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
fl – the lower frequency of the operation band.
fh – the higher frequency of the operation band.
fc – the center frequency of the operation band.
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]. i.e., one of interest rediscoveries of the biconical antenna and conical monopole is done by P. S. Carter in 1939  (See Fig.2). Carter improved upon Lodge’s original design by incorporating a tapered feed, one of the key steps towards the design of broadband antennas.
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. i.e., Agrawall et al.  carried out a bandwidth comparison of several planar monopoles with various geometries, such as circular, elliptical, rectangular, and trapezoidal monopoles. The results show that the circular and elliptical monopoles exhibit much wider bandwidth performance than those of others, and both can obtain the impedance bandwidth ratio exceeding of 10:1 (Circular monopole: 1.17~12 GHz, Elliptical monopole: 1.21~13 GHz). Evans et al.  proposed a trapezoidal planar monopole antenna above the ground plane, also achieved a bandwidth ratio exceeding of 11:1. Besides the regular structures, Suh et al.  proposed an interesting structure, the planar inverted cone antenna (PICA), which can provide an impedance bandwidth ratio of more than 10:1 and the pattern bandwidth ratio of about 4:1. To improve the pattern bandwidth ratio, two circular holes are added in the PICA, as shown in Fig.9(e). This alteration improves the radiation pattern performance dramatically without impairing the impedance performance, where the radiation pattern of the two-circular-hole PICA antenna provides a good omnidirectional performance over a bandwidth ratio up to 7:1 and has a very low cross polarization, 20 dB or less. Later, Bai et al.  presented a modified PICA, where a leaf-shaped metal plate with three circular holes is vertically mounted on the ground plate and is covered by a dielectric plate instead of the conventional metal plate. It achieves the impedance bandwidth ratio better than 20:1, covering the frequency range from 1.3 to 29.7GHz, as shown in Fig.9(f).
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 i.e., Su et al.  proposed a method of cutting a pair of notches at the two lower corners of the square planar monopole. With suitable dimensions of the notches chosen, the impedance bandwidth can be greatly enhanced to be about 3 times that of a corresponding simple square planar monopole antenna (2~12.7 GHz compared to 2~4.5 GHz). Antonino-Daviu et al.  proposed a method of double feed with aims to intense the vertical current distribution and suppress the horizontal distribution in the square planar monopole, which contributes to improvement in the impedance bandwidth and polarization properties. This method achieves an impedance bandwidth ratio of 9.5:1. However, in this double-feed design, an additional feeding network under the ground plane is required to excite the antenna at two separate feeding positions. To avoid using the external feeding network, Wong et al.  proposed a trident-shaped feeding strip structure. With the use of a trident-shaped feeding strip, the square planar antenna’s impedance bandwidth can be enhanced to be larger than 3.5 times that of a simple feeding strip (1.4~11.4 GHz compared to 1.5 ~3.3 GHz). Moreover, hybrid techniques are also proposed, i.e., Thomas et al.  used a sleeved transmission line as a transformer together with the bevelling technique, the impedance bandwidth ratio reaches about 12:1 (0.5 ~ 6 GHz). Ammann et al.  adopted a method of combining the bevelling and the shorting techniques, the square planar monopole antenna’s bandwidth ratio can be expanded to more than 13:1 (0.8 ~10.5 GHz).
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 et al. [20, 21] introduced this type of antenna with the wideband characteristics in 2004, various printed monopole antennas were studied in the following several years, mainly on the geometries of the monopole and the ground plane.
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,. i.e., Ooi et al.  introduced the two-layer octagon monopole antenna based on the low-temperature co-fired ceramic (LTCC) technique, also obtaining an impedance bandwidth ratio of 3.8:1 (3.76~14.42 GHz). Lizzi et al.  proposed the spline-shaped monopole UWB antenna able to support multiple mobile wireless standards, covering DCS, PCS, UMTS, and ISM bands, with the bandwidth ratio of 2.3:1 (1.7~2.5 GHz).
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. i.e., Huang et al. [28, 29] introduced an impedance matching technique by cutting a notch at the ground plane, and the impedance bandwidth can be enhanced by suitable size and position of notch chosen. Azim et al.  proposed to improve the impedance bandwidth by cutting triangular shaped slots on the top edge of the ground plane. The printed square monopole antenna with symmetrical saw-tooth ground plane obtains the impedance bandwidth ratio of 5.5:1 (2.9~16GHz). Considering high concentration of currents in the corners of the patch or ground, Melo et al.  studied a rounded monopole patch with a rounded truncated ground plane. It provides an impedance bandwidth ratio of larger than 4.7:1 (2.55 ~12 GHz).
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. i.e., Fortino et al.  introduced a CPW-fed printed triangular monopole antenna with a specific ground plane, such as the double folded structure, where the ground plane is optimized to obtain more constant radiation patterns versus frequency. Wu et al.  also pointed out the ground plane may affect the antenna’s H-plane radiation pattern as its width could be comparable with the wavelength at the higher operating frequency. Thus, a compact ground plane with a trident-shaped feed structure is proposed to significantly improve the H-plane radiation pattern in a very wide impedance bandwidth, as shown in Fig.14.
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 et al. , where a 40% size reduction is realized by simply exploiting its structural symmetry, as shown in Fig.15. It is found that the miniaturized bevelled planar monopole antenna exhibits a much wider impedance bandwidth, higher cross-polar radiation, and slightly lower gain at higher frequencies as compared with its un-miniaturized counterpart.
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. i.e., Jang et al. [40, 41] presented two rectangular wide-slot antennas fed by the microstrip line with a cross-shaped stub and a Π-shaped stub, respectively. The antenna bandwidth greatly depends on the length of the horizontal and vertical feed lines as well as on the offset position of the feedline. The two slot antennas achieve the impedance bandwidth ratios of 2.8:1 (1.7~ 4.9 GHz) and 3.5:1 (1.7~ 6.0 GHz), respectively. Later, Yao et al.  proposed a fan-shaped microstrip stub together with a strip, which contributes to a little wider with the bandwidth ratio of 3.6:1 (0.5~ 5.7 GHz). A rectangular slot with a rectangular tuning stub was also studied , but with the bandwidth ratio of 1.8:1.
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. i.e., Azim et al.  presented a tapered-shape slot excited by a rectangular tuning stub, which achieves an impedance bandwidth ratio of 3.1:1 (3~11.2 GHz) with a stable gain and radiation over the bandwidth. Denidni et al.  proposed a circular slot fed by a circular patch through a CPW feedline. This configuration offers a much larger bandwidth ratio of 6.1:1 (2.3~13.9 GHz). Furthermore, Angelopoulos et al.  investigated the elliptical slot with an elliptical tuning stub achieving an impedance bandwidth ratio of 15.4:1 (1.3~20 GHz), which is the widest bandwidth of the printed slot antennas in open literature.
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 et al.  introduced a tapered-slot-fed annular slot antenna. The tapered-slot feeding structure serves as an impedance transformer and guides the wave propagating from the slot line to the radiating slot without causing pernicious reflection. The radiating slot is curved to distribute part of the energy to the reverse side of the feeding aperture. This antenna achieves an impedance bandwidth ratio of 3.7:1 (2.95~11 GHz). Gopikrishna et al.  introduced a semi-elliptic slot antenna. The antenna features a CPW signal strip terminated with a semi-elliptic stub and a modified ground plane to achieve a wide bandwidth ratio of 7:1 (2.85~20 GHz). Pourahmadazar et al.  studied a special square slot antenna for circular polarization, which is composed of a square ground plane embedded with two unequal-size inverted-L strips around two opposite corners of the square slot. The antenna owns an impedance bandwidth ratio of 4.7:1 (2.67~13 GHz) and a circular polarization bandwidth ratio of 1.5:1 (4.9~6.9 GHz). Sim et al.  proposed a compact microstrip-fed narrow slot antenna design for UWB applications. By properly loading a notch to the open-ended T-shaped slot and extending a small section to the microstrip feedline, an impedance bandwidth ratio of 3.7:1 (3.1 ~11.45 GHz) is obtained.
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 h and relative permittivity εr, and with dimension of L × W. The slot’s outline size is denoted as l × w, where the coordinate of point A on the up-right of the slot is fixed to (w/2, l) and the edge is formed by a binomial curve function, expressed as follows:
where N is the order of the binomial curve function. The slot is excited by a CPW feedline, where the signal strip width is wf, and the gap spacing between the signal strip and the coplanar ground plane is g. To achieve an efficient excitation and a wide impedance matching, the signal strip is terminated to a tuning stub with the same shape as the slot but with a smaller size, which has an offset d away from the bottom edge of slot. The stub-to-slot’s outline size-ratio is denoted as τ. Therefore, the coordinate of point B on the upper-right of the tuning stub is denoted as (τw/2, τl+d), and the binomial curve function for the edge of the tuning stub can be rewritten as follows:
Several shapes for different N are given in Fig.20. As N equals to 1, both the slot and the tuning stub are the triangular shape. As N increases, the bottom widths of the slot and the tuning stub expand gradually, and their shapes look like bowls. As N approaches infinity, both the slot and the tuning stub are transformed to the rectangular shape. From Fig.21, it is found that the operation bandwidth range varies from 10~20 % to 20~40 %, 25~60 %, 60~90 %, 70~110% and 85~110%, as the order N increases from 1 to 2, 3, 6, 12 and ∞, respectively, meaning that the larger order N is selected, the wider bandwidth may be obtained. It is also known that a proffered wideband slot antenna can be easily designed based on suitable selection of parameters, such as w/l, N, according to the operation bandwidth in applications, which is convenient for the wideband antenna design.
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 i.e., Lapierre et al.  firstly proposed a hybrid antenna design by combining the properties of a quarter wavelength monopole antenna with an annular DRA. This design can be used to retrofit existing monopole antennas: by introducing an appropriate DRA, the original narrow-band monopole can be transformed to achieve an UWB performance, with bandwidth ratio of 2.6:1 (6.5~16.8 GHz). To enhance the antenna bandwidth further, Ruan et al.  proposed a double annular ring DR with different permittivity, which achieves an impedance bandwidth ratio of 3.7:1 (1.8~6.9 GHz). Later, Jazi et al.  proposed a skirt monopole antenna used to excite an inverted conical-ring-shape dielectric resonator. In this design, three different methods of impedance matching, dielectric, and ground plane shaping procedures have been applied to increase the antenna bandwidth. The results show that by shaping the dielectric structure and impedance matching method, the input impedance and location of the higher part of the frequency bandwidth can be controlled by exciting higher order mode (TM012+δ) of the same family with dominant resonant mode inside the DR (TM01δ). The lower part of the input impedance bandwidth can be adjusted using the ground plane shaping and matching method at the feed. This antenna achieves the bandwidth ratio of 3.8:1 (1.8~6.9 GHz).
Apart from the UWB hybrid DRA design, recently, Liang et al. [57-61] proposed a patch feed technique and DRAs with various alphabet structures, such as cross-T-shaped , L-shaped , U-shaped , and Z-shaped , as shown in Fig.23, where the DRA bandwidth ratios of 2.1:1~9.4:1 were obtained. Fig.24 presents the trapezoidal patch-fed L-shaped DRA design process. To explain the wideband operation of the patch-fed L-shaped DRA, three reference antennas are used as references. Fig.24 (a) is a rectangular DR on a ground plane; Fig.24 (b) is a rectangular DR on a single-sided copper-clad substrate, and Fig.24 (c) is an L-shaped DR on a single-sided copper-clad substrate. The three antennas are excited by the probe feed mechanism and their optimized numerical results in terms of bandwidth are compared with patch-fed L-shaped DRA, as shown in Fig.24. It is observed that the impedance bandwidth can be expanded by using an inserted intermediate substrate, an L-shaped DR, and an inverted-trapezoidal patch feed mechanism. Table 1 lists several proposed UWB DRAs, including the antenna geometry, the feed mechanism and the bandwidth.
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 et al.  proposed a UWB DRA with the configurable radiation pattern design, where a rectangular DR excited by dual bevel-rectangular patches. Two bevel-rectangular metal patches are attached on the opposite sides of the DR for excitation and both connect to the 50-ohm microstrip lines, as shown in Fig.25. The bevel-rectangular patch-fed DRA has been proposed to achieve an UWB operation. A reconfigurable radiation pattern performance in terms of the monopole-like radiation and the mushroom-like radiation is obtained through the in-phase feed and the out-phase feed of two input ports, respectively. For the in-phase feed, the antenna performs the monopole-like radiation pattern in the entire operation band. While for the out-phase feed, the antenna performs the mushroom-like radiation in the same operation band.
|No||Antenna Geometries||Feed mechanisms||εDR||VSWR ≤ 2|
|Frequency range||Bandwidth ratio|
|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 i.e., Kim et al.  proposed a CPW-fed planar UWB antenna with a hexagonal radiating element. By inserting a V-shaped thin slot with a length of λc/4 (λc is the wavelength of notched frequency) on the hexagonal radiating element, the narrow frequency band-notched is created, where the fractional bandwidth is approximately 8~10%. Chung et al. [64-66] introduced the printed UWB monopole antenna by inserting an inverted U-shaped, Π-shaped or rectangular slot. At the notched frequency, current is concentrated around the edges of the slot and is oppositely directed between the interior and the notched frequency. This leads to the desired high attenuation near the notched frequency. In , a band-notched printed monopole antenna is provided by using two modified U-shaped slots on the monopole. The U-shaped slot perturbs the resonant response and also acts as a half-wave resonant structure. At the notched frequency, the desired high attenuation near the notched frequency can be produced. Jiang et al. [68, 69] introduced a pair of inverted-L-shaped slots around the microstrip line on the ground plane; a frequency-notched response can also be achieved.
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 et al.  proposed a UWB rectangular slot antenna with a fractal tuning stub to realize the notched function. Chui et al.  proposed a branch with a length of a quarter of the wavelength adding on the tuning stub to obtain the band-notched property. Cai et al.  studied a pair of elliptic arc-shaped strips inserted into a suitable aperture region to disturb the field distribution that generates the resonance at the designed notched-band. The total length of this pair of strips is adjusted to about half-wavelength at the desired notched-band, resonance will occur at the strips.
For the printed UWB monopole design, Zhang et al.  introduced a segmented circular planar monopole antenna with a notched band. Through cutting apart a circular monopole patch with a pair of symmetrical slots, the patch is divided into three segments: the center patch and two side patches. Practically, the side patches function as two parasitic elements and work as bandstop filters. Then, the band-notched property is achieved. Wu et al.  introduced a square looped resonator and an end-coupled resonator to achieve the gain suppression in the notched band. The square-looped resonator consists of two square loops whose physical length approximates half a wavelength at the notched frequency. Meanwhile, the end-coupled resonator is composed of a strip line with a pair of quarter wavelength folded open stubs. Compared with the band-notched methods using thin slits and plastic strips, this resonator has a small size and a fast rolloff rate well as 10–25 dB gain suppression (generally, the gain suppression of thin slits and plastic strips are usually less than 10 dB).
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 i.e., Zhang et al.  proposed a U-shaped slot embedded in a microstrip feedline, and a band-notched characteristic was realized. Later, Nouri et al.  used the defect ground technique to realize the microstrip filter, where a vertical metal strip connected to the rectangular ring is embedded in the shovel-shaped slot that is located under the feedline at the center of ground plane. The notched frequency can be controlled by adjusting the dimensions of the filter structure. Moreover, the electromagnetic band-gap (EBG) structure has a characteristic of preventing wave propagation in special directions or at certain frequencies. In , square EBG cells are placed close to the microstrip feedline to obtain the desired notched bands. In , the dual band-notched characteristic has been achieved by introducing two open-circuited stubs from two sides of the microstrip feedline. By adjusting the length of two open circuited stubs approximately to one quarter of wavelength, a destructive interference of the current distribution takes place causing the antenna non-radiating at that notched frequency.
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 i.e., Abdollahavand et al.  studied the hybrid technique by adding parasitic strip and bandstop transmission line. A compound band-notched structure is formed by embedding Г-shaped stubs in the radiation patch and a modified G-slot defected ground structure in the feeding line, which can provide two filtering frequencies in a certain band and function as a second-order filter. Ye et al.  studied the hybrid technique of a parasitic strip and a parasitic slit, where the parasitic strip is embedded inside the polygon slot and an isolated slit employed in the beveled T-stub. The desired excellent band-notched UWB operation can be obtained by choosing the sizes of the parasitic strip and slit. Zhou et al.  presented the hybrid technique of adding parasitic and embedding slot to realize dual notched bands of WiMAX and WLAN. Firstly, the circular patch is cut to an annular ring and a pair of Y-shaped strips is connected to the annular ring, the notched band of WiMAX centered at 3.5 GHz is realized. Then, an inverted V-shaped slot is etched on the patch, a notched band of 5.2–5.98 GHz for WLAN band is achieved. Niu et al.  used the hybrid technique of CCRC resonator and embedding slot, where a CCRC is to realize the 5 GHz WLAN notched-band and an elliptic arc-shaped slot is to realize the WiMAX notched band. Kim et al.  suggested a triple-band notched hybrid technique, which is based on a geometric combination of a meander shaped stub and the two rectangular complementary split ring resonators (CSRRs) on the feedline, and an inverted U-shaped slot on the center of the patch.