A frequency-reconfigurable elliptical monopole antenna for cognitive radio networks

Turkish Journal of Electrical Engineering & Computer Sciences http://journals.tubitak.gov.tr/elektrik/ Turk J Elec Eng & Comp Sci (2017) 25: 2535 – 2546 c TÜBİTAK ⃝ doi:10.3906/elk-1604-45 Research Article A frequency-reconfigurable elliptical monopole antenna for cognitive radio networks Santasri KOLEY∗, Debjani MITRA Department of Electronics Engineering, Indian School of Mines, Dhanbad, India Received: 04.05.2016 • Accepted/Published Online: 20.09.2016 • Final Version: 29.05.2017 Abstract: In this paper, an elliptical disk monopole antenna with seven switchable states, including an ultrawideband (UWB) state and six narrowband states, is presented. It consists of an elliptical disk radiator to operate in the UWB mode for cognitive radio (CR) sensing. To operate in the narrowband mode, the antenna is fed through a defected ground structure (DGS) integrated microstrip line. Frequency reconfigurability is achieved by changing the DGS slot length using six electronic switches. The simple proposed design can cover a wide range of frequency bands from 0.75 to 12 GHz, has an omnidirectional radiation pattern with high gain, and is especially suitable as a CR front-end. The fabricated prototype shows good impedance matching in the specified frequency range. Key words: Cognitive radio, reconfigurable antenna, switchable filter, ultrawideband 1. Introduction The demand for limited spectrum resources is becoming too large to support various wireless services through traditional fixed frequency allocation. Cognitive radio (CR) systems are envisioned to be a revolutionary concept in overcoming the spectrum scarcity by dynamic spectrum allocation (DSA). In CR networks, secondary users (SUs) are enabled to access unused frequency bands of licensed primary users (PUs) without causing any harmful interference, which leads to remarkable improvements in spectrum efficiency [1]. Ultrawideband (UWB) is a promising technology that meets the requirements of CR networks very well when operating in the “underlay” mode. However, in the “overlay” mode, SUs are allowed opportunistic spectrum access to licensed bands in case they are not used by the PUs at a particular time and frequency. Thus, a CR front-end system requires a UWB antenna for wideband spectrum sensing and another reconfigurable narrowband antenna for communication purposes. In a CR network, an integrated wideband and reconfigurable narrowband antenna is preferable, where wideband is required for UWB spectrum-sensing and narrowband is desired for communication. Antennas are usually tuned by introducing switching techniques such as electronic switches, photoconductive switches, field effect transistor (FET) switches, radio frequency (RF) microelectromechanical systems (MEMS), p-i-n diodes, and varactor diodes. The literature specifies several antenna designs for sensing CR operations over a wideband. An UWB printed planar elliptical monopole antenna was reported in [1]. The antenna can be used for multiband operation with an omnidirectional radiation pattern in the azimuth plane. The antenna operates over an extremely wide impedance bandwidth in the range of 0.75–20 GHz. A CR reconfigurable multiple-input multiple-output (MIMO) and sensing antenna system was presented in [2]. The proposed sensing antenna is used to cover a ∗ Correspondence: 2535 KOLEY and MITRA/Turk J Elec Eng & Comp Sci wide range of frequency bands from 720 to 3440 MHz. An UWB compact planar rectangular folded monopole antenna with a bandwidth of 0.5–9 GHz was presented in [3]. A new wideband antenna was proposed in [4] to access a wide range of different entertainment, information, and data transfer services, including Bluetooth, Wi-Fi, GPS, DVB-H, and UWB. The antenna operates from 460 MHz up to frequencies in excess of 10.6 GHz. In [5], a planar disk antenna compatible with cognitive radio was developed to operate from 0.77 to 11.23 GHz. However, these antennas [1–5] are designed for wideband operations and they need to be reconfigured for narrowband operation. In general, there are two ways to achieve wideband to narrowband reconfiguration. One approach is to place a wideband antenna and a reconfigurable narrowband antenna side by side, using separate excitation ports. A second approach is to reconfigure a single-port wideband antenna into narrowband mode by introducing a filter property in the antenna structure or by switching parts of the structure [6]. In [7], a circular disk monopole was excited with two ports at opposite sides; one CPW feed port was used for wideband-sensing operation and the other port consisted of a microstrip feed line with DGS for narrowband communication operation. The DGS slots act as a bandpass filter that suppresses frequencies outside the desired band, and its operating frequency band can be tuned by varying the length of the slots. In [8], the wideband antenna proposed in [4] was integrated with a narrowband slot antenna. However, the antennas [7,8] were designed for fixed narrowband operation only. In [9], the antenna structure consisted of a UWB antenna and a switched subband antenna system. Here five different antenna patches were designed within a circular section, which connected each shape at different times with the rotation of the circular section via a stepper motor mounted on the backside. This antenna is able to tune throughout the whole band covered by the sensing antenna (2–10 GHz). The main demerits of these types of structures are their complexity and switching times due to rotational motor implementation. In [10], a photoconductive switch-based antenna was demonstrated to achieve frequency reconfigurability. The antenna comprises two monopoles: one for UWB operation and another in reconfigurable narrowband mode, which are placed in close proximity to one another. The narrowband reconfigurability is achieved by integrating laser diodes within the modified monopole antenna structure in order to control the switching state of photoconductive silicon switches. This scheme has the advantage of eliminating the use of optical fiber cables to guide light to the switches and enables easier integration of the reconfigurable antenna in a complete communication system. The antennas in [11] have two microstrip-fed monopole antennas on the same board and are spaced apart side by side. One of them is an UWB egg-shaped patch-sensing antenna. In the communicating antenna, it has a 1-mm-wide long line connected to the microstrip-fed section. This structure yields multiple resonances in the UWB sensing range due to harmonics. The antenna integration method suggested here may not be considered as truly integrated, as the parts do not occupy the same area. Dual-port antennas enable simultaneous sensing and communicating over the channel, but they have limitations in terms of their relatively large size and the coupling between the two ports [12]. These limitations are solved by using single-port antennas. In [13], a rec (...truncated)