broad band proximity coupled antenna

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Broadband Proximity-Coupled Microstrip Antenna

S Gao and A. Sambell

School of Engineering and Technology, Northumbria U niversity,

Newcastle Upon Tyne, NE1 8ST, United Kingdom

Email: [email protected]

Abstract-A simple broadband proximity-coupled microstrip antenna is presented. A

circular patch is used as the main radiator, and an H-shaped shot is cut in the ground

plane. A stepped-width microstrip line is used to feed the patch through proximity

coupling. The prototype antenna achieves an impedance bandwidth of 26%. Low cross-

polar levels below -20 dB are observed in both E- and H-planes. The antenna is simple in

structure and easy to be fabricated.

1. INTRODUCTION

Microstrip antennas have become the favorite choice of antenna designers because they

offer the attractive advantages of low profile, light weight, easy fabrication and easy

integration with circuits. The disadvantage of m icrostrip antenna is its narrow bandwidth.

During the recen t years, the capacity issue for w ireless communication applications is not

easily solved, with the expansion in mobile communication systems and the number of

people using their services. Much effort has been devoted to the bandw idth enhancement

of microstrip antennas, and many techniques have been proposed. These techniques

include the use of impedance matching network [I], multiple resonators arranged in

stacked or co-planar structure [2], lossy materials, the reactive loading using U-shaped

slot [3], the capacitively probe-fed structure

[4],

nd L-probe feeding

[ 5 ]

In many cases,

a thick foam substrate is required [3-51.

A few results of a broadband proximity-coupled microstrip antenna with an H-shaped

slot in the ground plane are given in [6]. The antenna has a simple configuration, and a

bandwidth o f 21

is

reported [6]. The purpose of this paper is to present a detailed study

of an imp roved design, which combines the techniques in [1,6].

2. ANTENNA

CONFIGURATION

Figure

1

shows the configuration of the broadband antenna proposed. The antenna

consists of an air layer having a thickness o f ho and a dielectric substrate layer I having a

permittivity of E and a thickness of h . The air layer is realized by using the plastic

spacers. Instead of using a rectangular patch in [6], here we use a circular patch. The

patch has a radius of

r.

Due to the existence of a thick air layer, the coupling between the

feedline and the patch is weak. To enhance the EM coupling between the patch and the

feedline, a slot is cut in the ground plane under the feedline. The slot could have the

shape of a narrow rectangle, be of U-shaped, or be realized as the H-slot. Here the

H-

shaped slot is used, which is defined by parameters lul lu2 wul and wu2. The slot is

located in the center of the patch. The feed line is a stepped-width microstrip line defined

by

w

Z w nd I,, where

w2

corresponds to the width of a 50 Ohm line. This stepped-

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width microstrip line is used

s

a kind of impedance matching network for further

broadening the antenna bandwidth [11.

3 RESULTS

To

demonstrate the concept, a prototype antenna is designed. For easy fabrication of the

metallic patch, a FR4 substrate is used, where the metal on one side is completely

removed while the other side carries the patch facing the feedline below. This is a kind of

inverted structure, where the substrate above the patch could act as a radome for

protection. The FR4 substrate (thickness of 1.6 mm and permittivity of 4.4 is also used

for substrate 1. T o understand the characteristics of this antenna, a parametric study of the

antenna is done first, and the simulation is carried out by using the software Ensemble

from Ansoft Corporation.Figure 2gives the input impedance results of the antenna with

different air layer thickness

ho.

As we can see, the input impedance vanes significantly

with the changing of ho. The double resonances are moved closer to each other, while

ho

is increased from 2 mm to 3.5 mm , and finally merged into a single resonance when

ho

is

5 mm. Thus, an appropriate choice of

ho

is very important for achieving a broadband

width.

The design paramete rs fo r the p roto type an tenna a re : ~ 8 . 5 m, h0=3.2 mm, lal=6

mm,

Za2=1

mm,

w a l = l

mm,

w a 2 4 . 4

mm,

w I = 2 m m I l = 7 .6 m m w 2

=3 m m a n d

I

=

2.5 . The measu red results of retum loss are given in Fig. 3.The retum loss is

below -10 dB in the frequency range of 5.12-6.65

GHz,

which corresponds to an

impedance bandwidth of 26 .

The measured results of the radiation pattems at 5.5

GHz

are given in Figure 4.As we

can see, the cross-polar levels are below -20 dB in both E and

H

planes. The backward

radiation is below -15 dB. Radiatio n patterns are also measu red at several other

frequencies, and it is observed that the radiation pattems are stable across the bandwidth.

4.CONCLUSIONS

A simple broadband antenna is proposed. By using only one patch, the designed

antenna achieves a bandwidth

of

26

.

Good broadside radiation pattems are observed,

and the cross-polar levels are below -20 dB at both E- and H-panes. The antenna is

simple in structure and easy to b e fabricated, thus promising for applications in wireless

communication systems.

ACKNOWLEDGEMENTS

The work is supported by EPSRC

(UK)

under the grant GWS42538/01, and the funding

from Nuffield Foundation (UK) nder the grant NAL/00673/G.

REFERENCES

[l] H.F. Pues and A.R. Van de Capelle, An impedance matching technique for

increasing the bandwidth of microstrip antennas, IEEE Trans. Antennas Propag at.

Vol.

37, pp. 1345-1354, NOV.1989

760


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[2] S.D. Targonski, R.B. Waterhouse and D. M. Pozar, Design of wideband aperture-

stacked patch antennas, IEEE Trans. Antennas Propagat. Vol. 46, pp. 1245-1251,

Sept.

1998

[3]

T. Huynh and K.F.

Lee

Single-layer single-patch wideband microstrip antenna,

Electron. Lef t. Vol. 3

1,

No. 16, pp. 13

10- 3

12, Aug. 1995

[4] M.A. Gonzalez de Aza,

J.

Zapata, and

J.A.

Encinar, Broadband cavity-backed and

capacitively probe-fed microstrip patch arrays, IEEE Trans. Antennas Propag af.

Vol. 48 pp. 784-789, May

200

[5] Y.X.

Guo, C.L. Mak, K.M.

Luk

and K.F. Lee Analysis and design of L-probe

proximity fed patch antenna, IEEE Trans. Antennas Propa gat. Vol. 49, pp. 145-149,

Feb.

2001

[6]

S Y e, Broadband proximity-coupled microstrip antennas with an H-shaped slot in

the ground plane, IEEE Antennas and Propagation Symp. Dig . pp.

530-533,2002

2r

50

Ohm input port

P-9

w

(a) Top view

Patch

(b) Side view

Figure

1

Configuration

of

the antenna

76


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a) Input resistance R

(b)

Input reactance

X

Figure

2

Input impedance results of the antenna with different

ho

solid line: h ~ 2m; dashed: h ~ 3 . 5 m; dash dot: h e 5 mm)

6

-10.

-

-I5

8 . 2 0 ~

~

25

30

35

6 6.5 7

4.5

5

5.5

f GHz)

Fig. 3 Measured retum lo ss o f the antenna

' ' j -.\i

i

,..

I

_^

fI

.

, .

\

-

. .i

-

. .

- i\.

j

\

-1

-100

do 0 Y

IW 150

4 0 '

' ,

05 t 150 - lW do

0

5

100 150

ll=bI~.pn.

a)

E

plane

Fig. 4 Radiation pattems at 5.5 GHz

762

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broad band proximity coupled antenna