Miniaturization of broadband modified dipole antenna based on complex meta-material substrate

Abstract

An yagi antenna miniaturization technique to increase Multiple Input Multiple Output(MIMO) system capacity has been proposed in antenna substrate which is composed of a mixture of a high-permittivity material and a meta-material. The proposed antenna contributes to the increase of channel capacity in two ways. First, by miniaturization of the conventional antenna structure, the relative distance between antenna elements is increased, therefore the mutual coupling between them is decreased, which makes the smooth signal transmission. Secondly, the proposed antenna makes the system’s volume reduction therefore the integration level is increased. So the channel capacity in restricted space is increased by using proposed antenna structure. The complex-substrate acts as a controller of antenna impedance, which makes the impedance matching at lower operating frequency. Therefore, more effective antenna miniaturization can be done. Also it coincides well to the antenna array expansion, therefore it is better to apply proposed antenna to the MIMO system compared to the conventional antenna. The proposed antenna operates at the center frequency of 1.8 GHz, and it radiates as the conventional dipole antenna structure. The proposed antenna controls the impedance matching with the complex substrate and makes the reduction of antenna structure’s volume, and this enables the antenna size reduction to be 60 %. Furthermore to restore the bandwidth reduction induced by using conventional antenna miniaturization, we tried the antenna design modification, which makes the increase of bandwidth up to 130 MHz. To check the performance enhancement of proposed antenna structure, the mutual coupling level and the channel capacity have been confirmed. In case of the mutual coupling, it was compared with $2 \times 4$ structure whose expansion distance is $\lambda /2$ , and the reduction of mutual coupling has been proven. Also, to show the channel capacity gain, the proposed antenna has been tested in 2 ways. The channel of the system is supposed as a Rayleigh channel, which maximizes capacity gain. The one way is that the proposed antenna is expanded to an array structure with same expansion distance like the conventional antenna array. With this configuration, the mutual coupling reduction makes the increase of channel capacity. With the variation of antenna element number, the increase of channel capacity has been shown. The other way is focused on the space usage reduction, which can make the higher integration level of antenna elements. By maximizing the antenna integration level, the channel capacities of conventional antenna and proposed antenna array have been compared. The proposed antenna configuration makes the increase of integrated antenna elements and decrease of mutual coupling, therefore the possible channel capacity is increased. To analyze the proposed antenna’s miniaturization level and the increase of bandwidth, the equivalent circuit model has been constructed. To make the precise equivalent circuit model, some important factors such as structural coupling and frequency characteristic are considered in the equivalent circuit model. With the identification of the accuracy in the equivalent circuit model, the impedance, frequency and S-parameter characteristic have been checked with simulated model. With the variation of inductance and capacitance by using proposed antenna structure, the increase of antenna bandwidth is confirmed.