## 1 Introduction  
Recent dramatic developments in wireless communications have resulted in an environment where many signals from various communication systems, such as 4G, 5G, Wi-Fi, Bluetooth, global positioning system (GPS), and Zigbee, exist simultaneously in a limited space [1; 2; 3; 4; 5; 6; 7]. Such signals from the antennas of various wireless devices can cause significant signal interference internally and externally. To mitigate interference, pattern nulling and forming techniques have been extensively studied and used in practical applications with array antennas by adjusting the weight of each antenna element [8; 9; 10; 11; 12; 13; 14; 15; 16]. However, the pattern nulling performance is significantly degraded when multiple antennas in the array are placed in a limited platform size. This nulling performance degradation is caused in part by increased mutual coupling of the array, where the distance between adjacent elements is too close. The spatial diversity of the array antenna is also reduced, since the radiation patterns of individual elements have a high pattern correlation, resulting in inferior pattern nulling or forming performance. To overcome these drawbacks, many studies have been carried out to minimize the mutual couplings and pattern correlations for compact array antennas, which include using parasitic elements [17; 18; 19], defected or extended ground planes [20; 21; 22], electromagnetic band-gap structures [23; 24; 25], and ferrite materials [26; 27; 28]. Although these previous studies can improve the isolation and maintain the low pattern correlations between adjacent array elements, these techniques are sometimes not feasible in small devices with limited spaces. Thus, research on mounting the array antennas in such compact spaces has been conducted by incorporating multiple radiating modes of each array element [29; 30; 31] or employing dissimilar antenna types such as a patch antenna with a monopole antenna [32; 33; 34; 35] and a patch type antenna with a dielectric resonating antenna [36; 37]. Furthermore, there is a study on using different antenna types (a circular patch antenna and a cylindrical monopole antenna) to operate in the higher order mode at multiband frequencies [38; 39]. However, in these studies, the electrical antenna size is still large because it is necessary to achieve higher order modes. In addition, it is difficult to control the mutual coupling and the pattern correlation due to the shape of the patch antenna when the array elements are close to each other with sharing a common ground [40; 41]. In this paper, we propose a novel design of a two-element array antenna on an electrically thick substrate with an extremely narrow array distance, where each element has a modal difference in the radiation pattern, causing a high isolation and a low correlation between the array elements. The proposed array consists of a rectangular ring patch printed on a thick substrate and a monopole wire in the center of the substrate. These two types of antennas share the ground plate, and the antenna ports are very close to each other while minimizing distortion in antenna performances. To demonstrate the feasibility of the proposed structure, the antenna is fabricated to measure the characteristics of the reflection coefficients, mutual couplings, and radiation patterns. For modal analysis of the antenna pattern, spherical mode decomposition is performed on the radiation patterns of the two elements to ensure that each antenna pattern has a significant modal difference with respect to spherical modes. Then, we define a modal difference metric (MDM) to indicate a quantitative value of the modal difference for the two-array elements. In addition, the envelope correlation coefficients (ECCs) of the proposed array in terms of the interelement spacing are observed and compared to the conventional patch and monopole arrays.