Design of Tunable Monopole Arm Planar Spiral Antenna for Cognitive Radio

This paper presents a spiral antenna design operating in the frequency range of 1–15？GHz having both selective notch bands and wideband response. The main feed arm of spiral antenna is configured as rectangular monopole of width quarter wavelength to achieve impedance matching with standard 50？？ excitation. Frequency tuning in the design is achieved by placing varactor diode at an appropriate position along the spiral arms and in the ground plane. The design offers a peak gain of 3.4？dB (simulated) and 3？dB (measured). The unique frequency response of antenna makes its suitable to be used for front-end system of cognitive radio for sensing the spectrum in various modes. 1. Introduction The development of antennas in the past five decades has gone through various design changes, in order to enhance the overall radiation characteristics. The antennas which are being used in the present wireless communication technology are either having a multiple frequency response or a wideband response. But the rapid growth in the field of microwave communication has put a demand to develop antenna with wideband response due to their high data rates, great capacity, simplex design, and low power consumption. However, an antenna with ultra-wide band response mainly finds its application in electronic warfare and military purpose. It is found in [1–3] that, frequency independent antennas like log-spiral, rectangular spiral and Archimedean-spiral are suitable to provide a wideband frequency response, but the problem that arises in such antennas is with the feeding system due to increased antenna impedance which is generally in the range of (140–200)？？. So special wideband balun is required to be designed [4–6] for providing an impedance matching in the wide microwave range of frequency thereby making the whole system much more bulky and putting a constraint in their application. Another main drawback of UWB antennas is their interference with existing communication channels like GSM (900–1800？GHz), UMTS (1.92–2.71？GHz), local area network (WLAN 5.15–5.825？GHz), and worldwide interoperability for microwave access (Wi-MAX, 3.3–3.7？GHz) IEEE 802.11a in the United States (5.15–5.35？GHz, 5.725–5.825？GHz). The interference produces a high impulse noise which is difficult to eliminate at the receiver system and thereby increases the complexity of whole communication system. The cognitive radio system is the upcoming wireless technology for very high data rates even beyond 100？Mbps. Such a system requires a few design specifications which the antenna should meet. For example,