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VC 2012 Wiley Periodicals, Inc.

NOVEL LOW-PROFILE FOAMDIELECTRIC OVER-THE-SHOULDERANTENNA BASED ON COUPLEDPATCHES TECHNIQUE

Milan Svanda, Milan Polivka, and Premysl HudecDepartment of Electromagnetic Field, Faculty of ElectricalEngineering, Czech Technical University in Prague, Technicka 2,16627 Prague 6, Czech Republic; Corresponding author:milan.svanda@fel.cvut.cz

Received 27 June 2012

ABSTRACT: The article presents a novel wearable antenna composedof two coupled patches excited by an overlapping folded dipole. In spiteof being operated in a relatively low frequency band 380 � 390 MHz,the antenna shows a very low profile and other dimensions areacceptable as well. Besides a very low absorption of the radiated RFpower by any nearby human body, it also ensures a very good immunityagainst influence on antenna parameters. In comparison to commonlyused simple conformal monopole antennas, the presented solutionprovides a double-side radiation pattern and a substantially higher gain.

The antenna can be manufactured using very light and twistable foamdielectric and conductive fabric, and can be worn as a strap placedover the shoulder. It is intended to be used together with personal radiocommunication transceivers operated in the given frequency band. VC

2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:593–597,

2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/

mop.27389

Key words: body centric communication; coupled patches; low-profileantenna; wearable antenna

1. INTRODUCTION

The research in the field of wearable antennas to be operated in

a close vicinity of a human body represents a topical issue.

Antennas of this kind should fulfil the following requirements.

First, it is required to provide as the highest possible immunity

from undesirable influences of a nearby human body on antenna

parameters. Accordingly, it should ensure as low irradiation of

the communicating person by the RF power as possible. Eventu-

ally, it is required to have small dimensions and show a low

weight and good flexibility.

To date, possible applications range from special RFID sys-

tems to body-centric communications intended especially for

paramedics, firefighters, military personnel, and so forth [1–3].

Although at higher RF and microwave frequencies, a number of

very good solutions fulfilling all the above-stated requirements

have been presented, so far, the design of flexible conformal

radiators for communications in the UHF band has not been

tackled in a satisfactory manner.

In case of common antennas (e.g., monopole or dipole

types), the presence of a nearby human body causes significant

frequency detuning and absorption of the radiated or received

RF energy, which results in low radiation efficiency and gain

[4–7]. Both problems can be, in principle, solved by inserting a

screening metallic plate. The latter can act as an additional

shielding or an inherent part of the antenna; [4]. The coupled

patches structure presented by the authors in Ref. 8 represents

the mentioned shielding principle.

This technique can be applied for minimizing the majority of

disadvantages of simple (2n þ 1) k/4 long monopole antennas

Figure 1 Underarm holder with radio transceiver and simple monop-

ole antenna hidden in shoulder strap

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 3, March 2013 593

that are frequently used in connection with personal radio trans-

ceivers. An example of such antenna and 380 � 390 MHz per-

sonal transceiver can be seen in Figure 1. The radio transceiver

is placed in the underarm holder, which is fixed to a shoulder

strap. An approximately 3=4k-long wire installed in the strap is

used as an antenna (see the first connector from the left).

The operation of this type of monopole antennas in a very

close vicinity to the human body gives rise to several major

problems. Usually, a nonnegligible absorption of the RF power

by the human body, together with the detuning of antenna input

impedance and significant decrease in its radiation efficiency

result in a substantial drop in the antenna gain; see Table 1.

Moreover, the resulting radiation pattern is hard to predict and

often attains minima in the forward and backward directions,

where the maxima of communication traffic can be expected.

The presented coupled patches structure solves the majority

of the aforementioned problems. This technique enables to

design and manufacture low-profile wearable antennas with sat-

isfactory radiation efficiency and a very low level of frequency

detuning even if it is located directly on the human body.

Besides, the antenna shows two identical radiation maxima tar-

geted at the mostly required forward and backward directions.

2. COUPLED PATCHES TECHNIQUE

As mentioned above, the employment of coupled patches struc-

ture enables to design extremely low profile antennas with a

very good immunity from the influence of a human body situ-

ated in the close vicinity. The structure was derived from the

standard patch antenna. Despite many virtues, the latter suffers

from a significant fall in its radiation efficiency (under 50%) if

the relative thickness of the used substrate drops below �0.01

k0 (i.e., 8 mm at 385 MHz); see Ref. 4. This phenomenon can

be eliminated provided that two half-wavelength long patches

that are strongly coupled by a narrow gap are used; see

Figure 2.

Radiation properties of this coupled structure are significantly

enhanced even in case of low-profile substrates with thicknesses

below 0.01 k0 and are, to a large extent, insensitive to the width

of coupling gap. The electric field distribution of the coupled

structure is demonstrated in Figure 3. Due to the strong coupling

of the two half-wavelength patches, three radiating slots are

present, (out of which two are located on the outer sides,

whereas the third one is situated within the coupling gap).

The structure is excited by the folded dipole, etched on a

very thin (0.25 mm) separate upper substrate. For these pur-

poses, even other radiator shapes might be considered, such as

meander dipole, loop antenna, and so forth; see Refs. 8–10.

Change of length of the folded dipole enables to tune the

antenna input impedance to the required level (50 X).

The following paragraph shows that antenna based on the

coupled patches structure with a profile thinner than 0.003 k0

enables to reach �50% radiation efficiency, which is about four

times better than the radiation efficiency of the common patch

antenna made of comparable substrate and showing similar

dimensions.

3. DESIGN, REALIZATION, AND MEASUREMENT

To verify parameters of the coupled patches antenna for the

over-the-shoulder application in the operational 380 � 390 MHz

frequency band, the test sample was designed and manufactured.

The antenna takes the form of a strap to be placed over the

shoulder. To ensure its low weight and high flexibility, the foam

TABLE 1 Comparison of Maximum Gain and 26 dBBandwidth for Simple Monopole Antenna and ProposedCoupled Patches Antenna in the Over-the-ShoulderApplication at f 5 385 MHz

Gain (dBi) BW�6 dB (%)

Stimulated Measured Stimulated Measured

Monopole in free space 1.2 – 16 –

Monopole folded over

human shoulder

�11.3 – 0.3 –

Coupled patches antenna

in free space

�1.3 �1 2.9 3.1

Coupled patches folded

over human shoulder

�2.5 �2.2 3.4 2.9Figure 2 Coupled half-wavelength patches excited by folded dipole

Figure 3 Electric field distribution of coupled half-wavelength patches antenna. Grey arrows indicate orientation of fictive magnetic currents in radiat-

ing slots

594 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 3, March 2013 DOI 10.1002/mop

dielectric was used as a substrate; see Figure 4. Although the

metallic layers of the test sample were manufactured using a

copper adhesive tape, the practical implementation is intended

to be based on more flexible and resistant conductive fabric.

The total size of the unfolded antenna amounts to 790 � 50

� 5.6 mm3 (1.014 � 0.064 � 0.0072 k0), while the expected di-

ameter of the fold is equal to 150 mm. The thicknesses of the

lower and upper substrates reach 4.8 and 0.8 mm, respectively.

The dielectric constant of the foam dielectric accounts for er ¼1.3, while its loss tangent reaches tan d ¼ 0.02. The width of

coupling slot equals g ¼ 2.5 mm and the folded dipole footprint

is 570 � 6 mm (0.73 � 0.0077 k0). The antenna feeding is

ensured by a thin RG-174 flexible micro-coaxial cable termi-

nated with a SMA connector.

Figure 5 involves the photograph of manufactured sample in

the intended position over the human shoulder. Figure 6 com-

pares the simulated and measured input reflection coefficients,

both in free space and in the close vicinity of a human body. A

very good agreement between the simulation and measurement

as well as a high immunity from the impact of a nearby human

body can be observed.

Figure 7 present the simulated antenna radiation patterns,

both in free-space and in the close vicinity of a human body,

which were modelled as a definite volume that is merely slightly

exceeding the dimensions of folded antenna. They are filled

with a material having in principle the same parameters as

human body. The maxima of radiation in the forward and back-

ward directions are very well-defined and are nearly identical in

both cases. Figure 8 shows the simulated and measured radiation

patterns in the horizontal (xz) plane. Because it was impossible

for the person wearing the antenna to stand at the antenna turn-

table, the measurements of radiation pattern were performed

only from several discrete angles. Taking into account a small

tilt caused by the asymmetrical position of antenna with respect

to the body, the agreement between the measurement and simu-

lation is also fairly good. The antenna feeding cable was

Figure 4 Drawing of designed coupled patches antenna (a) and detail

of excitation folded dipole (b)

Figure 5 Photograph of manufactured coupled patches antenna placed

over shoulder during tests

Figure 6 Measurement and simulation of reflection coefficient of

coupled patches antenna in a free space and in the close vicinity to the

human body

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 3, March 2013 595

connected directly to ends of the folded dipole. Because the

antenna is operated in a very complex environment, the imple-

mentation of symmetrization was neither used nor advisable.

Table 1 provides comparison of simulated and measured pa-

rameters of both, the coupled patches antenna and simple

monopole antenna described in Section 1. The obtained results

show a very good agreement of simulations and measurement.

Furthermore, in comparison to simple monopole types, the novel

antenna shows obvious advantages.

4. CONCLUSION

The novel wearable antenna based on coupled patches technique

for the over-the-shoulder application in the 380 � 390 MHz fre-

quency band was developed and its parameters were subject to

verification. Despite having a very low profile (0.0072 k0), it

shows the satisfactory radiation efficiency and very good immu-

nity from the presence of nearby human body. The resulting

gain reaches �1 dBi in free space and �2.2 dBi in the over-the-

shoulder application, which can exceed by up to 8.8 dB the gain

of the simple monopole antenna operated under similar condi-

tions. The antenna radiation patterns have two nearly identical

maxima oriented to the forward and backward directions, where

the communication traffic maxima can be expected. The antenna

is very light, twistable, and can be worn as a separate fixable

over-the-shoulder strap. Alternatively, it can be incorporated

into the article of clothing. It is intended to be used together

with personal communication transceivers operated, for example,

by paramedics, firefighters, or military personnel.

ACKNOWLEDGMENTS

The present research was undertaken at the Department of Electro-

magnetic Field at the Czech Technical University in Prague. It was

jointly supported by the Czech Science Foundation—(project No.

P102/12/P863 ‘‘Electromagnetic Properties of Radiating Structures

and Artificial Screening Surfaces in the Close Vicinity of the Human

Body’’) and the COST project (No. LD 12055 AMTAS: ‘‘Advanced

Modelling and Technologies for Antennas and Sensors’’) that repre-

sents a subpart of the COST project No. IC 1102 VISTA: ‘‘Versatile,

Integrated, and Signal-aware Technologies for Antennas.’’

REFERENCES

1. P.S. Hall and H. Yang, Antennas and propagation for body-centric

wireless communications, Artech House, Norwood, MA, 2006.

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bedded UHF RFID label antenna for TAGing metallic objects, In:

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antenna performance on various materials using radio link budgets,

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Figure 7 Simulated three-dimensional radiation pattern (directivity) of

coupled patches antenna in free space (a), in the close vicinity of human

body phantom (b)

Figure 8 Comparison of simulated and measured radiation patterns in

horizontal (xz) plane of coupled patches antenna operated in over-the-

shoulder position

596 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 3, March 2013 DOI 10.1002/mop

7. D.M. Dobkin and Weigand, Environmental effects on RFID TAG

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ton, USA, July 2005.

8. M. Svanda and M. Polivka, Two novel extremely low-profile slot-

couplet two-element patch antennas for uhf RFID of people,

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intended for fastening on human body, Czech Republic Patent,

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quency identification of persons in buildings and open areas, IEEE

Trans Microwave Theory Tech, 57 (2009).

11. G. Marrocco, RFID antennas for the UHF remote monitoring of

human subjects, IEEE Trans Antennas Propag 55 (2007).

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communications, In: European Conference on Antennas and Propa-

gation (EuCAP), November 2006.

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VC 2012 Wiley Periodicals, Inc.

UWB POWER DIVIDER WITH ONENARROW NOTCH-BAND AND WIDESTOP-BAND

Lei Chen,1 Feng Wei,2 Chang Jia Gao,2 and Wei Qiang Liu21 School of Electronic and Information Engineering, Xi’anTechnological University, Xi’an 710032, People’s Republic ofChina; Corresponding author: sumter1108@163.com2 National Key Laboratory of Antennas and Microwave Technology,Xidian University, Xi’an 710071, People’s Republic of China

Received 3 July 2012

ABSTRACT: A compact ultrawideband (UWB) microstrip powerdivider (PD) with one sharply rejected notch-band and wide stopband isanalyzed and designed in this article. The proposed UWB PD is based

on conventional Wilkinson PD, while interdigital hairpin resonators areintroduced in two symmetrical output ports to widen passband. The

stepped impedance resonator is studied and used to generate onedesired notched band. Defected ground structure is introduced toimproved stopband performance. Theoretical and simulated results are

presented, which are in good agreement with the measured results.VC 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:597–

600, 2013; View this article online at wileyonlinelibrary.com. DOI

10.1002/mop.27365

Key words: ultrawideband; power divider; stepped impedanceresonator; defected ground structure

1. INTRODUCTION

Since the ultrawideband (UWB: 3.1–10.6 GHz) spectrum was

regulated for unlicensed use in 2002, many UWB devices and

circuits have been presented and investigated extensively. Power

dividers (PDs) are key components of many microwave systems,

such as balanced mixers and antenna array feed networks. The

most popular PD is the Wilkinson divider, which has good isola-

tion between two output ports while the fractional bandwidth is

less than 20%. Owing to the growth of UWB technology, some

new types of UWB PDs with different structures have been

recently designed [1–3]. In Refs.1 and 2, a novel UWB PD with

UWB bandpass filtering response based on multilayer slotline

structure is proposed, but it requires complex technology and is

hardly compatible with the existing microwave-integrated circuit

(MIC). A UWB microstrip PD with the sharp roll-off skirt using

a pair of stepped-impedance open-circuited stubs and parallel

coupled lines in two output ports is developed in Ref. 3. The

aforementioned PDs exhibit good performance over the UWB

range.

However, the existing undesired narrow band radio signals

such as WLAN signals may interfere with the UWB system. As

a result, a PD designed for UWB applications should also incor-

porate one notch band to suppress interference. UWB PD with a

notched band characteristic is achieved by using stepped imped-

ance resonator (SIR) in Ref. 4. However, the proposed UWB

PD has narrow stopband, which cannot suppress the high-order

harmonics and limits its applications.

A compact UWB microstrip PD with wide pass-band, one

highly rejected notched band, and wide stopband is designed

and implemented in this article [4]. The proposed UWB PD is

based on single-layer microstrip topology structure and one iso-

lation resistor. Good performances in terms of equal power split-

ting and high notch-band rejection are achieved over the UWB

range. Measured results agree well with the simulated results.

2. INTERDIGITAL HAIRPIN RESONATOR

Figure 1 shows the configuration of the microstrip interdigital

hairpin resonator unit, which is composed of three identical

microstrip coupling fingers. The width of the coupling finger is

W2 and the distance between the adjacent coupling fingers is

W3. The coupling-finger length is about one-quarter guided

wavelength with respect to the center frequency of 6.85 GHz.

The microstrip interdigital hairpin resonator unit can be seen

as a six-port network. The network can be equivalent to a two-

port admittance inverter circuit, as shown in Figure 2 [5].

The frequency characteristics of the interdigital hairpin reso-

nator unit are simulated by HFSS 11.0, as shown in Figure 3. It

can be seen from the simulation results that the interdigital hair-

pin resonator can achieve a wide pass-band performance, but the

cutoff frequency response is gradual and the stopband is relative

narrow, which limits its application in UWB system. Figure 4

shows the frequency response of the interdigital hairpin resona-

tor unit with various dimensions. It can be seen that the width

of passband is decreases as L1 increases and increases as W3

decreases [6].

3. STEPPED IMPEDANCE RESONATOR

The proposed folded SIR is constructed by cascading a low-

impedance section in the center accompanied with the two high-

impedance sections in the two sides, and two low-impedance

sections are located in the two terminals. The two

Figure 1 Configuration of the microstrip interdigital hairpin resonator

unit

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 3, March 2013 597