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Since the Federal Communications Commission (FCC) declaration of the frequency band 3.1 to 10.6 GHz for commercial communication applications in 2002 , the ultra wideband UWB technology has experienced many significant developments in recent years. Like all wireless technology, however, there are still some unique research challenges which have to be addressed. One major challenge is the UWB antenna design.
To be useful for mobile communications, the motivation of the UWB antenna design has to design an electrically small planar antenna which possesses wideband properties. Besides its small size, the antenna has to provide sufficient directivity and efficiency for the given radio channel . The use of an array of micro-strip patches can improve directivities by providing a predetermined scan angle. However, undesirable coupling between the antenna elements increases, leading to a reduction of the antenna gain and a distortion of its radiation patterns as discussed in  and .
Since UWB systems operate in a very large bandwidth, they need to share the spectrum with other users as well as with the existing communication systems this results in interferences. Therefore, as reported in  and , the design for the UWB communication antenna should include a band-rejection filter to avoid interference with existing wireless LAN and Hiper LAN service band between 5.15 GHz and 5.825 GHz. However, this will provide complications for UWB system. So another requirement is the integration of UWB antenna with the UWB device. UWB antenna for wireless communications should be an integral part of the system and not a stand-alone element. This is an important issue in the successful implementation of UWB technology for wireless communication applications.
The main purpose of this research is to develop novel compact types of UWB antennas, to investigate methods for diversity scheme of UWB communications and also to analyze the possibility of having a built-in notch band antenna array to satisfy the requirements set by FCC. Therefore the main objectives include:
- studying and understanding the concept of decoupling and directivity in small mobile platforms like notebook or handheld computers,
- optimizing the directivity and the side lobe level of the proposed antennas without degradation of array performance, and
- investigating techniques available for good impedance matching by optimizing the antenna configuration to obtain good time domain behaviors.
Brief Literature Review
For many applications, it is desirable to design antenna with very directive characteristics to fulfill the requirements of long distance communication. To meet such a requirement, an antenna array could be one of the solutions. An antenna array consists of more than one antenna element and these radiating elements are assembled in geometrical configuration to form an array with desired characteristics.
Antenna superdirectivity (also known as supergain) is the directivity higher than that obtained with the same antenna arrangement uniformly excited (constant amplitude and linear phase). However, problems such as, low radiation resistance, sensitive excitation and position tolerances, and narrow bandwidth are created when there is excessive array superdirectivity.
While Oseen  is the pioneer researcher to explore the possibility of superdirectivity, Hansen and Woodyard  researched and developed a limited endfire superdirectivity using a monotonic phase function. Franz  is also another earlier researcher but Schelkunoff  who came four years later published a paper on linear arrays which discussed, about array spacing less than ?/2. Schelkunoff described that equal spacing of the array polynomial zeros over that portion of the unit circle represented by the spacing gives superdirectivity.
When La Paz and Miller  claimed that a given aperture would allow a maximum directivity the field received wide attention but Bouwkamp and De Bruijn  demonstrated that La Paz and Miller had made an error and that there was no limit on theoretical directivity. The error pointed out by Bouwkamp and De Bruijn lead to a discovery of an important theorem: a fixed aperture size can achieve (in theory) any desired directivity value. Unfortunately the above theorem was less significant form practical point of view.
Bloch et al.  is of view that although the theorem has been rediscovered several times, the practical limitations of superdirectivity have surprised the systems engineers and others year after year. In 1946, Reid  generalized the Hansen-Woodyard endfire superdirectivity to include an element pattern and the endfire directivity was derived by Uzkov  as d ? 0. Thus superdirective aperture design has constraint such as superdirective ratio, sidelobe level, quality factor, tolerance, or efficiency.
A half-wave spaced array yields maximum directivity for a given sidelobe ratio when all sidelobes are of equal height. Dolph  recognized that Chebyshev polynomials were ideally suited for this purpose. However, Dolph's derivation and the formulas of Stegen are limited to d = ?/2. Riblet  showed that this restriction could be removed, but only for N odd. For spacing below half-wave, the space factor is formed by starting at a point near the end of the Chebyshev Â±1 region, 1 tracing the oscillatory region to the other end, then retracing back to the start end and up the monotonic portion to form the main beam half. Because the Mth-order Chebyshev has M - 1 oscillations, which are traced twice, and the trace from 0 to 1 and back forms the center sidelobe (in between the trace out and back), the space factor always has an odd number of sidelobes each side, or an even number of zeros. Hence only an odd number of elements can be formed into a Chebyshev array for d =?/2.
In another case of modest superdirectivity published by Sanzgiri and Butler , stepwise sidelobe constraints were employed, and the optimum directivity was formulated as the ratio of two Hermetian quadratic forms, as previously described. In this Lagrangian methods were used to solve for max D. The array was broadside with nine elements at d/? = 0.6. Several sidelobe envelopes were used; the case with constant SLR = 20 dB was typical. Directivity was 14.85, with SDR of 1.55.
This very modest value was due to the large element spacing; significant SDR for a broadside array requires d/? much less than 0.5. Multiple power pattern constraints were used by Kurth  with directivity optimization. Constraints on both main beam and sidelobe were used, leading to the common ratio of Hermitian quadratic forms solved by Lagrangian multipliers. A circular array of dishes was used as an example. Cox  obtained a modest superdirectivity for an acoustic endfire array for various angular distributions of white noise. He also discussed “oversteering” past endfire to increase directivity. Apparently, the acoustics community was not familiar with Hansen-Woodyard. Dawoud and Anderson  used Chebyshev polynomials to optimize the ratio of beam peak value for a superdirective array to beam peak value for a uniformly excited cophasal array. As the beamwidth narrows, this ratio rapidly decreases.
Another polynomial approach, by Dawoud and Hassan , used Legendre polynomials instead of Chebyshev polynomials. The former yields slightly greater directivity for a broadside array with small spacing in wavelengths. The calculated directivities (SDR = 6.2) seem to be much too high for the 3 dB beamwidth shown.
Typical UWB Antenna Types
As reported in , there are various types of UWB antennas which can be grouped into the following classes according to form and function:
- Frequency dependent antennas
- Small-element antennas
- Horn antennas
- Reflector antennas
The log-periodic antenna is an example of this type of antennas where the smaller scale geometry of antenna contributes to higher frequencies and the larger scale contributes to the lower frequencies.
These antennas are suitable for commercial applications since they are small and omni-directional. The bow-tie or diamond dipole antennas are typical examples of small-element antennas.
Horn antennas are the simplest form of antennas that concentrate energy in a given direction by means of electromagnetic funnel. They have large gains, narrow beams and are heavier than small-element antennas.
The reflector antenna can offer much higher gains than horn antennas. They are relatively large but easy to adjust by manipulating the antenna feed. Reflector antennas radiate energy in a particular direction and Hertz's parabolic cylinder reflector is an example of this type.
Research Scope and Methodology
The research scope is focused on UWB antennas designs which can provide ultra wide bandwidth characteristics. One way is to add a partial ground plane flushed with feed line, to improve the impedance matching. The superdirectivity antennas characteristics are also achieved by varying the length of the radiating elements. Therefore in order to achieve the above objectives, a number of tasks have been identified, as outlined below:
- Investigate the time domain characteristics of the proposed UWB antenna by simulation or measurement or by both.
- Simulate the UWB antenna design model using antenna simulation software before the actual prototype is built.
- Develop a novel design prototype of UWB antenna array to achieve very high directivity.
- Optimize and evaluate the antenna performance