Various bandwidths of UWB DRAs.
Ultra-wideband (UWB) antennas are gaining prominence and becoming very attractive in modern and future wireless communication systems, mainly due to two factors. Firstly, people increasingly high demand for the wireless transmission rate and UWB properties such as high data rate, low power consumption and low cost, which give a huge boost to the UWB antennas’ research and development in industry and academia since the Federal Communications Commission (FCC) officially released the regulation for UWB technology in 2002. Secondly, now the wireless portable device need antenna operated in different frequencies for various wireless transmission functions, and operation bands and functions are increasing more and more, which may result in challenges in antenna design, such as antenna space limitation, multi antennas interference, and etc. One UWB antenna can be used to replace multi narrow-band antennas, which may effectively reduce the antenna number.
The bandwidth is the antenna operating frequency band within which the antenna performances, such as input impedance, radiation pattern, gain, efficiency, and etc., are desired. The most commonly used definitions for the antenna bandwidth are the fractional bandwidth (for narrow or wideband definition) and the bandwidth ratio (for ultra-wideband definition).
The fractional bandwidth is defined as
The bandwidth ratio is defined as
2. History of UWB antennas
In 1898, Oliver Lodge  firstly introduced the concept of UWB antenna design, such as spherical dipoles, square plate dipoles, triangular or “bow-tie” dipoles, and biconical dipoles. Fig.1 depicts Lodge’s biconical antennas which are unmistakenly used in transmit-receive links. After that, a number of types of UWB antennas were developed in the following several years [2-7].
In 1940, S. A. Schelkunoff  proposed elaborate conical waveguides and feed structures in conjunction with a spherical dipole (see Fig.3). Unfortunately, his design of the spherical dipole antenna was not very useful. Almost at that time, the most well-known UWB antenna was the coaxial horn proposed by N. E. Lindenblad . In order to make the antenna more broadband, Lindenblad took the design of a sleeve dipole and introduced a continued impedance change, as shown in Fig.4.
In 1940, J. C. Kraus  also developed an antenna similar to the Lindenblad’s coaxial horn and named it volcano smoke antenna (See Fig.5), which played a significant role as the cornerstone of television development. Investigations carried out on this antenna showed that this bulbous monopole-like structure yields an impedance bandwidth ratio of 5:1. During that period, coaxial transitions became one of the design techniques for other antenna researchers and designers. In 1948, L. N. Brillouin  developed omni-directional and directional coaxial horns, as shown in Fig.6. But these two antennas are difficult to manufacture and use because of their widely structure. Thus, some aspects such as manufacturing cost and complexity of procedures become the important considerations in the design of broadband antennas. The well-known “bow-tie” antenna reveals those benefits, which was originally proposed by Lodge and later rediscovered by G. H. Brown and O. M. Woodward. In 1947, R. W. Masters  proposed a similar type of antenna, the inverted triangular dipole, which was later referred to as the “diamond antenna”. More recent, other UWB antennas were also developed. W. Stohr  introduceed the ellipsoidal monopole and dipole antennas in1968, as shown in Fig. 8. P. J. Gibson proposed the Vivaldi antenna  as an amalgamation of slot and Beverage antenna, collectively called tapered slot antenna, in 1979.
The conventional UWB antennas have been wide used in the broadcast communication applications, but they are not suitable for some high frequency applications in modern and further due to their solid structure and un-integration. In the following sections, some new types of UWB antennas will be introduced for high frequency applications..
3. Omni-directional UWB antenna and design
Along with the wireless system miniaturization and operation frequency increasing, some novel types of onmi-directional UWB antennas have been developed in the last decade. Mainly consisting of two types, the UWB planar monopole antenna and the UWB printed monopole antenna, both types are basically developed from the principles of conventional UWB antennas, such as the biconical antenna, the cone-disc antenna, the cage antenna, and etc. Based on several techniques in terms of bandwidth enhancement, omni-directional radiation improvement and size reduction, they can provide almost the same bandwidth and radiation performances as the conventional UWB antennas but with much smaller volumes.
3.1. UWB planar monopoles
The planar monopole antenna was firstly reported in 1976 by G. Dubost and S. Zisler . It can be realized by replacing a conventional wire monopole with a planar monopole, where the planar monopole is located above a ground plane and commonly fed using a coaxial probe. Up to now, many planar monopole antennas have been introduced due to their wideband performance. Several representative structures are shown in Fig.9, and these antennas achieve the impedance bandwidth ratio from 2:1 to more than 10:1.
Among various planar monopole antennas, the square planar monopole is the simplest in geometry, and its radiation pattern is usually less degraded within the impedance bandwidth. These favourable features attract many studies, mainly on the bandwidth enhancement since the square planar monopole only owns an impedance bandwidth ratio of 2:1. From the antenna geometry, the feed gap, the feed point location and the shape of the monopole’s bottom, all may affect the impedance matching. Thus, several techniques such as notching, bevelling, double feed, trident-shaped feed, and etc., were proposed to expand the bandwidth of the square monopole antenna, as shown in Fig.10
3.2. UWB printed monopoles
The aforementioned planar monopole antennas achieve an ultra-wideband performance based on various techniques, but they all need a perpendicular ground plane, resulting in increasing of the antenna volume and inconvenience for integration with monolithic microwave integrated circuits (MMICs). For the portal wireless device applications, the printed UWB monopole antennas are more popular due to their easier integration than the planar UWB monopole antennas.
The printed UWB monopole antenna commonly consists of a monopole patch and a ground plane. Both of them are printed on the same or opposite side of a substrate, and a microstrip or CPW feedline is used to excite the monopole patch. Since Choi
For geometry of the monopole patch, Fig.11 presents several representative structures. These antennas achieve the impedance bandwidth ratios from 2.3:1 to 3.8:1. Among various geometries of the monopole patches, the printed circular monopole antenna is one of the simplest , which achieves the impedance bandwidth ratio of 3.8:1 (2.69~10.16 GHz) with satisfactory omnidirectional radiation properties. Other monopoles such as octagon monopole , spline-shaped monopole , U-shaped monopole , knight’s helm shape monopole  and two steps circular monopole , as shown in Fig.11, were also proposed and studied,.
For geometry of the ground plane, several representatives are also shown in Fig.12, and obtain the impedance bandwidth ratios from 3.8:1 to more than 10:1.
One of interesting UWB printed monopole antenna designs is a trapeziform ground plane with a rectangular patch monopole aroused from the discone antenna, where the rectangular patch is used to replace the disc, the trapeziform ground plane is used to replace the cone, and the CPW is used to replace the coaxial feed, as shown in Fig.13 . It is found that the printed rectangular antenna with a trapeziform ground plane achieves an impedance bandwidth ratio of 5.1:1, which is similar to that of a discone antenna. To enhance the bandwidth further, the input impedance is investigated by comparing bandwidths for various characteristics impedance of CPW feedline. The impedance bandwidth ratio expands to 12:1 when the characteristic impedance of CPW feedline is about 100Ω, which means the impedance bandwidth is enhanced by a factor of about 2.3. In order to match 50Ω SMA or N-type connectors, a linearly tapered central strip line is used as an impedance transformer, and an impedance bandwidth ratio of 10.7:1 (0.76~11 GHz) is obtained. Moreover, various printed monopoles and feed structures are also studied to enhance the bandwidth further [33-36].
In fact, geometries of the monopole and the ground plane not only affect the antenna impedance bandwidth but the antenna radiation pattern and phase center over a wide bandwidth, which is an important phenomenon, especially for pulsed devices that need minimum signal distortion. Thus, the relation between antenna structure and its radiation are also studied.
Moreover, the wireless portable devices become smaller and smaller, which contributes the printed UWB antenna design on miniaturization. One of creative miniaturization techniques is proposed by Sun
4. Directional UWB antenna and design
Comparing to the omni-directional UWB antenna, the directional UWB antenna with a much higher gain is also required to meet various applications. In this section, several types of directional UWB antenna design such as the UWB printed wide-slot antennas, the UWB DRAs, and the DRAs with radiation reconfiguration, etc., are detail introduced.
4.1. UWB printed wide-slot antenna
The printed wide-slot antenna is another type of the most suitable candidates for UWB applications. This type of antenna is commonly consists of a wide-slot and a tuning stub connected with a microstrip or CPW feedline. Up to now, many wide-slot antennas, including different wide-slots or tuning stubs, have been extensively studied on the antenna operation bandwidth. Among various shapes of slot, the rectangular wide-slot is the simplest structure. Based on the rectangular wide-slot, several shapes of the tuning stub are studied, where the representative geometries are shown in Fig.16 and the bandwidth ratios vary from 1.8:1 to 3.6:1.
It is also noted that the slot shape plays more important on affecting the antenna bandwidth compared to the tuning stub shape. Fig.17 gives several shapes of wide-slots, such as the tapered slot, the circular slot, the hexagonal slot, and etc. These antenna can provide the impedance than ratios from 3.1:1 to 15.4:1, which are much wider bandwidth those of rectangular wide-slot antennas.
Different from above regular shapes of the slot or tuning stub, several special geometries of printed slot antennas, such as dual annular slot, semi-elliptic slot, and etc., were also introduced for UWB applications, as shown in Fig.18. These antennas achieve the impedance bandwidth ratios from 3.7:1 to 7:1. For instance, Ma
Commonly, the operation bandwidth depends on the requirement of the wireless communication system which needs various bandwidths. From above mentioned slot antennas, it is known that the printed wide-slot antenna may achieve a wide range bandwidth based on design of various special slots or stubs. Unfortunately, it maybe wastes a lot of time for antenna designers to find a suitable slot antenna structure according to a required operation bandwidth. So the investigation on the relationship between the slot structure and the bandwidth becomes very useful. For this purpose, reference  presented an interesting deep study on printed binomial-curved slot antennas, where the slot and the tuning stub both formed by a binomial curve function, thus various bandwidths can be obtained based on the structure with different binomial curves.
The CPW-fed printed binomial-curved slot antenna is shown in Fig.19. It consists of a wide slot, a tuning stub, and a CPW feedline, all printed on a single-layer metallic substrate of thickness
Several shapes for different
4.2. UWB dielectric resonator antenna
Dielectric resonator antenna is a new type of directional UWB antenna. It owns a much smaller size and higher efficiency than the UWB printed monopole and wide-slot antennas. Recently, many studies have been proposed to expand the DRA’s bandwidth and promote it into the UWB antenna. One of effective methods to expand the DRA’s bandwidth is the hybrid technique, which combine the DRA and the monopole antenna. Both antennas provide the similar radiation patterns but with different operation frequency bands. Several representative UWB hybrid DRAs are shown in Fig.22
Apart from the UWB hybrid DRA design, recently, Liang
The above mentioned UWB antennas could provide the monopole-like radiation or mushroom-like radiation in an ultra-wideband. While some portal UWB wireless devices need both the monopole-like radiation and the mushroom-like radiation since their position are not fixed in communication. For this purpose, Liang
|1||Annular ring DR+monopole ||probe||10||6.5~16.8 GHz||2.6:1|
|2||Double annular-ring DR+monopole ||probe||4&36||3~11.2 GHz||3.7:1|
+ skirt monopole 
|4||L-shaped DR ||Trapezoidal patch||9.8||3.87~8.17 GHz||2.1:1|
|5||Cross-T-shaped DR ||Trapezoidal patch||9.8||3.56~7.57 GHz||2.1:1|
|6||U-shaped DR ||Triangle patch||9.8||3.1~7.6 GHz||2.4:1|
|7||Z-shaped DR ||Beveled rectangular patch||9.8||2.5~10.3 GHz||4.1:1|
|8||Circular DR ||Crescent patch||35||1.6~15 GHz||9.4:1|
|9||Rectangular DR||Bevel-rectangular patch||9.8||3.9~12.2 GHz||3.1:1|
5. Band-notched UWB antenna and design
The Federal Communication Commission released the frequency band 3.1~10.6 GHz for the UWB system in 2002. But along with the UWB operating bandwidth, there exist some narrowband wireless services, which occupy some of the frequency bands in the UWB band. The most well-known among them is wireless local area network (WLAN) IEEE802.11a and HIPERLAN/2 WLAN operating in 5.15~5.35 GHz and 5.725~5.825 GHz bands. Apart from WLAN, in some European and Asian countries, world interoperability for microwave access (WiMAX) service from 3.3 to 3.6 GHz also shares spectrum with the UWB. This may cause interference between the UWB system and other exist communication systems. To address this problem, one way is to use filters to notch out the interfering bands. However, the use of an additional filter will result in increasing the complexity of the UWB system and also the insertion loss, weight and size for the UWB trans-receivers. Therefore, various UWB antennas with notched functions have been researched to overcome this electromagnetic interference. This section concludes the existing band-notched techniques, which can be classified into the following categories: embedding slot, parasitic stub, bandstop transmission line, and hybrid techniques.
5.1. Embedding slot
Among various proposed techniques on the band-notched UWB antenna design. One common and simple way is to etch slots on the radiation patch or ground plane. Up to now, many shapes of embedding slots were studied, and some representatives are shown in Fig.26
Since split-ring resonator (SRR), electric-LC (ELC) resonator, complementary split-ring resonator (CSRR) and complementary electric-LC (CELC) resonator are commonly used to design a material with negative permittivity and permeability, all these structures can also be applied in UWB antennas for the notched band design. Several representatives are shown in Fig.27. The SRR is generally composed of two concentric split ring strips. It has a favourable aspect in size since it can be designed as small as one-tenth of the resonance wavelength. In , a dual reverse split trapezoid slots, instead of the conventional strip-type SRRs, was proposed and implemented for a bandstop application. In , a slot-type CSRR is etched inside the tuning stub of the printed elliptical slot antenna, and implemented for a band-stop application. It was found that an alterable notched band could take place by adjusting the radiuses of the CSRR. In , the CELC resonator is etched inside the circular patch of the monopole antenna to achieve the notched frequency band. The CELC could provide a predominantly magnetic response. At the notched frequency, the current flows into the CELC region so that the desired high attenuation near the notched frequency would be produced. In , a fractal-binary tree slot embedding technique for the band-notched characteristics design was introduced. By etching a dual band-notched resonance slot using a four-iteration fractal binary tree, two additional filters are applied to the radiating element of the antenna. The fractal, which effect increases the possible length of isolated current paths on the radiating element, has clear and useful properties for band-stop design within small antenna footprints.
5.2. Parasitic stub
Similar to the embedding slot technique in the UWB antenna design, another commonly used technique is a parasitic strip or stub in the aperture area of the antenna or a nearby radiator that forms a resonant structure and leads to a sudden change in the impedance in the notched band. Many parasitic strips or stubs were studied and several representative structures are presented in Fig.28.
For the UWB printed wide-slot antenna design, Liu
For the printed UWB monopole design, Zhang
5.3. Bandstop transmission line
The above mentioned notched-band techniques, such as embedding slot or ELC resonator, parasitic stub, will result in affecting the antenna radiation, especially for increasing of the cross-polarization. A transmission line with a bandstop characteristic to feed the UWB antenna can be considered as an integration design of the printed UWB antenna and the filter, which may have little affection to the antenna radiation. Several designs of microstrip feedline with the notched-band function are proposed, as shown in Fig.29
5.4. Hybrid techniques
Using one notched-band technique will face two problems. Firstly, it is relatively difficult to create multiple frequency notches with a sharp and narrow stop band. Secondly, multi-notched bands do not have any means to control independently because of the same technique. Therefore, various notched-band techniques have been together used to realize the WiMAX and WLAN bands rejection. The representative hybrid techniques are shown in Fig.30
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