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D U A L - B A N D M I C R O S T R I P P A T C H A N T E N N A E L E M E N T

W I T H D O U B L E U S L O T S F O R

GSM

Jose Rosa, Rui Nunes, Alexandre Moleiro and Cust6dio Peixeiro*

Instituto de TelecomunicaqBes

Instituto Su perior TBcnico

AV. Rovisco Pais, 1

1049-001 Lisboa Portugal

E-mails: joprosa l 

@madrid.ecid.cig.mot.com ,

custodio peixeiro@lx it pt

Abstrac t

A new dual-band microstrip patch antenna element is presented. The

element configuration consists on a rectangular patch with double U slots

and coaxial feeding. A prototype was designed, fabricated and tested to be

used in a GSM 900/1800 base station antenna. Good agreement has been

obtained between theoretical and experimental results. To provide the

required bandwidth for both the GSM bands a double-layer structure is

under analysis.

1 In t roduc t ion

The main purpose of this work is to develop a dual-band microstrip patch

antenna eleme nt to be used in GSM 900/1800 base stations.

Some of the well known characteristics advantages of microstrip patch

antenna elements such as low profile, lightweight and low cost

are

very

important in mobile and personal communication system applications. Due

to the enormous increase in the number of G SM users and traffic many base

stations are introduced each year mainly in highly populated areas. The use

of dual-band antennas allows a significant reduction in the number of

antennas providing an important improvement in terms of cost and

preservation of the environment.

2

Element Conf igura t ion

The configuration presented in this work is based on the rectangular

microstrip patch element with an U slot proposed in [I]. The ideia is to

design a single-layer small rectangular element resonating at 18 00 MH z

inside a larger element resonating at 900 MHz. At first the two elements

were coupledh eparate d by a 1 mm wi de rectangular slot. The design proved

to be inadequate as the outer element could not get enough coupling from

the coaxial feed probe located inside the inner element. Therefore two small

openings were made to allow current flow to the outer element. The

geometry of the element with double U slots is shown in figure 1.  The

structure is fed through a

50 s

coaxial probe located near the center of the

inner element.

0-7803-6369-8/00/ 10.0002000

E E E

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A 62 mils (1.57 mm) thick RTD uroid 5880 substrate has been used. The

analysis of the structure was carried out using Ensemble software [2] The

optimization folowed an intuitive procedure based on the results

of

the

analysis.

56.75

A

6.55

34.35

38.57

II - ,

8.5

Figure

1

Element

geometry

(dimensions n

mm)

3 Experimental

Resuls

The prototype shown in Figure 2 was fabricated using a conventional

photolithography printing technique. The input return loss was measured

using a HP 8720 vector network analyzer. The radiation pattem was

measured using an anechoic chamber and standard antennas for reference.

A

good agreement w as obtained between theoretical and experimental results.

The experimental input return loss results are shown in figure 3. The

required resonance frequencies are obtained at 932 MH z and 1 776 MH z.

However the bandwidth is not enough to cover the two GSM bands,

therefore one should look for a more com plex structure.

The experimental results as far as radiation pattem is concemed are shown

in figures 4 and 5 Very similar broad radiation patterns with low cross

polarization level are obtained in

E

and

H

planes.

A

gain

of

4.5 dBi was

measured at 1776 MHz.

A summary of the predicted and measured results is shown in table

1. 

Resonance frequencies, bandwidth (VSW R<2) and half-power E plane and

H plane beamwidths are included.

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2

-4

a

-10

-12

-14

-16

-18

-20

7

900 1100 1300 1500

1700

1900

Frequency MHzl

Figure

3

Experimental input return loss

igure 2 Antenna prototype.

0

5

- 1 0

-30

-3 5

-40

0

5

-10

-30

-3 5

-40

Crosspolar

E

plana

C o p o l a r

H

p l a n e

- 1 8 0 - 1 5 0 - 1 2 0 -90

-60 -30

30

60

90 1 2 0 1 5 0 1 8 0

~ n g l t

[ I

Figure4 -Experimental radiation pattern at

932

MHz.

180 - 1 5 0 - 1 2 0

.90

-60 30 0 30 60 90 1 2 0 1 S O 1 8 0

A n g l c 9 I

C r o s s p o l a r

E

l a n e

Copo la r

H

p l a n e

- C r o s s

olar

H

lane

Figure 5 -Experimental radiation pattern at 1776

MHz

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H plane

Predicted 97.6 80.0

99.2

80.0

Measured

4.4

73.5

1776 6 0

1n4

5 96

0

Table 1 Sum mary of theoretical and experimental results

4 Futurework

The configuration presented shows the required GSM, 900 MHz and 1800

MHz, resonances but it does not provide the specified bandwidth. Therefore

the work will carry on until the required bandwidth is obtained. Two

approches are followed. The simplested is to use a thick low dielectric

constant substrate layer (foam or even air) between the RTlDuroid substrate

and the ground plane. Another possibility is the use of a satcked-patch

configuration [3].

5. Conclusions

A new dual-band m icrostrip patch anten na element is presented. The

element configuration consists on a rectangular patch with double U slots

and coaxial feeding. A prototype was designed, fabricated and tested to be

used in a GSM 900/1800 base station antenna. Good agreement has been

obtained between theoretical and experimental results. Typical broad

radiation patterns have been obtained for the two GSM bands. However the

bandwidth specifications are not fulfilled. To provide the required

bandwidth for both the GSM bands a double-layer structure is under

analysis.

References

T. Huynh and K.-F. ee, Single-layer single-patch wideband microstrip

antenna , Electronics Letters vol. 31, no 16, pp. 1310-1312, August

1995.

ENSEMBLE Design Review &

I

Array Synthesis Version 5.1, User's

guide, Ansoft Corporation, January 1998.

D M. Pozar, A Review of Bandwidth Enhancement Techniques for

Microstrip Antennas , in Microstrip Antennas The Analysis and Design

o Microstrip Antennas and Arrays D M. Pozar and D. H. Schaubert

(editors), IEEE RESS, New York, 1995.

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