Polarisation synthesis and beam tilting using a dual port circularly polarised travelling wave antenna array

P.H. Rao and V.F. Fusco

Abstract: The paper describes the application'of a dual port annular sector radiating line antenna (ANSERLIN) element, designed to provide a radiation bandwidth of 35% (1.7GHz to 2.4GHz) for a VSWR < 1.5. A four-element array consisting of such elements is shown to provide more than 18" of beam tilt without the need for phase shifters. A method to reduce on-axis cross-polarisation in the circularly polarised array is also presented. In addition, it is shown how the array can be used to produce slant 45" linear polarisation. The resulting array can operate over the personal communication network (PCN) through universal mobile telecommunication system (UMTS) frequency bands, from I .7 GHz to 2.2 GHz.

1 Introduction

Generally, an antenna radiates elliptical polarisation, which can be defined by axial ratio, tilt angle and sense of rotation [I]. In the extreme limit this takes the form of linear and circular polarisation (CP). Many types of compact CP antennas exist; for example, [2-51 describe wire, circular sector, annular ring and annular sector microstrip antennas, respectively. The annular sector radiating line (ANSER- LIN) two port antenna [6] is interesting since it offers the prospect of wideband impedance matching and 3 dB axial ratio handwidth. Since it has two ports, either sense of circular polarisation (LHCP or RHCP) is possible by exciting one port of the antenna at a time while properly terminating the other port. Linear polarisation rotated to any required tilt angle is also possible by simultaneously exciting both ports of the antenna with a suitable differential phase delay applied between them.

In this paper, in order to demonstrate the flexibility that a two port circularly polarised antenna of this type can offer a designer, we illustrate how an array of four ANSERLIN elements can be used to form a k e d beam tilt by relative physical rotation of the antenna elements. This is useful since service coverage often requires a fixed amount of down tilt for operational reasons and the inclusion of phase shifting devices adds expense. In addition, their power handling can reduce operating power levels. We also show how classical base station f4S" slant linear polarisation can be synthesised using this type of antenna. This offers the advantage of much wider operating handwidth than is available using a conventional wideband patch element solution for the same problem [7].

In another development, we show how deliberate rota- tional misalignments can be introduced to the orientation of

Q IEE, 2003 IEE ProceedvryS onlinc no. 20030746 doi: 10.1049/ip-map:ZW30746 Paper first received 14th November 2002 and in revised form 21s: Wily 2003. O h e publishing date: 16 October 2003 The authon are with High Frequency Labomtoties, Sfhod of Electtical and Eletronics Enginering. The Queen's University of Belfast, Ashby Building. Slranmillis Road, Belfast BT9 5AH, UK

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the CP elements when placed in an array in order to improve its on-axis cross-polarisation level.

2 antenna

Maintaining constant impedance along the antenna struc- ture from the input port to the output port while producing a travelling wave results in wideband performance of the antenna. Normally, the antenna element shown in Fig. 1 is fed at one port and terminated with a non-reflecting load placed at the other port; in this way, circular polarisation can be generated, albeit at the expense of enerby dissipated into this load (Fig. 2, S12). By interchanging the input port and the terminating port, either sense of circular polarisa- tion can be realised. If both the antenna ports are excited simultaneously then linear polarisation and, specifically for mobile communication applications, slant 45" linear polarisation is possible.

The annular sector antenna is designed as a strip conductor placed over a ground plane as a transmission line with a phase shift of one degree per degree of traverse on the annular sector, as required for travelling wave operation [2]. The antenna is designed as a three-dimen- sional tapered transmission line, which is created by maintaining a constant width (w) to height (b) ratio along the structure. For 500 characteristic impedance W/h ratios should be of the order of 3 [PI. For a design centre frequency of 2.0GHz, [q gives the initial geometric parameters for the antenna. These were subsequently refined using EM simulation software [9]. The outer to inner radius (Rl/R2) ratio and the circumference of the antenna are optimised to provide circular polarisation with good axial ratio. The design parameters of the antenna are outer radius R1 35" and inner radius R2 9.5". The width to height ratio of the antenna structure is 3.6 giving the height of the annular =tor from the ground plane as 7.2mm.Thesizeofthegroundplaneis 11Smmx 115" (less than l & x I & at the centre frequency), the other parameters are I , = 20 mm, 1, = 8 mm, W, = 33 mm and W2=3mm (Fig. la).

Design of the annular sector radiating line

321

0.

-10 -

g -20-

, . -30 - ;.. : . :

4 0 1 1.0 1.5 2.0 2.5 3.0 3.5 4.0

frequency, GHz

Meawed scattering parameters for single ANSERLIN Fig. 2 anfenna

4 Experimental results

Figure 2 shows the measured return loss and transmission loss at port 1 and port 2 for the isolated element shown in Fig. la. It can be seen that the antenna has a wideband impedance match over a frequency range I .W.0GHz for VSWR<I.S. The minor discrepancy between the retum' loss at port 1 and port 2 is due to physical feed fabrication spacing differences at port 1 and 2.

4.1 Circular polarisation By exciting port one while matching port two, left-hand circularly polarised antenna radiation is obtained (Fig. 3).

semi-rigid cables with SMA connectors

'ji

b

Fig. 1 ANSERLIN anfenna U Geometry h Photograph

3 Linear polarisation synthesis

It is well known that linear polarisation can be synthesised using two contrarotating circularly polarised signals. When an ANSERLIN antenna is excited simultaneously at both of its ports with 0" phase difference between them, then RHCP and LHCP signals are generated, resulting in linear polarisation along the y-direction (Fig. Ib). When this type of antenna is excited simultaneously at both ports with 2y" differential phase, then again RHCP and LHCP signals are simultaneously generated but with relative phase rotations that result in slant yo linear polarisation. For example, with 90" differential phase applied, slant 45" linear polarisation is produced since

1 - lhcp -rhcp I

theta, degrees

a

- meas. lhcp ........ Sim. lhcp - meas. rhcp

-180 -135 -90 4 5 0 45 90 135 180

theta, degrees

b

Fig. 3 a Single element h 4 x I a m y

LHCP mtenm radiation puttems at 2.06GHz

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Figure 3a shows the LHCP radiation patte'ms at band centre 2.0GHz for an isolated element The antenna exhibits a front-to-back ratio of more than 22dB, and the cross- polar level is low resulting in an on-axis axial ratio of less than 2dB over the entire band, I.7GHz to 2.4GHz. The gain of the antenna element was measured to be 2dB at 2.0 GHz. Across the frequency band the patterns are nearly symmetrical except for a small shift in the main beam- pointing angle, f2", at the band edges. This is due to the fact that the antenna mean circumference in terms of wavelength changes with frequency. This, in turn, disturbs the necessary condition required for circular polarisation [2].

Using simulation results, the optimal spacing for the antenna array configuration was investigated using [IO]; here, pattern multiplication of the measured single antenna element radiation pattem in Fig. 3a was included. In this way, an array of four ANSERLIN elements was designed with interelement spacing d of 0.76A; at 2.0GHz. This spacing was selected as a useful compromise between the ininimum available spacing due to array element sue and larger spacing where grating lobe effects occur.

When all of the port Is of the antenna elements are excited in parallel with equipower/phase signals, and all port 2s are terminated in matched loads, the resulting LHCP far field pattern is shown in Fig. 3h at band centre 2.0GHz. Fig. 36 shows that, since all the elements are excited uniformly, the expected sidelobe response should be around -13dB. For brevity, the measured characteristics of the array across its operating bandwidth are summarised in Table I . The results show that the array can be used for wideband circular polarisation application with good front-to-back ratio and low axial ratio.

Table 1: LHCP array

Frequency. 3dE FIB ratio, Cross-pol., Axial Sidelobe GHr beamwidth, dE dE ratio, level SLL,

deorees dB dB _____ ____

1.5 23 -22 -15 3.2 -11

1.7 21 -24 -14 3.3 -12

2.0 17.5 -27 -18 2.2 -12

2.4 14.5 -29 -21 1.6 -13

4.2 Linear 4 5 operation According to (I), when both ports of an individual element are excited simultaneously, hut with 90" phase difference between them, we achieve slant 45" polarisation. Figure 4 shows the slant +45" (CO-) and -45" (cross-) polarisation patterns at band centre, 2.0GHz. In the 4 x I array, the necessary condition for generating slant 45" linear polarisa- tion requires that the input signal is divided into eight equal magnitude signals, with each pair having 90" phase difference (obtained using commerical broadband 30%, 90" hybnds) between them. Measured radiation pattems at band centre 2.0GHz for this configuration is shown in Fig. 4, while Table 2 summarises the sidelobe level, cross- polarisation level and beamwidth response of the array with frequency.

4.3 Phase shifterless beam tilt Steering the main beam of a CP array without providing any phase shifters or delay lines in the array is accomplished by the physical rotation of circularly polarised antenna

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n l- slant 45 degrees 1

-180 -135 -90 -45 0 45 90 135 180

theta, degrees

Fig. 4 2.0 GHz

Slant 45" linear polarised anrema radwtiun patterns at

Table 2: Slant 45" polarised array

Frequency, 3dE beamwidth, FIB ratio, Cross-pol., SLL, dE GHz degrees dE dB

1.7 21 -23 -11 -14

2.0 18 - 30 -9 -11

2.4 15 -40 -13 -11

elements [Ill. To provide beam tilt, the rotation of the antenna elements should be progressive in order to achieve a symmetrical radiation pattem with dehed sidelobe levels. Assuming no mutual coupling between elements, the relation between the angle of rotation and the beam steering is given by [I I]

where = progressive phase shift, = main lobe point- ing angle of the steered pattern, k = 2n/& and d = 0.76 I*.

Each element of the array fed for LHCP is sequentially rotated by 45" relative to the first element. This in turn provides the progressive phase shift required for a k e d beam tilt to a pre-specified angle. The measured radiation pattems at 2.0 GHz are shown in Fig. 5 and a summary of

-180 -135 -90 4 5 0 45 BO 135 180

theta, degrees

Fig. 5 Parrem with 45'progressiur r/enzent rorarioir at 2.0 CHr

antenna performance with frequency is given in Table 3. The measured beam tilt at 2.0GHz is 13" (14" predicted using (2)). The measured results for 60" sequential element rotation are summarised in Table 4; here, beam tilt of 19" (predicted 19') was achieved at band centre.

323

Table 3: 46" progressive rotation

Frequency, Beam tilt FiB ratio, Cross-pol., 3dB SLL, GHz angle, dB dB beamwidth, dB

degrees degrees

1.7 14 40 -36 21 -9

2.0 13 38 -25 18 -10

2.4 11 32 -24 15 -10

Table 4 60" Progressive rotation

Frequency, Beam tilt FIB ratio, Cross-pol., 3dB SLL GHz angle, dB dB beamwidth, dB

dearees degrees

1.7 18 30 -26 21 -9

2.0 19 33 -28 18 -10

2.4 17 31 -18 15 -9

4.4 Cross-polar improvement By adapting the beam tilting strategy of the previous section, the on-axis axial ratio of a circularly polarised array can be improved if the orientation of the elements in the array are optimised to negate mutual coupling phasing effects. Here, we introduce relative phase differentials between the adjacent array elements while maintaining symmetry with respect to the centre of the array. For simplicity, we have arranged that the offset rotation angle is the same for each element in the array and that mirror symmetry is maintained with respect to the centre of the array. Typical cross-polar radiation reduction of about -3dB at 2.0GHz for lo" rotation offset of the antenna elements is shown in Fig. 6. The copolar far field patterns remain largely the same as those given in Fig. 3b.

O 1

-180 -135 -90 4 5 0 45 90 135 180

theta, degrees

Fig. 6 M e m r e d cross-polar response with offset elements, a 2.0 GHz

5 Conclusions

A single ANSERLIN antenna element was designed that has a useful radiation pattern bandwidth and VSWR< 1.5 over the frequency range 1.7GHz-2.4GHz. The antenna has a low profile and requires only a small ground plane making it a suitable element for array deployment. The flexibility of the antenna in situations where CP or slant linear polarisation is required has been demonstrated. Fiwed beam tilt from boresight is achieved by physical rotation of the antenna elements. This e l ina t e s the need for phase shifters in the array. This is often very useful if, for example, it is required to enhance coverage, or minimise interference, in array deployment by providing a fixed down tilt in the radiation pattern. By exploiting the phase dependence that rotary orientation of the antenna elements in the array allows, then axial ratio can he improved by negating deleterious mutual coupling effects in the array. In addition, we have shown that by proper arrangement, slant 45" linear polarisation is possible by virtue of the simultaneous production and spatial re-combination of two counter rotating CP signals. The antenna array described in this paper should find application in the wideband mobile communications environment due to its simple construction and small s i .

...

6 Acknowledgments

This work was carried out under British Commonwealth Scholarship/Fellowship programme and EPSRC grant GR/ R15139/01. Discussions with Dr R. Cahill of Queen's University of Belfast are gratefully acknowledged.

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References

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pp. 861~867

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