**6. Conformal UWB antennas**

In several occasions UWB antennas need to be integrated or mounted on nonplanar surfaces, and consequently the use of conformal UWB antennas has been investigated by several researchers. In [22, 29] three distinct UWB antennas are used in order to test their matching and radiation characteristics when the antennas are

mounted on cylindrical surfaces of different radii. The first is a polygonal monopole antenna which will be referred to with the acronym PM for brevity. The second is a CPW-fed, elliptical slot antenna or ES for brevity, and finally a more compact size elliptical monopole or EM as it will be referred as, for brevity. Since one of the most popular solutions for a variety of UWB applications is the fat monopole, two such prototypes (PM and EM), significantly different in size, are investigated. Liquid crystal polymer (LCP) is used as fabrication substrate because LCP has relatively low dielectric constant (εr = 3) and low loss (tanδ = 0.002). For the two monopole antennas, the PM and the EM, the commercially available substrate, with thickness of 100 μm, is used. The fabricated prototypes are tested in planar shape and also when they are mounted on two different Styrofoam cylinders with radius either 25 or 15 mm. The used prototype antennas in planar shapes are presented in **Figure 11**.

The prototypes are measured in planar shape (R = inf), and then the measurements are repeated with the antennas mounted on custom-made cylinders with (a) radii 25 mm (R = 25 mm) and (b) 15 mm (R = 15 mm). The used cylinders were custom-made at a machine shop, and Styrofoam (εr = 1.03) material was used to resemble free space radiation conditions. When an antenna is kept in planar shape, it may be assumed that it is mounted on a cylinder with infinite radius, and this is the description (R = inf) used in the legends for the following figures. The measured S11 results for the three antennas are presented in **Figure 12**. In every one of the three cases presented in **Figure 11**, it can be observed that the measured reflection coefficient for the three different radius values is almost identical. Radiation pattern measurements can be also seen in **Figures 13**–**15**. Considering the high accuracy and the pattern measurements, in almost every compared pair of

**Figure 11.** *Polygonal monopole (PM), elliptical slot (ES), and elliptical monopole (EM) UWB antennas in planar form.*

#### **Figure 12.**

*Measured S11 under three different bending conditions for (a) polygonal monopole (PM), (b) elliptical slot (ES), and (c) elliptical monopole (EM) [22].*

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**Figure 14.**

**Figure 13.**

radiation patterns, the agreement between the patterns deduced from the planar and folded antennas is noteworthy. Both the shape of the patterns and the maximum directivity remain mostly steady, verifying the good agreement between the

*Measured radiation patterns and gain for the elliptical monopole (EM) in planar form and when it is folded around a Styrofoam cylinder with a 25 mm radius. (a) Conformal EM radiator, (b) E-plane at 5 GHz, (c) H-plane at 5 GHz, (d) measured gain (dBi) vs. frequency, (e) E-plane at 9 GHz, and (f) H-plane at 9 GHz [22].*

*Measured radiation patterns and gain for the polygonal monopole (PM) in planar form and when it is folded around a Styrofoam cylinder with a 25 mm radius. (a) Conformal PM, (b) E-plane at 5 GHz, (c) H-plane at 5 GHz, (d) measured gain (dBi) vs. frequency, (e) E-plane at 9 GHz, and (f) H-plane at 9 GHz [22].*

*Antennas for UWB Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.86985*

*Antennas for UWB Applications DOI: http://dx.doi.org/10.5772/intechopen.86985*

**Figure 13.**

*UWB Technology - Circuits and Systems*

mounted on cylindrical surfaces of different radii. The first is a polygonal monopole antenna which will be referred to with the acronym PM for brevity. The second is a CPW-fed, elliptical slot antenna or ES for brevity, and finally a more compact size elliptical monopole or EM as it will be referred as, for brevity. Since one of the most popular solutions for a variety of UWB applications is the fat monopole, two such prototypes (PM and EM), significantly different in size, are investigated. Liquid crystal polymer (LCP) is used as fabrication substrate because LCP has relatively low dielectric constant (εr = 3) and low loss (tanδ = 0.002). For the two monopole antennas, the PM and the EM, the commercially available substrate, with thickness of 100 μm, is used. The fabricated prototypes are tested in planar shape and also when they are mounted on two different Styrofoam cylinders with radius either 25 or 15 mm. The used prototype antennas in planar shapes are presented in **Figure 11**. The prototypes are measured in planar shape (R = inf), and then the measurements are repeated with the antennas mounted on custom-made cylinders with (a) radii 25 mm (R = 25 mm) and (b) 15 mm (R = 15 mm). The used cylinders were custom-made at a machine shop, and Styrofoam (εr = 1.03) material was used to resemble free space radiation conditions. When an antenna is kept in planar shape, it may be assumed that it is mounted on a cylinder with infinite radius, and this is the description (R = inf) used in the legends for the following figures. The measured S11 results for the three antennas are presented in **Figure 12**. In every one of the three cases presented in **Figure 11**, it can be observed that the measured reflection coefficient for the three different radius values is almost identical. Radiation pattern measurements can be also seen in **Figures 13**–**15**. Considering the high accuracy and the pattern measurements, in almost every compared pair of

**68**

**Figure 12.**

**Figure 11.**

*(ES), and (c) elliptical monopole (EM) [22].*

*Measured S11 under three different bending conditions for (a) polygonal monopole (PM), (b) elliptical slot* 

*Polygonal monopole (PM), elliptical slot (ES), and elliptical monopole (EM) UWB antennas in planar form.*

*Measured radiation patterns and gain for the polygonal monopole (PM) in planar form and when it is folded around a Styrofoam cylinder with a 25 mm radius. (a) Conformal PM, (b) E-plane at 5 GHz, (c) H-plane at 5 GHz, (d) measured gain (dBi) vs. frequency, (e) E-plane at 9 GHz, and (f) H-plane at 9 GHz [22].*

#### **Figure 14.**

*Measured radiation patterns and gain for the elliptical monopole (EM) in planar form and when it is folded around a Styrofoam cylinder with a 25 mm radius. (a) Conformal EM radiator, (b) E-plane at 5 GHz, (c) H-plane at 5 GHz, (d) measured gain (dBi) vs. frequency, (e) E-plane at 9 GHz, and (f) H-plane at 9 GHz [22].*

radiation patterns, the agreement between the patterns deduced from the planar and folded antennas is noteworthy. Both the shape of the patterns and the maximum directivity remain mostly steady, verifying the good agreement between the

**Figure 15.**

*Measured radiation patterns and gain for the elliptical monopole (EM) in planar form and when it is folded around a Styrofoam cylinder with a 25 mm radius. (a) EM radiator, (b) E-plane at 5 GHz, (c) H-plane at 5 GHz, (d) measured gain (dBi) vs. frequency, (e) E-plane at 9 GHz, and (f) H plane at 9 GHz [22].*

**Figure 16.** *Conformal cylindrical array of (a) four EM elements and (b) eights EM elements.*

planar and folded antennas' radiation behavior. In conclusion and considering the testing results for all three prototypes, it can be claimed that the radiation patterns of omnidirectional UWB antennas are not significantly affected by conforming the antennas around a cylinder when the axis of the cylinder is parallel to the feeding line direction.

The use of conformal UWB antennas allows their direct use for the implementation of conformal cylindrical arrays. Such arrays are used for microwave imaging systems that are used for high-accuracy breast tumor detection devices that can be found in hospitals or even for wearable lightweight low-cost devices. **Figure 16** presents a schematic for the design of two cylindrical arrays that consist of four and eight conformal elliptical monopole UWB radiators, respectively. The radiated power is focused toward the axis of the cylinder, and the involved elements can radiate simultaneously, one at a time, or in various combinations implementing multistatic radars with different focus characteristics.

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**8. Conclusion**

*Antennas for UWB Applications*

**Figure 17.**

*a realistic breast phantom.*

*DOI: http://dx.doi.org/10.5772/intechopen.86985*

**7. Microwave imaging radar antenna element**

Recently microwave imaging was considered as an alternative promising technology for medical imaging especially since the required cost is much lower than the most prominent medical imaging methods such as computational tomography (CT) or magnetic resonance imaging (MRI). For the use of microwave imaging devices like the ones used for breast tumor detection in clinical trials [19, 20], the number or radiating elements is important. Generally, it is desired to use high number of elements which should have unidirectional radiation patterns, while the crosscoupling among the radiating elements must be as small as possible. As a result, the use of compact unidirectional UWB radiators is considered. An effective solution is the use of cavity-backed slot antennas. Such an element is presented in **Figure 17**. An implementation of a microwave imaging device consisting of similar such radiators is described in [21, 30]. The front face can be made conformal to better match the non-planar surface, while the feeding cable can pass through the metallic cavity that is used to cancel the back radiation. The desired low profile of the cavity and the intermediate gap between the radiating antenna elements and the target make the design of such an antenna a challenging task. In order to avoid additional reflections, the target is immersed in a liquid with controlled dielectric constant since the UWB antennas' matching can be severely degraded when the antenna radiates in close proximity or in touch with the human body. The high effect of the human body with the complex electromagnetic characteristics on the UWB antenna performance necessitates the use of accurate human phantoms in the full-wave simulations. **Figure 17c** demonstrates a cavity-backed slot UWB antenna in close proximity with a realistic breast phantom in a setup used in a full-wave simulator in order to ensure the good performance of the antenna when it radiates in close proximity with human body parts. Although the metallic cavity increases the profile of the receiver, it is very

*(a) Cavity-backed slot UWB antenna, (b) prospective view, and (c) multiple elements in close proximity with* 

useful since it blocks signals which are not coming directly from the target.

Different vendors [19, 20] use customized UWB antennas which serve better the preferred reconstruction algorithms that they use; however, the presented cavitybacked slot radiator is one of the best candidates for medical microwave devices.

Selected UWB antennas for personal area network communication systems, for positioning and location tracking, and for numerous radar applications are presented. It is evident that depending on the application, different antenna

*Antennas for UWB Applications DOI: http://dx.doi.org/10.5772/intechopen.86985*

**Figure 17.**

*UWB Technology - Circuits and Systems*

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**Figure 16.**

**Figure 15.**

line direction.

*Conformal cylindrical array of (a) four EM elements and (b) eights EM elements.*

multistatic radars with different focus characteristics.

planar and folded antennas' radiation behavior. In conclusion and considering the testing results for all three prototypes, it can be claimed that the radiation patterns of omnidirectional UWB antennas are not significantly affected by conforming the antennas around a cylinder when the axis of the cylinder is parallel to the feeding

*Measured radiation patterns and gain for the elliptical monopole (EM) in planar form and when it is folded around a Styrofoam cylinder with a 25 mm radius. (a) EM radiator, (b) E-plane at 5 GHz, (c) H-plane at 5 GHz, (d) measured gain (dBi) vs. frequency, (e) E-plane at 9 GHz, and (f) H plane at 9 GHz [22].*

The use of conformal UWB antennas allows their direct use for the implementation of conformal cylindrical arrays. Such arrays are used for microwave imaging systems that are used for high-accuracy breast tumor detection devices that can be found in hospitals or even for wearable lightweight low-cost devices. **Figure 16** presents a schematic for the design of two cylindrical arrays that consist of four and eight conformal elliptical monopole UWB radiators, respectively. The radiated power is focused toward the axis of the cylinder, and the involved elements can radiate simultaneously, one at a time, or in various combinations implementing

*(a) Cavity-backed slot UWB antenna, (b) prospective view, and (c) multiple elements in close proximity with a realistic breast phantom.*
