A Circular Polarized Self Tracking L Band Array

with High Bandwidth and Scan Beamwidth for

Inmarsat BGAN Applications. N.B. Buchanan

*, V.F. Fusco

*, M. Van Der Vorst

+

*The Institute of Electronics, Communications and Information Technology (ECIT), Queen's University Belfast, Northern

Ireland Science Park, Queens Road, Queens Island, Belfast, BT3 9DT, Tel +44 2890 971721, Fax +44 28 9097 1702,

n.buchanan@ecit.qub.ac.uk +European Space Agency, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

Abstract This paper presents a 4x5 element antenna array for

Inmarsat BGAN applications. This array addresses an important

phenomenon which appears to have been little reported in the

literature. This is the degradation of circular polarization

characteristics when a CP array is steered electronically. The

design of this array presents significant challenges, as not only

deos it have to maintain a high quality of CP when scanning up

to 40, but also has to do this over an 8.5% frequency coverage

of 1.525 to 1.66 GHz. The resulting array was able to fulfill these

requirements, with element sequential rotation being employed

to further enhance the CP characteristics.

I. INTRODUCTION

Despite there having been numerous studies on antenna

arrays for electronic beam steering regarding linear

polarization, e.g. [1], what seems to have been much less

studied, is the performance of circular polarized (CP) arrays

with regard to electronic beam steering, especially for self-

tracking array applications. Obtaining a high quality of CP,

over a wide range of electronic beam steering angles, is

challenging, as axial ratio can rapidly degrade as beam

steering angles move away from boresight [2,3]. If the array is

also required to operate over a reasonably large bandwidth the

polarization problem is further compounded, as a significant

number of well known planar CP element designs only offer

high quality CP over a reasonably small bandwidth [4, 5].

In this paper we are showing results from a 4x5 element

planar electronically steered circularly polarised array,

intended for an advanced self-steering application for

applications such as the L band (1525 to 1660MHz) Inmarsat

BGAN system. When combined with novel retrodirective

circuits, developed at QUB, it will offer a simple and energy

efficient self steering alternative to fixed beam planar antenna

arrays used for portable ground terminals. Using the new

retrodirective technology ground terminals can be setup in

seconds.

II. DESIGN METHOD

The antenna array was designed to the specification shown

in Table 1. Existing planar arrays operating in this frequency

range were of fixed beam type, so as long as the individual

elements offered good CP characteristics at boresight, they

fulfilled the requirements. The challenges occurred with the

requirement to obtain high quality CP at angles of up to 40

away from boresight. In addition the L band Inmarsat BGAN

system operates over a frequency range of 1525 to 1660MHz,

equating to an 8.5% bandwidth, whereas most planar patch

type antennas only offer a few percent coverage.

TABLE I

BGAN ANTENNA ARRAY SPECIFICATION

Frequency coverage: 1525 1660 MHz

Polarization: RHCP

Beam steering Capability: 40 Azimuth/Elevation

Gain: > 17.5 dBi

Polarization isolation: < -10dB

Antenna geometry planar

A. CP Element Contending Options

A number of different planar antenna elements were

studied to attempt to find one suitable for use in the L band

retrodirective array. The crucial factors for the selection of the

antenna element were: (1) It required a reasonably flat

amplitude beamwidth over a 40 azimuth/elevation range,

(2) High quality CP was required over the 40

azimuth/elevation range and (3) Points 1 and 2 needed to be

fulfilled over the 8.5% (1525 to 1660MHz) frequency

coverage. To begin the antenna selection only those with the

potential for the 8.5% frequency coverage could be considered.

If a patch element is considered, its bandwidth can be

enhanced by placing an air layer between the patch element

and groundplane [6]. Regarding the feed structure, probe

feeding [7] or aperture feeding [6] can be employed to

enhance bandwidth. In this paper we will mention the most

significant findings of the positive and negative characteristics

of the different antenna options.

First we looked at the possibility of a simple patch design

which required a single feed structure to produce CP. A patch

element with perturbation segments [4,5], which is often used

for GPS applications, only offered acceptable axial ratios at

the centre of the frequency coverage, degrading fairly rapidly

outside this. This type of antenna has also been shown as an L

band array in [8] but was only of fixed beam configuration.

This polarisation bandwidth problem can be alleviated by

using a dual 90 fed patch element. Three methods were

studied to facilitate the feed structure (1) Direct pin fed,

(2) Probe Fed, (3) Aperture coupled. The aperture coupled

option (Fig 1(a)), appeared, at first, to be the most attractive

one, since a CP array for similar L band applications had

already been shown in [6] where the return loss bandwidth of

the elements was well within what was required. The

interesting discovery was made of this structure when applied

to simulations using Agilent Momentum and CST Microwave

Studio. What was found was an asymmetry in the axial ratio

Vs scan angle (Fig 1(b)) where at -40 coverage the axial ratio

has increased beyond 3 dB, which would be considered

unacceptable for the application. This characteristic may have

seemed previously unimportant if the antenna was only being

considered for a fixed beam application at boresight.

Axial Ratio

m1THETA=dB(ARcp)=4.744

-40.000m5THETA=dB(ARcp)=1.315

40.000

-150 -100 -50 0 50 100 150-200 200

5

10

15

20

25

30

35

40

45

0

50

THETA

Mag.

[dB

]

m1

m5

m1THETA=dB(ARcp)=4.744

-40.000m5THETA=dB(ARcp)=1.315

40.000

Asymmetry (>3dB)

(a) Aperture Coupled CP

Patch

(b) Asymmetry of Axial Ratio

Fig. 1 Aperture coupled patch showing Asymmetry on Axial ratio Vs

Azimuth Angle

B. Selected Element for Circular Polarised Array

The Antenna element chosen for the array was a probe fed

circular patch element (Fig. 2(a)). This was simulated using

Agilent Momentum which showed that an axial ratio of

degraded. This shows that, when combining elements to form

a circularly polarised array, despite the fact that acceptable

performance is achieved from a single element, does not

always guarantee a high level of performance when combined

as an array. One contributing factor to this is the mutual

coupling between elements and the next section will show

methods to reduce this.

Fig. 4 4x5 array of probe fed CP patches

E_left E_right

-80

-60

-40

-20

0 20

40

60

80

-10

0

10

0

-50

-40

-30

-20

-10

-60

0

THETA

Ma

g. [d

B]

Ma

g. [d

B]

Axial Ratiom1THETA=dB(ARcp)=4.769

0.000

-80 -60 -40 -20 0 20 40 60 80-100 100

5

10

15

20

25

30

35

40

45

0

50

THETA

Ma

g. [d

B]

m1

m1THETA=dB(ARcp)=4.769

0.000

(a) CP radiation patterns at 1.6 GHz (b) Axial Ratio at 1.6 GHz Vs Azimuth Angle

Fig. 5 Radiation characteristics of 4x5 element array including mutual

coupling, all elements at the same rotation orientation

D. Circular Polarised Array With Sequential Element rotation

To improve the overall performance of the probe fed CP

patch array, a well known technique to mitigate the effects of

mutual coupling is to sequentially rotate the elements of the

array. Some studies were carried out on this in [9] which

showed that the highest level of CP performance for a CP

patch array could be achieved using the sequential rotation

scheme shown in Fig. 6. This was applied to the 4x5 array of

probe fed patches, and the feed phases adjusted accordingly to

account for the phase difference induced by the element

rotations. An EM simulation of the entire array, carried out

using Agilent Momentum, produced the radiation patterns of

Fig. 7 (a)&(b). These show a polarisation isolation of 28dB at

boresight, and an axial ratio of 0.661 dB. Boresight gain was

predicted to be 17.99dB. The results show that the sequential

rotation has offered a significant improvement on the 4x5

array, considering that the same array with no sequential

rotation had already degraded, in terms of axial ratio, to 5 dB,

and offered only 12dB polarisation isolation.

90 0 0 180

270 180 90 270

180 270 270 90

0 90 180 0

90 0 0 180

180 270

0 90

90 0 0 180

270 180 90 270

180 270 270 90

0 90 180 0

90 0 0 180

180180 270270

00 9090

Fig. 6 4x5 Element array sequential element rotation

Circular Polarization

E_left E_right

m3THETA=dB(Erhp)=-0.007

0.000

-80 -60 -40 -20 0 20 40 60 80-100 100

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

-60

0

THETA

Ma

g. [d

B]

m3

m3THETA=dB(Erhp)=-0.007

0.000

Axial Ratio

m1THETA=dB(ARcp)=0.661

0.000

-80 -60 -40 -20 0 20 40 60 80-100 100

5

10

15

20

25

30

35

40

45

0

50

THETA

Ma

g. [d

B]

m1

m1THETA=dB(ARcp)=0.661

0.000

(a) CP radiation patterns at 1.6 GHz (b) Axial Ratio at 1.6 GHz Vs Azimuth Angle

Fig. 7 Radiation characteristics of 4x5 element array with element sequential

rotation

E. Beam Steering Simulated Performance

After verifying that the 4x5 array gave a high level of

performance at boresight, the main purpose of this study was

to ensure that adequate circular polarization characteristics

could be maintained over the entire 40 beam scanning

coverage. The results of Fig. 8 show that a polarization

isolation of -15dB is maintained when scanning out to

azimuth angles of 40 at the 1.6 GHz centre frequency. This

easily fulfils the

To verify that the array also produced acceptable CP

performance over the 1525-1660 MHz frequency coverage,

the plots of axial ratio for azimuth and elevation beam steering

angles were produced (Fig. 9 (a&b)). These include the 1.6

GHz centre frequency and also include the results for the

upper and lower frequencies (1525 and 1660 MHz). The

results show that axial ratio increases as the scan angle is

moved away from boresight, although the array is still able to

maintain an axial ratio of approximately < 3dB over all the

required scan angles and frequency coverage.

0

0.5

1

1.5

2

2.5

3

3.5

4

-60 -40 -20 0 20 40 60

Azimuth Beam Steering Angle (degrees)

Ax

ial R

ati

o (

dB

)

1.525 GHz 1.6 GHz 1.66 GHz

(a) Azimuth

0

0.5

1

1.5

2

2.5

3

3.5

4

-60 -40 -20 0 20 40 60

Elevation Beam Steering Angle (degrees)

Ax

ial R

ati

o (

dB

)

1.525 GHz 1.6 GHz 1.66 GHz

(b) Elevation

Fig. 9 Axial Ratio Vs Beam steering Angle over the BGAN frequency range

1.525GHz to 1.66GHz

F. Measured CP Array Results

To provide confidence that the simulated results were

accurate, verification was provided by measuring a 1x2 array

(Fig. 10) inside an anechoic chamber. The array employed a

90 element rotation to enhance the CP characteristics. The

same antenna array size was then applied to the ADS

momentum simulation for comparison. The measured CP

radiation patterns of Fig. 11(a) show that the 20dB of

polarization isolation at boresight is in close agreement with

the simulation (-22dB), the overall trend of the results

providing excellent agreement. The same level of agreement

is also evident for the axial ratio measured and simulated

results of Fig. 11 (b).

Fig. 10 Fabricated CP Array

-60

-50

-40

-30

-20

-10

0

-90 -45 0 45 90

Angle (deg)

Re

lati

ve

Po

we

r(d

B)

Measured LHCP Measured RHCP

Simulated LHCP Simulated RHCP

(a) CP Radiation Patterns

0

10

20

30

40

50

60

-90 -45 0 45 90

Angle (deg)

Re

lati

ve

Po

we

r(d

B)

Measured Axial Ratio Simulated Axial Ratio

(b) Axial Ratio Vs Azimuth

Fig. 11 Measured Results of 1x2 CP Array

III. CONCLUSIONS

A 4x5 element antenna array has been presented for

Inmarsat BGAN applications. This array has addressed an

important phenomenon which appears to have been little

reported in the literature. This is the degradation of circular

polarization characteristics when a CP array is steered

electronically. The design of this array presented significant

challenges, as not only did it have to maintain a high quality

of CP when scanning up to 40, but also had to do this over

an 8.5% frequency coverage of 1.525 to 1.66 GHz. The

resulting array was able to fulfill these requirements, with

element sequential rotation being employed to further enhance

the CP characteristics.

ACKNOWLEDGMENT

This project is supported by the European Space Agency

project AO/1-6168/09/NL/JD, Self-Focussing Retro-

Reflective Tx/Rx Antennas for Mobile Terminal

Applications.

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