## 1 Introduction  
The rapid development of wireless technologies and new communication services requires a large amount of frequency availability. Cognitive radio (CR) is a hierarchical spectrum-sharing solution for this high spectrum demand [1]. According to the Federal Communications Commission, a cognitive radio is \"a radio that can change its transmitter parameters based on interaction with the environment in which it operates\" [2]. To detect changes in occupied bandwidth and communicate with other wireless devices, transceivers are designed to have both ultra-wideband (UWB), for sensing purposes, and narrow band (NB) antennas for communication [1]. Various UWB antennas [3; 4; 5; 6; 7; 8] have been studied to obtain the wideband characteristic required for UWB communications. In [9; 10], a reactive impedance surface (RIS) was used to broaden the impedance bandwidth, especially at low frequencies. A meta-material structure was utilized to achieve the wide bandwidth and high gain characteristics in [11; 12]. In these research works, however, the radiator and ground plane are not coplanar, meaning that these UWB antennas are not easy to integrate with NB communication antennas within a compact space. In terms of fabrication, a coplanar waveguide (CPW)-fed structure is considered as a better choice than conventional microstrip structures [13]. Therefore, we choose to use a CPW-fed wide-slot UWB antenna for easy integration with three NB ground radiation antennas [14; 15] on a single ground plane and to achieve good isolation characteristics. Recently, dual-port integrated UWB and frequency-reconfigurable NB antennas for CR applications were studied in [16; 17; 18; 19; 20; 21; 22; 23]. To achieve frequency reconfigurability, additional switches, such as PIN diodes, step motors, and even photoconductive elements, are implemented. However, these approaches have drawbacks, such as fabrication complexity, large antenna dimensions, low mode switching speeds, and mutual coupling from biasing lines [23]. More importantly, it is difficult to fully cover the entire UWB communication band (3.1 GHz to 10.6 GHz) using only one NB antenna because of its narrow bandwidth characteristics. To fully cover the entire UWB communication band, some researchers [20] have been trying to integrate multiple NB antennas into the UWB antenna. However, that causes a low isolation problem between the UWB antenna and the NB antennas due to the compact antenna dimensions, especially in the low-frequency band. To further improve isolation performance, the UWB antenna and the NB antennas should be separated by greater than half a wavelength. Thus, the enlargement of antenna size becomes another inevitable problem. Recently, to realize high isolation characteristics, three types of novel decoupling methods were introduced, such as using defected ground structures [24], applying additional decoupling structures [25], and placing antenna elements orthogonally to each other to realize pattern diversity [26]. However, the aforementioned methods are not practical for the following reasons. First, it is not desirable to separate the ground plane in practical designs. Secondly, to apply the decoupling network and/ or the orthogonally arranged antennas, a sufficient ground size is necessary. Therefore, it is desirable to design a UWB antenna considering a compact ground size and isolation problems. To overcome these drawbacks, this paper proposes a compact four-port coplanar antenna with a CPW-fed wide slot UWB antenna and three broadband loop-type ground radiation antennas for CR applications. This antenna is based on an excitation switching reconfigurable mechanism. This concept for CR systems was suggested in [20] by integrating multiple NB antennas and a reconfigurable switching mechanism. Depending on the sensed frequency spectrum hole, the corresponding NB antenna communicates while the other NB antennas are idle.