Design of band-notched ultra wideband antenna for ... - IEEE Xplore

www.ietdl.org Published in IET Microwaves, Antennas & Propagation Received on 9th June 2014 Revised on 22nd August 2014 Accepted on 29th August 2014 doi: 10.1049/iet-map.2014.0378

ISSN 1751-8725

Design of band-notched ultra wideband antenna for indoor and wearable wireless communications Masood Ur-Rehman1, Qammer Hussain Abbasi2,3,4, Muhammad Akram3, Clive Parini4 1

Centre for Wireless Research, University of Bedfordshire, Luton, UK Department of Electronic Engineering, Texas A & M University at Qatar 3 University of Engineering and Technology, Lahore, Pakistan 4 School of Electronic Engineering and Computer Science, Queen Mary University of London, UK E-mail: [email protected] 2

Abstract: Design of a tapered-slot ultra wideband (UWB) band-notched wearable antenna is presented in this study. The antenna operation covers the whole UWB frequency spectrum of 7.5 GHz ranging from 3.1 to 10.6 GHz, while rejecting the wireless local area network operation at 5.25 GHz band. The performance of the antenna is analysed through simulations and validated through measurements. The antenna makes use of ultra-thin liquid crystal polymer (LCP) substrate. The presented return loss and radiation pattern results show that the antenna offers excellent performance in the UWB frequency band in free space. Use of the LCP substrate makes the antenna to efficiently mitigate the bending effects. Moreover, the antenna performs well in on-body configurations and its working is little affected in adversely hot and humid weather conditions. Furthermore, it offers good on-body communication link and pulse fidelity. These features make the proposed antenna design a well-suited choice for hand-held and wearable UWB applications.

1

Introduction

Ultra wideband (UWB) has emerged recently as a promising radio technology because of its inherent features of high data rate and efficient bandwidth by utilisation spectrum overlaying employing transmitted power control. The Federal Communications Commission (FCC) has authorised it to operate primarily in frequencies between 3.1 to 10.6 GHz with a 7.5 GHz band, maximum power spectral density of −41.25 dBm/MHz and a maximum transmit power of −2.5 dBm [1]. UWB is an extreme case of spread spectrum technology offering flexibility, robustness, high-precision, radio reusability, high throughput and extended communication range, location determination performance and ranging ability with accuracy in the sub-centimetre range [2]. Demand of portable and wearable wireless devices is ever increasing. Extremely low power consumption and scalable data rates make UWB very attractive for wireless personal area networks (WPANs) and wireless body area networks (WBANs) applications. Antenna is an indispensable component of any wireless device. The optimal performance of a radio system depends greatly on efficient design of the antenna. The human body is an inherent part of the WPAN/WBAN applications. In addition to the general characteristics of antennas, the WPAN/WBAN requires high performance antennas with small size, less weight, body-conformal and inexpensive designs that avert degrading effects of the human body presence [3]. Moreover, the antenna should provide stable IET Microw. Antennas Propag., 2015, Vol. 9, Iss. 3, pp. 243–251 doi: 10.1049/iet-map.2014.0378

electrical and mechanical characteristics even in a harsh weather and bending conditions with no interference to other radio systems. Design of UWB antennas is a well-studied topic [4, 5, 7–22]. Schantz, has presented a clear and concise overview of the major design approaches with comprehensive analytical analysis employed by researchers in recent past [4]. Aldek has proposed a novel microstrip antenna design for the UWB operation with a bandwidth of 103%, low cross polarisation, end fire radiation pattern and high gain [5]. A diamond shaped microstrip patch antenna is proposed by Foudazi et al. [6] Multiple narrow strips are used to create quad-band operation at GSM, GPS, wireless local area network (WLAN) and UWB. The antenna has a small size and stable omni-directional patterns across the bands of operation. Guo et al. have investigated a miniaturised quasi-self-complementary UWB antenna using half-circular disc with an electrical size of 0.24l [7]. Tapered shapes are used very commonly in impedance matching networks to increase the bandwidth. Tapered slot configurations offer a smooth radiation pattern with low cross-polarisation and low cost. Yamaguchi et al. have based their UWB slot antenna on this concept [8]. A UWB antenna design using combination of two crossed exponentially tapered slots and a star-shaped slot is proposed by Costa et al. [9]. The human body is an integral part of the WPAN/WBAN applications. Studying the effects of the human body presence in the vicinity of the UWB antennas has therefore attracted much interest of researchers [11–14, 23]. The UWB 243

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www.ietdl.org antennas suffer from impedance mismatch, radiation pattern degradations, gain variation and detuning, when operating near/on the human body. The proximity effects of the human head on the impedance and radiation performance of the two types of planar UWB antennas have been studied numerically and experimentally by Chen et al. [11]. Klemm et al. [12] numerically characterised the performance of an aperture-stacked UWB patch antenna in free space and on-body. Performance of an all-textile, ‘full-ground-plane’, microstrip UWB antenna designed by combining several techniques to achieve the broadband operation is evaluated both in free space and on body in [13]. A UWB antenna consisting of a compact microstrip-fed monopole printed on a 1.6 mm thick substrate is designed and tested for UWB on-body applications by Chahat et al. [14]. IEEE 802.11a directs that a UWB antenna working for wearable and indoor applications should not interfere with the WLAN applications operating at 5.15–5.35 GHz [24]. Incorporating this band rejection requirement directly in the antenna design avoids the addition of band rejection filters in the UWB system module. Design of band-notched UWB antennas is therefore getting much attention recently. Band-notched function of the UWB antennas can be realised in many ways. Some of the proposed methods include cutting a slot on the patch [15, 16], using parasitic patch elements [17, 18], inserting a strip on the patch [19] or by inserting a slot in the patch and adding two half wavelength stepped impedance resonators around the feed-line [20]. Most of these band-notched UWB antenna designs are rigid (etched on a PCB). They are neither suitable for wearable configurations (that are supposed to undergo bending) nor tested to operate in adverse weather conditions. Hence, design of a UWB antenna having a compact size, light weight, reliable electrical and mechanical characteristics to sustain the bending conditions and adverse environments (i.e. extreme heat or humidity) and offers no interference to existing WLAN systems is much needed for WPAB/WBAN applications. In this work, a band-notched flexible UWB antenna design is proposed that meet all these requirements sufficiently. Radio propagation aspects of the proposed band-notched UWB antenna are studied considering various on-body channels. Time domain performance of the proposed antenna is also evaluated using fidelity and received pulse analysis. Moreover, a parametric study is also carried out to

investigate the effects of the dimensions of the H-shaped slot on the working of the antenna. Following the introduction, this paper is organised in six sections. In Section 2, a brief description of the antenna design process is given, highlighting the process of simulation and fabrication. Section 3 describes the experimental setup, whereas Section 4 presents the simulated and measured results of the proposed antenna structure. On-body antenna performance is also analysed in addition to its performance in extreme working conditions. Section 5 discusses the radio propagation and time domain characteristics of the designed antenna. Section 6 presents the parametric studies for the H-shaped slot and finally conclusions are drawn in Section 7.

2

Antenna design

The proposed antenna design employs the tapered-slot design as its base [21]. It uses liquid crystal polymer (LCP) as the substrate with relative permittivity of 2.9 and a thickness of 0.05 mm. Thickness of the copper layer on the LCP is 0.018 mm. This ultra-thin, flexible, light weight, and low-cost substrate has an extremely low water absorption factor of 0.004, low dissipation factor of 0.002 and low moisture permeability [1, 25], which make it suitable for antennas operating in all kind of environments and different bending configurations. A smooth impedance transition has been achieved using two curved tapered slots as ground. The gap between the patch and the ground plane is optimised to be 0.28 mm. The ratio of semi-major to semi-minor axis of the two ellipses plays the most important role in the impedance matching [21, 22, 26]. The optimised antenna dimensions for the UWB operation result in an overall size of 26 × 16 × 0.068 mm3. It makes this antenna one of the lightest antennas present in open literature for the operation in the frequency range of 3.1 to 10.6 GHz. Fig. 1 shows the simulated model and fabricated prototype of the designed antenna. The antenna is fed using a 50 Ω coplanar waveguide feed. The higher WLAN frequency band (5.15–5.35 GHz) is filtered out using a slot in H-shape placed near the feeding point. The slot has two arms of equal length of 13.5 mm separated by a gap of 1.75 mm. The total length of the two arms and the gap is approximately equals to lg/2 at 5.35 GHz [27].

Fig. 1 Designed antenna (all units are in mm) a Simulated model b Fabricated prototype 244 & The Institution of Engineering and Technology 2015

IET Microw. Antennas Propag., 2015, Vol. 9, Iss. 3, pp. 243–251 doi: 10.1049/iet-map.2014.0378

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Fig. 2 Simulated and fabricated antenna design at different bending angles (15°, 30°, 45°, 60°); a Simulated model b Fabricated prototype

3

Experimental setup

The proposed antenna is modelled numerically and simulated using CST Microwave Studio® which is based on the finite integration technique for the solution of Maxwell’s equations [28]. An Agilent HP8720ES vector network analyser (VNA) is used for measurements in this study. The proposed antenna is tested in different operational scenarios including free space, bent configurations, adverse environments and body mounted configurations. 3.1

Free space

First of all, the antenna is tested for its working in free space. The simulated and measured results for different parameters are compared and analysed in the UWB frequencies. 3.2

Bent configurations

The proposed antenna design is meant to be used in body-worn applications. It is difficult to keep a flat placement of the antenna in most of the wearable scenarios because of the human body shape and movement. Therefore an antenna that operates well in both the conformal form factor and at different bending angles is necessary for such applications. The proposed antenna is therefore bent at different angles including 15°, 30°, 45°, 60° and its performance is observed and compared both in the simulation and the measurement. The simulated and fabricated models of the antenna in the bent configurations at different angles are illustrated in Fig. 2.

3.3

Adversely harsh environments

Ideally, antennas should be able to maintain their performance in different environments, but most of the conventional antennas lack in this regard. Potential major applications of the proposed UWB antenna include its use by the fire-fighters, life-guards, water-divers, emergency and disaster management services etc. Hence, this antenna is expected to cope well with the degrading factors of the harsh environmental conditions especially very high and low temperature and humid conditions. Excellent dielectric stability of the LCP at high temperatures and very low absorption factor in humid conditions should make this antenna to fulfil this criterion well. The performance of the proposed antenna is therefore tested in terms of its return loss for heat resistance and low water absorption. High temperature environment is created by placing the antenna in front of a room heater for 20 min. The temperature of this environment was measured to be 160–180°C using a digital thermo-probe as shown in Fig. 3a. The return loss of the antenna is measured in this environment to observe the heating effects. Then the antenna was submersed in water for 30 min (Fig. 3b) and the return loss was measured after taking it out. 3.4

On-body placement

The antenna design for the wearable applications necessitates the characterisation of the human body presence effects on their performance. The on-body operation of the proposed band-notched UWB antenna is therefore measured by placing it on the left chest of the human subject, that

Fig. 3 Measurement setup for testing the antenna in adverse environments and wearable position a Antenna placed in front of heater and temperature measurement using thermoprobe b Antenna placed in water for 30 min c Antenna positioned on-body at left chest of the human subject IET Microw. Antennas Propag., 2015, Vol. 9, Iss. 3, pp. 243–251 doi: 10.1049/iet-map.2014.0378

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Fig. 4 Comparison of simulated and measured return loss for the antenna in free space

Fig. 6 Comparison of free space return loss with the measurements of the antenna in adverse environment

replicates the most common wearable position as shown in Fig. 3c. These measurements were taken in an anechoic chamber. Low loss coaxial cable with an insertion loss of 0.5 dB is used in these measurements.

4.1.2 Bending angles: Fig. 5 depicts the simulated and measured return loss for different bending angles. A good agreement of the two results is observed. It is evident from these results that the antenna maintains its impedance performance well by exhibiting the −10 dB return loss for whole of the UWB range, while keeping the notch at the required frequency when it is bent at different angles. The return loss response is nearly identical to the no bending (0°) condition for the four considered bending angles. Small fluctuations though appear at extremely bent angle of 60° because of changes in the direction of the impedance transition.

4

Antenna performance evaluation

Antenna performance in various configurations is investigated in terms of return loss, bandwidth, radiation pattern, efficiency and gain as follows: 4.1

Return loss

4.1.1 Free space: Comparison of the simulated and measured return loss responses of the antenna in free space is presented in Fig. 4. The antenna covers the whole of the UWB frequency range with a return loss below than −10 dB level, while filtering out the WLAN band at 5.25 GHz as required. These results show that the antenna exhibits satisfactory performance to fulfil the required specifications. However, small discrepancies are present in the simulated and measured return loss curves. These discrepancies are associated to the fabrication errors because of the ultra-thinness of the substrate. Also the use of coaxial cable and the SMA connector in the measurements affect the operation of this electrically small antenna. A commercial level expert fabrication of the prototype would reduce these errors significantly. Overall, the −10 dB bandwidth is achieved well by the antenna in simulation as well as in measurements.

4.1.3 Adverse environments: The return loss of the antenna at high temperatures and in wet and humid conditions is measured. A comparison of these results with the antenna response in free space is given in Fig. 6. Results clearly show that stark variation in the operational environment has little impact and the antenna preserves its impedance performance well at these two extreme conditions. Hence, it can be deduced from these results that the antenna has attractive temperature stability and water absorption properties for the UWB applications. 4.1.4 On-body placement: The measured on-body return loss of the antenna is compared with the free space response in Fig. 7. A good agreement between the two results indicate that the antenna is matched to the human body and sustains the degrading factors well. Small discrepancies between the two results arise from the change in the electrical properties of the substrate because of the presence of the human body tissues underneath the antenna.

Fig. 5 Simulated and measured return loss at different bending angles a Simulated results b Measured results 246 & The Institution of Engineering and Technology 2015


www.ietdl.org radiations. These effects are more visible at higher frequencies. It is because of the fact that with increase in the frequency, the human body backscatters higher amount of energy giving rise to the directivity and changing the induced current on the antenna structure. Moreover, antenna efficiency decreases because of absorptions in the lossy human body tissue. 4.3

Fig. 7 Return loss comparison of free space and on-body antenna configurations

4.2

Radiation pattern

The antenna radiation pattern is also studied in different scenarios including free space, different bending angles (15°, 30°, 45° and 60°) and on-body placement. The radiation pattern of the fabricated prototype is measured in both the E-plane and H-plane at different frequencies. Figs. 8 and 9 show E-plane and H-plane radiation patterns at 3, 5.25, 6.5 and 9 GHz, respectively. The antenna has nearly omni-directional radiation in the H-plane over the entire UWB frequency range. The antenna maintains this feature stably even in conformed configurations at various bending angles. In the E-plane, the antenna works such as a dipole radiating a donut-shaped pattern at 3 GHz. Small fluctuations in the pattern are observed though at the higher end of the band particularly at the frequencies of 6.5 and 9 GHz. The radiation characteristics of the antenna in the bent configurations remain identical to the planar (non-bent) structure throughout the frequency range. Although, small variations between the two scenarios are observed at 5.25 GHz however, gain of the major lobe is at the same level for various bending angles at this frequency. The antenna radiation patterns in on-body configuration are also measured. Results in Figs. 8 and 9 show that the human body presence deteriorates the antenna performance substantially with higher directivity and lower back

Antenna gain and efficiency

Fig. 10 shows the simulated and measured results for the peak gain and simulated results for the total efficiency of the proposed band-notched UWB antenna. It is evident from the results that the antenna offers good gain performance with values ranging between 2–3 dBi in the whole frequency band of interest. Maximum value of the gain is noted at 6.5 GHz. The antenna also has good efficiency varying from 94% at 5.25 GHz to 99% at 9 GHz.

5 Radio propagation and time domain characterisation The proposed band-notched UWB LCP antenna is characterised for radio propagation and time domain performance in the following sections. A pair of the fabricated antenna is used in these studies. 5.1

Radio propagation characterisation

Radio propagation of the designed antenna is characterised by measuring the mean path loss (S21) for various on-body channels. In these measurements, the transmitting antenna was fixed on left waist, while the receiving antenna was placed at various positions on the body of the human subject as shown in Fig. 11a. The measurements were taken in an anechoic chamber. Three bending orientations including planar (reference), vertically bent and horizontally bent as shown in Fig. 11b and c are used. The VNA was set to use 1601 sampling points and five sweeps at a sweep rate of 800 ms were performed and then averaged. The path loss values for each channel are obtained by performing averaging across the whole UWB band [29]. Table 1 summarises the path loss for different on-body communication channels. Results show that the proposed

Fig. 8 Measured radiation patterns of band notched CPW-fed LCP antenna at different frequencies and bending angles in E-plane a 3 GHz b 5.25 GHz c 6.5 GHz d 9 GHz IET Microw. Antennas Propag., 2015, Vol. 9, Iss. 3, pp. 243–251 doi: 10.1049/iet-map.2014.0378

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Fig. 9 Measured radiation patterns of band notched CPW-fed LCP antenna at different frequencies and bending angles in H-plane a 3 GHz b 5.25 GHz c 6.5 GHz d 9 GHz

Fig. 10 Simulated and measured peak gain and total efficiency values of the proposed UWB antenna a Gain b Efficiency

Fig. 11 Setup for on-body path loss measurements a On-body position of transmitter and receivers b Horizontally bent orientation c Vertically bent orientation

antenna establishes good on-body communication link. The weakest channel is observed to be the waist and right head link with a path loss of −80.69 dB, whereas the link between the waist and left wrist appears to be the strongest

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channel with a path loss of −52.90 dB. These results also show that bending the antenna vertically or horizontally brings no significant distortion to the path loss value for all the considered on-body channels.


www.ietdl.org Table 1 Path loss measurement for different orientation and channels On-body channel

Average path loss, dB

waist-back waist-left ankle waist-left chest waist-left head waist-left wrist waist-right ankle waist-right belly waist-right chest waist-right head waist-right wrist

5.2

Reference, Planar

Horizontally bend

Vertically bend

−79.78 −75.87

−80.36 −77.31

−80.38 −77.62

−62.04

−63.85

−63.08

−65.23 −52.90 −72.57

−69.11 −53.83 −73.94

−69.80 −53.50 −73.45

−74.76

−75.23

−75.22

−75.69

−76.68

−76.26

−80.69

−80.71

−80.84

−72.37

−73.83

−73.31

Time domain characteristics

The UWB systems often employ short pulses to deliver information. The overall performance of a UWB antenna cannot be assessed only by its frequency domain characteristics. Therefore study of UWB antennas from a time domain perspective is indispensable. Ideally, the received UWB signal should maintain exactly the same shape as the source pulse. Practically, the received signals normally are distorted in shape and sometimes have the ringing effect. The time domain performance of a UWB antenna can be characterised by a well-defined parameter termed fidelity and is given by [30] ⎡

+1

⎤

⎢ −1 st (t).sr (t − t).dt ⎥ F = maxt ⎣ ⎦ +1 2 +1 2 (t).dt s (t). s −1 t −1 r

(1)

where st (t) is the source pulse and sr(t) is the received pulse. Fidelity is the correlation between the source pulse and the radiated electric field denoting the extent of the similarity between the two pulses. It ranges from 0 to 1 where 1 indicating maximum similarity between the two signals. It is usually described as a normalised coefficient to factor out any scaling effects. For the time domain characterisation of the antenna, the antenna is connected to the VNA and a UWB pulse is generated from the VNA. The source signal in this case is

Fig. 13 Channel impulse response for face to face configuration

the radiated signal. The measured frequency domain response (S21) is transformed into time domain using Hermitian processing [30]. The impulse response is measured and fidelity is being calculated using this data for face-to-face (i.e. antennas are placed face-to-face at different angles) and side-to-side (i.e. antennas are placed side-to-side at different angles) configuration as shown in Fig. 12. The separation between the two antennas is kept at d = 1.2 m. For the two cases, received pulse waveforms generally follow the shape of the source pulse with slight distortions. The received waveforms at different angles for face-to-face configuration are illustrated in Fig. 13. The measured fidelity F is 0.91 for face-to-face scenario and 0.86 for side-by-side scenario. This makes the designed antenna a promising candidate with excellent time domain characteristics [3].

6

Parametric study

This section investigates the effects of the dimensions of the H-shaped slot on the performance of the band-notched UWB antenna. 6.1

Side arms

Impact of the geometry of the two side arms of the H-shaped slot has been studied. Results in Fig. 14 show that the frequency of the band notch operation depends greatly on the length of the two side arms, whereas it is less affected by the width of the two arms. 6.2

Middle arm

The impact of the variation in the width and position of the middle arm on the antenna performance has also been studied. The return loss curves presented in Fig. 15 depict

Fig. 12 Measurement setup for time domain characterisation of the proposed antenna a Face-to-face configuration b Side-by-side configuration IET Microw. Antennas Propag., 2015, Vol. 9, Iss. 3, pp. 243–251 doi: 10.1049/iet-map.2014.0378

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Fig. 14 Effects of length and width of the side arms of H-shaped slot a Length of side arm b Width of side arm

Fig. 15 Effects of width and position of the middle arm of H-shaped slot a Length of middle arm b Width of middle arm

that the position of the middle arm plays a vital role in the antenna matching and the WLAN frequency filtering. The optimised position is taken as reference. It is noted that the notch shifts to higher frequencies, when the position is varied in upward direction (i.e. away from the feed). Change in the width of the middle arm also shifts the notch upward, but its effect is substantially low in comparison. It is evident from the parametric study that the band notch operation of the antenna is highly dependent on the optimised dimensions of the H-shaped slot.

7

Conclusion

A novel antenna design for wearable UWB communication devices having a notch at higher WLAN frequency has been proposed. The antenna structure is small, low profile and very flexible. The presented results show that the proposed antenna performs excellently in free space, upon bending at different angles, under adverse weather conditions (extremely high temperature and humidity) and on-body configurations maintaining −10 dB bandwidth and radiation characteristics. The antenna also exhibits good time domain performance. Excellent band-notched UWB operation with reliable electrical and mechanical characteristics makes the proposed antenna a well-suited candidate for indoor and wearable applications.

8

References

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