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A Novel Folded UWB Antenna for Wireless Body Area Network
Cheng-Hung Kang, Sung-Jung Wu, and Jenn-Hwan Tarng
Abstract—A novel folded ultrawideband antenna for Wireless Body Area
Network (WBAN) is proposed, which can effectively reduce the backward radiation and proximity effects of human bodies. The proposed antenna has a low-profile 3D structure that consists of a bevel-edge feed structure and a metal plate with folded strip. The bevel edge feed structure achieves broadband impedance matching and the metal plate acts as the main radi-ator. Moreover, the folded strip not only extends the lower frequency band but also provides additional resonant frequency around 6 GHz. The final bandwidth covers from 3.1 GHz to 12 GHz. The proposed antenna shows the directional patterns with low backward radiation due to the patch-like structure and the ground plane also prevents from the proximity ef-fects of human bodies. Furthermore, the simulated SAR values of the pro-posed antenna are lower than the values of omnidirectional disc planar monopole. These features demonstrate that the proposed antenna is suit-able for WBAN application.
Index Terms—Body-area network, 3-D antenna, ultrawideband.
I. INTRODUCTION
Wireless body area network (WBAN), a communication system which transmits large amount of information/data near the human body, is attracting more and more attention in wireless communi-cations [1]. WBAN integrated with proper sensors can observe and transmit vital signs of patients, police or fire personnel without cables. With increasing attention directed toward WBAN, the ultrawideband (UWB) technology becomes an active solution for these applications because of its low transmission power and high data rates.
The antenna and propagation measurements for WBAN are dis-cussed in [2], [3]. The results show that the path loss and the rms delay spread are highly related to antenna structure. Several studies provide various antenna designs used in WBAN [4]–[7]. In [2], a 3-D monopole antenna placed perpendicular to the human body is designed for WBAN. However, the 3-D monopole antenna is too high so it
Manuscript received May 21, 2010; revised May 15, 2011; accepted July 25, 2011. Date of publication October 24, 2011; date of current version February 03, 2012. This work was supported by the National Science Council, R.O.C., under Grants NSC-99-2219-E-009-001 and NSC-99-2221-E-009-028-MY2.
The authors are with National Chiao Tung University, Hsinchu, Taiwan 300, ROC (e-mail: chkang.cm97g@g2.nctu.edu.tw).
Color versions of one or more of the figures in this comunication are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2011.2173101
1140 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 2, FEBRUARY 2012
Fig. 1. (a) Configuration of the proposed ultrawideband antenna. (b) The an-tenna unbent into a planar structure.
Fig. 2. Measured and simulated return loss of the proposed antenna.
will obstruct the human daily activities when applied to the human body. Therefore, planar monopole antennas with low antenna height are widely developed in the UWB community. However, when these antennas lie on the human body, the operating frequency, bandwidth and radiation efficiency are easily interfered with by the human body. Except the aforementioned influences, specific absorption rate (SAR) is another important issue in WBAN. In [5], it has been shown sim-ulated results that the antenna with omnidirectional pattern exhibits low radiation efficiency and high SAR values compared to directional antenna when placed on human model.
Low backward radiation, low height with compact form, and low mutual effect between the antenna and the human body are three major requirements [5]–[7] for WBAN antennas. These features increase the difficulty of antenna design. To solve this issue, some studies propose using a reflector in antenna design to reduce the backward radiation and enhance directionality. In [5], a reflector was added to a 3 GHz–6 GHz slot antenna and this additional reflector enhanced the directionality and radiation efficiency. But the additional reflector affects the antenna bandwidth which becomes 4 GHz to 6.5 GHz and is not wide enough for the UWB system.
In this communication, a novel directional UWB antenna is proposed for the WBAN application. The proposed antenna consists of a bevel
Fig. 3. Simulated return loss of various lengths ofL and L .
Fig. 4. Simulated return loss of various lengths ofH .
Fig. 5. Simulated return loss of various ground size.
edge feed structure and a truncated metal plate with folded strip. The size of the proposed antenna is 252 22 2 10 mm3with ground plane 502 50 mm2 and the bandwidth covers from 3.1 GHz to 12 GHz. Section II presents the geometry and design concept of the proposed antenna. The design parameters and simulated SAR values are also in-troduced in Section II. Radiation patterns are shown in Section III. Fi-nally, Section IV draws some conclusions.
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 2, FEBRUARY 2012 1141
Fig. 6. Current distribution of the proposed antenna at (a) 4 GHz, (b) 7 GHz, (c) 10 GHz.
II. ANTENNADESIGNCONCEPT ANDPERFORMANCE
Fig. 1 shows the geometry of the proposed antenna. The proposed antenna consists of a bevel edge feed structure and a truncated metal plate with folded strip. To enlarge bandwidth, the bevel edge structure achieves slow impedance variation by using traveling wave concept. The metal plate is designed as a main radiator. The truncated edge and the folded strip of the metal plate extend the current path to lower the operating frequency. In addition, because of the patch-like struc-ture, the wave radiates toward the z direction and the ground plane re-duces the backward radiation. The whole antenna size is 25 mm2 22 mm2 10 mm with a 50 mm 2 50 mm ground plane. The final param-eters of the proposed antenna areH1 = 4 mm, H2 = 6 mm, H3= 3 mm,W1 = 20 mm, W2=W3 = 5 mm, L1= 22 mm, L2= 8 mm, L3 = 7 mm, L4 = 10 mm.
Fig. 2 shows the simulated and measured return losses. The sim-ulation was performed using a commercial simulator while the mea-surements were taken by an E8364B network analyzer. The measured bandwidth covers from 3.1 GHz to 12 GHz and agrees with the simu-lated results. The minor discrepancies of simulation and measurement may be attributed to the connector, which is not considered in the sim-ulation. Fig. 2 also shows the simulated result of the proposed antenna without the folded strip. It is evidenced that the folded strip not only determines the lower operating frequency from 3 to 4 GHz but also cre-ates an additional resonant frequency around 6 GHz. The folded strip is the key factor of antenna design.
The effect of folded strip is presented in Fig. 3. The folded strip extends the current path and creates lower resonant frequencies. Ac-cording to Fig. 3, the length ofL3affects the lowest frequency and when the length ofL4increases, the impedance in the middle frequency be-comes mismatched. By suitably adjusting the length of the strip, we can make the whole frequency band under the 10 dB return loss condition. The effect of the feed structure is shown in Fig. 4. The tapered pro-file of the feed structure achieves the slow impedance variation for ob-taining the ultra-wide bandwidth. The slope of the bevel edge should be carefully designed to achieve wideband matching. In our experiments, theH1 should be 4 mm to obtain better impedance matching for the whole operating frequency.
Fig. 5 shows the parametric simulations with regard to the different ground plane size. For the proposed antenna which radiates as a patch antenna, the ground plane should be large enough to resonant the desired frequency. In the simulating results, it can be observed that the impedance match is interfered and the bandwidth becomes narrow when the ground plane becomes 302 30 mm2which is close to the main radiator. Therefore, the ground plane size of the proposed antenna should be larger than 402 40 mm2 to generate the wanted resonance mode. In the final design, 502 50 mm is chosen for the ground plane by the dimension and better impedance match.
Furthermore, the current distribution of the proposed antenna in 4 GHz, 7 GHz and 10 GHz is exhibited in Fig. 6. In the low frequency, the current distributes along the edge of the truncated plate and the folded strip, which is like a patch antenna. Therefore, the truncated
Fig. 7. Comparison measured return loss between in free space and on the body.
part and the length of the folded strip determines the lowest frequency. In addition, a resonance mode can be observed at the folded strip in middle band. For high band, the tapered profile fed structure travels the energy to the plate and radiate as combination of general monopole and patch antenna. Moreover, the shape of the tapered profile crucially affects the impedance match within the whole band. The proposed antenna combines the patch-like radiator and traveling wave concept to achieve the ultra-wide bandwidth and directional patterns.
In order to verify the proximity effect of human body, return loss of the proposed antenna in free space and on the body are measured, as shown in Fig. 7. The spacing between the proposed antenna and the human skin is 2 mm. The proximity effect of the human body slightly affects the impedance matching of the proposed antenna because that directivity of the proposed antenna is outward from the human body.
Furthermore, a truncated body model is considered in simulation to estimate the specific absorption rate (SAR) value and radiation efficiency by software SEMCAD X. Two different body model is con-sidered in the simulations, one is single layer muscle model and another is three layers body model with skin, fat and muscle according to the [5]. Full dimensions of the models are skin: 1202 110 2 1 mm3with"r= 38, = 2:7 [S/m], fat: 120 2 110 2 3 mm3with"r = 5:1; = 0:18 [S/m] and muscle: 1202 110 2 40 mm3with"r = 50:8; = 3 [S/m] according to [5]. Moreover, in order to reveal the SAR values in relative way, a planar disc monopole antenna with omnidirection pattern, as shown in Fig. 8, is also involved in the simulation for comparison. For keeping the same distance from the model to the top of the antenna (an-tenna height: 10 mm), the proposed an(an-tenna is 1 mm away from the body model and the planar disc monopole antenna (antenna height: 1.6 mm) is 10 mm away from the body model. Table I shows the simulated results of peak SAR values and the total radiation efficiency included the muscle and body models. As expected, the SAR values of the proposed antenna are lower than one of the planar UWB monopole antenna due to the patch-like structure and directional patterns of the proposed antenna.
1142 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 2, FEBRUARY 2012
Fig. 8. Geometry of the planar disc monopole antenna and its simulated return loss.
TABLE I
PEAKSAR VALUES ANDRADIATIONEFFICIENCY(NORMALIZE TO1 W)
III. RADIATIONPATTERNS
The antenna power gain radiation patterns in free space are measured in an anechoic chamber with an Agilent E362B network analyzer and NSI2000 far-field measurement software. The xz- and yz-plane radia-tions at 4 GHz, 7 GHz and 10 GHz are illustrated in Fig. 9. The mea-sured patterns agree with the simulated patterns while some minor dis-crepancies of simulation and measurement in xy-plane and yz-plane can be attributed to the interference of the coaxial cable and the ab-sorber. The radiations perform directional patterns in xz-plane with peak gains of 5.8 dBi, 4 dBi, and 3 dBi for each frequency, respectively. In the yz-plane, the radiation patterns on the whole operating frequency are nearly directional patterns. The power levels of backward radiation are less than05 dBi. It is evidenced that the proposed antenna is de-sirable in WBANs applications to reduce the backward radiation.
IV. CONCLUSION
A novel folded UWB antenna for WBAN applications has been pro-posed. The proposed antenna utilizes the bevel edge feed structure and the truncated metal plate with folded strip to achieve ultrawide band-width from 3.1 GHz to 12 GHz. The effects of the feed structure and the truncated metal plane are discussed in order to provide brief guidelines. The measured results show that the antenna is only slightly affected by the proximity effect of human body. Moreover, the simulated SAR
Fig. 9. Simulated and measured radiation patterns (a) at 4 GHz XZplane, (b) at 4 GHz YZplane, (c) at 7 GHz XZplane, (d) at 7 GHz YZplane, (e) at 10 GHz XZplane, (f) at 10 GHz YZplane.
values are lower than the omnidirectional disc planar monopole. The patch-like structure can reduce the backward radiation and enhance the directionality. These features demonstrate that the proposed antenna is suitable for WBAN applications.
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