A technique to decrease the size of log-periodic dipole array (LPDA) antennas employing inductive loads on its elements is introduced. This technique consists of adding a short-circuited cylindrical hat cover at the end of each arm of the antenna. Simulation and measurement results show that the total area, if considered as a trapezoid, of a fabricated LPDA antenna, based on the proposed technique, is approximately 50% smaller than the size area of a reference antenna using a conventional technique. The parameters of the reduced antenna are slightly modified, preserving the bandwidth, impedance, polarisation and radiation pattern shape. The drawback of this technique is the small reduction of gain, from 7.5 dB for the conventional antenna to 6.4 dB for the reduced prototype in the middle operation band.

In this paper, a proposal of a four-port antenna for MIMO applications is presented with high interport isolation. The isolation among elements is implemented by limiting the surface current distribution of each radiator by placing them on different substrate layers, leading to a size optimization. The array is implemented by four drop-shaped slot radiators. The antenna performs from 1.7 to 7.2 GHz, presents a very low Envelope Correlation Coefficient (ECC), reaching values of 0.0025 tops, and a stable Total Active Reflection Coefficient (TARC) over the entire operational bandwidth. These parameters demonstrate the feasibility of the antenna for MIMO systems. The peak average gain is 5 dB, and the port isolation between adjacent radiators, S₁₂ and S₁₄, and opposite radiators, S₁₃ and S₂₄ are below −20 dB, reaching values even less than −30 dB in some bands without adding extra structures, avoiding an increment in the total area.