Abstract
In this chapter, a compact dual band notched Ultrawideband (UWB) antenna with fractal shaped Hilbert curve slots (HCS) is presented. The antenna covers the frequency band from 2.5 GHz to 12 GHz for VSWR=2 and also shows stable radiation patterns throughout the operating frequency band. By introducing Hilbert Curve fractal Slots (HCS) in the antenna, band notch characteristics have been achieved. The HCS renders the capability to reject 5.15-5.825 GHz band assigned for IEEE 802.11a and HYPERLAN/2 and also 7.9-8.4 GHz band assigned for X-Band uplink satellite communication systems where the gain is suppressed very well in the desired WLAN and X-Band. The antenna gain varies from 3dBi to 5dBi over the operating band. Novelty of this design lies in achieving miniature notch structure which has higher degree of freedom for adjusting notch parameters and unsusceptible to coupling with other notches. The antenna can be used for various mobile communication services such as DCS, IMT-2000, UMTS, DMB and UWB.
TopIntroduction
The inspiration for the development of fractal geometry (Sagan, 1994) came largely from an in depth study of patterns from nature. Fractals have been successfully used to model such complex natural objects like cloud, galaxies, mountain ranges, coastlines, trees, leaves, ferns and much more. Mandelbrot realized that it is very often impossible to describe nature using only Euclidian geometry in terms of straight lines, circles and cubes. He coined the term ‘fractal’ about 20 years ago in his book named ‘The fractal geometry of nature’ (Sagan, 1994). He proposed that fractals and fractal geometry could be used to describe real objects such as trees, lightening, river meanders and coastline and so forth. Recent efforts by several researchers around the world have combined fractal geometry with electromagnetic theory which have led to a plethora of innovative antenna designs (Karmakar, Verma, Pal, & Ghatak, 2012; Jahromi, Falahati & Edwards, 2011; Azad & Ali, 2005; Bor, Lu, Liu & Zeng, 2009; Chi, Chen, Wei, Lien, Hung, Bor & Chen, 2013; McVay, Engheta & Hoorfar, 2004) and led to an era of fractal antenna engineering which is primarily focussed in two broad areas: the design of fractal antenna elements and the application of fractal to the design of antenna arrays.
The adopted term Ultrawideband (UWB) in 1989 by the U.S department of defence has re-emerged the century-old concept for modern applications. The release of the 7.5 GHz unlicensed spectrum by the U.S. Federal Communications Commission (FCC 02-48, ET-Docket 98-153, 2002) for commercial usages, sparked a renewed interest of UWB in industries, universities and governments. In recent years, research in the area of Ultrawideband (UWB) system has generated a lot of interest among microwave engineers. February 2002 witnessed the allocation of the frequency band between 3.1 GHz to 10.6 GHz and the Equivalent Isotropic Radiated Power (EIRP) less than -41.3 dBm/MHz for the unlicensed indoor UWB wireless communication system which sparked a renewed interest in industries, universities and governments. According to the released regulation, UWB technology which is based on transmitting ultra short pulses with duration of only a few nanoseconds or less, has received great attention in various fields for the short-distance (< 10 m) wireless communications. Because of the ultra-wideband property, UWB technology has many benefits which are enlisted below.
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High data rates and large channel capacity
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Immunity to multipath interference
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Low complexity and minimal cost
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Low power consumption
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Coexistence with other wireless communication systems
The reasons causing high data rates in UWB are high signal power and low noise power. The maximum achievable data rate for ideal band limited additive white Gaussian noise channel (AWGN) is related to the bandwidth and signal to noise ratio (SNR) by Shanon-Nyquist criterion as shown in (1):
(1) Key Terms in this Chapter
Band Notch: To avoid the electromagnetic interference between the UWB system and the wireless system, band notch functionality is necessary. It is achieved by etching different notches in UWB monopoles. The length of the band notch element is equal to half wave length at notch frequency.
Hilbert Curve: Hilbert curve fractal geometry was first proposed in the year 1891 named after the germen mathematician ‘David Hilbert’ as space filling curve. Hilbert curve geometry in antenna design has been configured to reduce the size of antenna as well as to get multiple resonances. Total length of the Hilbert curve increases when increasing the iteration stage, while keeping the overall space of the entire geometry fixed. This phenomenon mainly stems from electrical miniaturization of the geometry.
Fractal Antenna: The amazing realization towards combination of fractal geometry with electromagnetic theory has led to the development of novel class of antennas called fractal shaped antennas which have received significant attention in applied electromagnetic research community. Fractals in antenna engineering are based on analysis and design of fractal radiating elements and incorporation of fractal shaped antenna arrays. In antenna community, it is known as fractal antenna engineering. As fractal geometry is an extension of classical geometry, its appearance provides engineers with the unprecedented opportunity to explore number of configurations for possible use in the development of new and innovative antenna designs.
UWB (Ultrawideband): February 2002 witnessed the allocation of the frequency band between 3.1 GHz to 10.6 GHz with an Effective Isotropic Radiated Power (EIRP) less than -41.3 dBm/MHz for the unlicensed indoor UWB wireless communication system by the Federal Communication Commission (FCC), USA which initiated research in component development. According to the released regulation, UWB technology which is based on transmitting ultra short pulses with duration of only a few nanoseconds or less, has recently received great attention in various fields for the short-distance (< 10 m) wireless communications.
Frequency Independent Antenna: Rumsey’s principle suggests that the impedance and pattern properties of an antenna will be frequency independent if the antenna shape is specified only in terms of angles. To satisfy the equal-angle requirement, the antenna configuration needs to be infinite in principle, but is usually truncated in size in practice. This requirement makes frequency-independent antennas quite large in terms of wavelength. Rumsey’s principle has been verified in spiral antennas, conic spiral antennas and some log periodic antennas.
Mandelbrot: Mandelbrot realized that it is very often impossible to describe nature using only Euclidian geometry in terms of straight lines, circles and cubes. He coined the term ‘fractal’ about 20 years ago in his book named “The fractal geometry of nature”. He proposed that fractals and fractal geometry could be used to describe real objects such as trees, lightening, river meanders, and coastline and so forth.
Fractal Curve: Recursive structures describe many real-world objects like clouds, mountains, trees, coastlines and Brownian motion, which do not correspond to simple geometric shapes. Fractal geometry, which was introduced in 1975 by B.B.Maldelbrot, mathematically defined these extremely intricate self-similar shapes. His work inspired interest and has made fractal geometry and its application a very popular field of study. A fractal is a rough or fragmented geometric shape that can be subdivided in parts, each of which is a reduced copy of the whole.
Monopole Antenna: The monopole is a resonant antenna where the radiator acts as an resonator for radio waves, oscillating with standing waves of voltage and current along its length. Therefore the length of the antenna is determined by the wavelength of the radio waves it is used with.