MICROSTRIP ANTENNA ARRAY WITH MULTI-POLARIZATION CAPABILITIES

IN THE GSM1800 FREQUENCY BAND

Adrian Metelica(1), Piotr M. Słobodzian

(1)

(1)Wroclaw University of Technology, ITTA, Wyb. Wyspiańskiego 27,

50-370 Wrocław, Poland, Email: piotr.slobodzian@pwr.wroc.pl

ABSTRACT

The paper describes a linear microstrip antenna array,

which is capable to operate with four variants of linear

polarization, i.e. slant ±45°, vertical and horizontal one.

The antenna is design to operate in the GSM1800

frequency band, and contains two separate input ports,

which are sensitive to two mutually orthogonal slant

polarizations. The principal matter under investigation

concerns antenna radiation patterns, which can be

obtained for an impinging EM wave of various linear

polarizations. As usual, the patterns have been measured

at both the antenna ports. In addition to this, the sum

and difference antenna pattern have been measured. The

paper reports a set of characteristics, which can be used

to asses and better understand polarization capability of

a multi-polarization antenna.

1. INTRODUCTION

Presently, dual-polarization antennas (e.g. so-called

X-pol antennas) are in everyday use in various wireless

communication systems to combat the multipath fading

problem [1-4]. A variety of single and dual-band

aperture-coupled patch antennas for achieving dual-

polarization radiation have been reported in the

literature (see, e.g. [5-7]). Although in practice, and

especially in cellular systems, most dual-polarized

antennas have the form of linear antenna arrays, there is

relatively less material in the open literature available,

which discusses and describes extensively the

performance (i.e. polarization) characteristics of such

antennas. The existing material, e.g. [8] or the material

published by antenna manufacturers, also does not

contain much information. Most of the published works

deals with various geometries of a radiating element in

order to optimize its performance in terms of ports

decoupling and cross polarization level.

In this work, the principal matter under investigation

concerns more extensive antenna polarization

characteristics, which can be assessed upon analysing

antenna radiation pattern obtained for an impinging EM

wave of various linear polarizations. The subsequent

sections of the paper describe antenna array design and

fabrication and antenna prototype measurements, which

are focused mainly of the antenna radiation patterns.

2. ANTENNA DESIGN AND FABRICATION

In the course of the multi-polarization antenna concept

development it has been decided to apply, in contrast to

most solutions presented in the literature, a pair of

linearly polarized microstrip rectangular patch antennas

(radiating elements). In order to achieve the assumed

polarization characteristics, the radiating elements have

been suitably rotated in relation to vertical polarization,

i.e. one of them has been rotated by the angle of +45°

and the other by −45°, as illustrated in Fig. 1. This

configuration enables two orthogonal slant polarizations

at two separate ports.

Vert

ical direction

Figure 1. A pair of linearly polarized microstrip

rectangular patch antennas rotated

by the angle of +45° and −45°

Such choice has been caused by two reasons: first, an

expected increase in ports separation, and second, to

verify performance of a linear antenna array composed

of such radiating elements.

In order to meet requirements for broadband

performance of the radiating elements in the GSM1800

frequency band (relative bandwidth of ca. 10%) we

decided to use the conventional aperture coupled

microstrip patch antenna. In addition, in order to reduce

the level of back radiation the antenna has been

equipped with a back ground plane. The structure of the

radiating element is shown in Fig. 2.

The configuration of radiating elements, shown in

Fig. 2, occupy quite big surface, and hence is practical.

Additionally, due to a shift of radiation elements in

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Proc. ‘EuCAP 2006’, Nice, France 6–10 November 2006 (ESA SP-626, October 2006)

relation to the vertical axis of the antenna array it is

expected that its horizontal radiation pattern will exhibit

considerable squint. In order to mitigate both the effects

the radiating elements have been shifted inwards, as

illustrated in Fig. 3.

Figure 2. The structure of the radiating element used

in the antenna array design (substrate: MC5, εr=3.86,

tgδ=0.003, t=0.787 m; foam: Rohacell HF31)

Vert

ical direction

Figure 3. Modified configuration of a pair

of rotated radiating elements

Using the modified configuration of the radiating

elements a linear antenna array has been designed and

fabricated. Four pairs of such elements have been used

to set up antenna array prototype. The structure of the

antenna is shown in Fig. 4. In fact, the antenna consists

of two linear arrays (with separate ports), and each of

them contains four radiating elements (effectively, we

have 2x4 array). The overall size of the array is

265x458 mm (DxL, when referred to Fig. 4). The

distance S between each radiating element is 0.53λo. All

radiating elements are fed by means of a T-junction

based corporate feed network, which provides uniform

power distribution.

The antenna parameters have been optimized by means

of computer simulations. The obtained results have

shown that the proposed structure should works in the

required frequency band with the return loss less than

-16 dB. The isolation between the two input ports shoul

be better than 30 dB and for some frequencies even as

high as 38 dB. The actual antenna prototype perfor-

mance has been verified experimentally.

Figure 4. The structure of the antenna

array prototype

3. MEASUREMENT RESULTS

All relevant electrical parameters of the antenna array

prototype have been measured at both antenna ports

separately. The measurements at Port 1 have been made

with Port 2 terminated in a matched load, and vice-

versa. First, the input VSWR has been tested and the

results are shown in Fig. 5. As we can see, the required

frequency bandwidth (e.g., VSWR<1.5) has been

achieved in excess.

As regards the antenna ports isolation, the measurement

has revelled rather moderate performance. The isolation

between the Port 1 and 2 of the prototype antenna in the

operating frequency band is better than 22 dB, and is

shown in Fig. 6. The isolation level is poor compared to

levels reported in the literature, and probably this fact

results from stronger mutual coupling, which has been

increased due to the modification of the radiating

elements configuration.

1

1,25

1,5

1,75

2

1500 1600 1700 1800 1900 2000 2100

Frequency [MHz]

VSWR

Port 1 Port 2

Figure 5. VSWR at the ports of the antenna array

prototype

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-25

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-10

-5

0

1500 1600 1700 1800 1900 2000 2100

Frequency [MHz]

Isolation [dB]

GSM1800

Figure 6. The measured isolation between the ports of

the antenna array prototype

Vertical plane

(elevation)

Port 1 Port 2

Horizontal plane

(azimuth)

Figure 7. Definition of two principal planes

for antenna pattern measurements

In the next step the radiation characteristics of the

antenna prototype have been investigated. The

measurements have been carried out in two principal

planes (as indicated in Fig. 7): in the horizontal plane

(azimuth angle) and vertical plane (elevation angle).

As usual, the patterns have been measured at both

antenna ports. In addition to this, a sum and difference

pattern have been measured using a power combiner

and a simple 180° hybrid connected to the antenna

ports, respectively. In this way, it has been possible to

perform very simple algebraic operations (addition and

subtraction) on the signals appearing at two separate

ports of the antenna array. Such solution enables

verifying the antenna performance for various types of

polarization of the impinging EM wave (received

signals) as well as various antenna array polarization. In

this case, the antenna can be polarized in four different

linear ways, namely: horizontally (for the difference

pattern), vertically (for the sum pattern), slantwise +45°

(at Port 2), and slantwise −45° (at Port 1). The results

of antenna array radiation pattern measurements are

shown in Fig. 8 – 10 (see, subsequent pages). Please

note pattern description given in the figures. For each

pattern the antenna polarization has been designated by

black bold arrow, and the impinging field polarisation -

by black thin arrow.

4. CONCLUSIONS

The antenna prototype, described in this paper, exhibits

very similar performance for various polarizations. In

fact the antenna is capable to operate quite well with

any type of linear polarization. Upon analyzing the

obtained radiation patters for the direction normal to the

antenna aperture (azimuth and elevation angle equal 0°)

we can conclude that by selecting suitable antenna

polarization it is possible to hold the received signal

almost on the same level (in the worst case it can

experience 1 dB loss). As it was expected, all horizontal

patterns exhibit squint of about ±18°, depending on

which polarization impinges the antenna array.

In order to predict the antenna performance in a real

communication system it is required to determine the

antenna polarization discrimination in an angular range

covering the whole serviced area. This question can be

resolved for by means of the presented antenna radiation

characteristics.

5. REFERENCES

1. Wahlberg U., et al., The performance of polarization

diversity antennas at 1800 MHz, IEEE-APS, vol. 8,

1368-1371, 1997.

2. Lempiainen J.J.A., Laiho-Steffens J.K., The perfor-

mance of polarization diversity schemes at a base

station in small/micro cells at 1800MHz, IEEE

Trans. Vehicular Tech., vol. 47, 1087-1092, 1998.

(for cont. ref. see, the last page)

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-20

-15

-10

-5

0

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Azymuth [deg]

E/Emax [dB]

Figure 8. The measured sum radiation pattern of the antenna in the horizontal plane

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-20

-15

-10

-5

0

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Elevation [deg]

E/Emax [dB]

Figure 9. The measured sum radiation pattern of the antenna in the vertical plane

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-10

-5

0

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Azymuth [deg]

E/Emax [dB]

Figure 10. The measured difference radiation pattern of the antenna in the horizontal plane

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-30

-25

-20

-15

-10

-5

0

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Elevation [deg]

E/Emax [dB]

Figure 11. The measured difference radiation pattern of the antenna in the vertical plane

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-15

-10

-5

0

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180

Azymuth[deg]

E/Emax[dB]

Figure 12. The measured radiation pattern at Port 2 of the antenna in the horizontal plane

Cont. references:

3. Kar M., Wahid P., Two-branch space and polariza-

tion diversity schemes for dipoles, IEEE-APS, vol. 3,

364-367, 2001.

4. Holma H., Tolli A., Simulated and measured perfor-

mance of 4-branch uplink reception in WCDMA,

IEEE-VTC, vol. 4, 2640 – 2644, Spring 2001.

5. Lindmark B., A novel dual polarized aperture

coupled patch element with a single layer feed

network and high isolation, IEEE-APS, vol. 4, 2190-

2193, 1997.

6. Chiou T.-W., Wong K.-L., A compact dual-pola-

rized aperture-coupled patch antenna for GSM

900/1800MHz systems, APMC, vol. 1, 95-98, 2001.

7. Kin-Lu Wong K.-L., et al., Broadband dual-

polarized aperture-coupled patch antennas with

modified h-shaped coupling slots, IEEE Trans.

Antennas Propag., vol. 50, no. 2, 188-191, 2002.

8. Lee B., et al., Polarization diversity microstrip base

station antenna at 2GHz using T-shaped aperture

coupled feeds, IEE Proc. Microw. Antennas

Propag., vol. 148, no. 5, 334-338, 2001.