulations for the corrected array: the co-polar component is shown, since the cross-polar one (i.e., jE h j) is well below 230 dB. First of all, as in the previous case it can be noticed that simulations and measurements from the realized prototype (represented in Fig. 11) agree. It can be seen that the main lobe direction changes from 08 to 98 with respect to the linear case, and that the side lobe level has consistently increased from 213.8 dB to 26 dB. The gain of the 1 3 4 undistorted linear array was 11:1 dB, when the array is deformed it becomes 10.5 dB in the new direction of maximum at h59 while it is only 8.2 dB for h50 (the original direction of maximum); after compensation the gain in the desired direction of maximum (i.e., h50 ) increases to 10.8 dB. Applying the projection method the radiation pattern can be recovered: the main lobe direction moves back to 08 and the side lobe level decreases to 211 dB. CONCLUSION

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A new compact multiple-input multiple-output (MIMO) ultra-wideband (UWB) antenna array is presented. The antenna array initially consisted of two monopoles placed side by side at a distance of 4 mm. A strong mutual coupling was observed so the design was modified by rotating the second radiator at 90° at a distance of 1 mm. Wideband isolation is achieved by exploiting polarisation diversity of antenna elements. Simulation in HFSS and printed prototype results validate the high isolation, over 21 dB on the entire 2.5-12 GHz frequency range. A prototype was fabricated on a low loss substrate of Rogers TMM4 measuring 23 × 39.8 mm2. To evaluate the diversity performance, the envelope correlation coefficient was calculated resulting below -20 dB, thus ensuring good diversity performance. The compactness of the proposed UWB-MIMO design is finally compared against alternative solutions already present in the literature