Beam steered reactively loaded printed microstrip loop antennas

R.L. Li and V.F. Fusco

Abstract: Printed microstrip loop antennas loaded with reactive components are analysed to demonstrate the capability of beam steering by reactance adjustment. The geometry of the printed microstrip loop including reactive element loading is represented as a superquadratic function so that the current distribution, hence radiation pattern for various antenna loop shapes, can be calculated using a unified method of moment formulation. The effects of the loop shapes and dielectric substrates on the beam steering capability and input impedance characteristics are investigated. It is found that continuous beam steering can be achieved and that a compromise has to be made between the maximum beam steering and minimum input VSWR. A 1.0GHz experimental verification is provided, which demonstrates that beam steering of 24" can be achieved by control of a single reactive element, while low crosspolarisation (- -20 de) and VSWR (- 1.7) can be simultaneously maintained.

1 introduction

Printed microstrip loop antennas have been investigated by many researchers [l-51. Usually these antennas are used to radiate maximum power at broadside. In mobile commu- nications systems beam steering or pattern shaping may be required during operation. More general practice is to employ an antenna array to control the bedm direction. For applications involving mobile communications, a small antenna size is desirable and the physical space for deployment of a beam steering array may not exist, this aspect is particularly true for the mobile handset. This means that it is desirable to use fewer than normal antenna elements for pattern control in these situations, ideally the number of elements should be reduced to one. In [6], pp. 16&163, it was shown theoretically that a two element 0.5 wavelength spaced reactively loaded dipole array can be made to exhibit 28" of beamsteer. The input impedance associated with this arrangement which required four reactive elements varied for approximately 1 kQ to 1.5 kQ and beam steering was in one direction, + 4 only. Owing to the high VSWR of this and other proposed arrangements based on reactively loaded dipoles, this technique remains largely experimentally unvalidated to date.

Recently it was found that a reactively loaded one- wavelength square loop antenna placed in air a quarter- wavelength above a groundplane can achieve continuous 60" beam steering, simply by varying the reactance of a single reactive loading component [7]. The concept is extended in this paper to printed loop antennas in order to reduce the antenna size and improve their input impedance characteristics. This problem is considerably more complex to analyse than that in [7], due to the

0 IEE, 2002 IEE Proceeding.7 online no. 20020360 DOI: 10. 1049/ip-map:20020360 Paper first received 14th August 2001 and in revised fonn 15th February 2002 The authors are with the High Frequency Electronics Laboratories, Department of Electrical and Electronic Engineering, Queen's University of Belfast, Ashby Building, Stranmillis Road, Belfast BT9 5AH, Northern Ireland, UK

presence of the dielectric substrate onto which the loop is placed.

This paper will demonstrate the pattern control capability of reactively loaded loop antennas printed on various dielectric substrates whch exhbit well controlled VSWR and far field radiation characteristics. To facilitate the modelling of various shapes of the printed loop, the geometry of the loop is represented as a superquadratic function. This permits a wide variety of loop configurations, such as a square, rectangle, circle, ellipse, rhombus and so on, to be defined by a single set of parametric equations. First, a unified method of moment (MOM) formulation is summarised briefly, for the current distribution and radiation pattern computations, which incorporates the inclusion of arbitrary lumped impedance loading. The feature of beam steering for a printed square loop is then described, followed by a comprehensive investigation of the effects of various loop shapes and dielectric substrates on the beam steering capability and input voltage standing- wave ratio (VSWR). Experimental results are presented to demonstrate the performance of a reactively beam steered printed loop antenna.

2 MOM formulation

2. I Antenna structure The reactively loaded printed wire loop antenna under consideration is shown in Fig. 1, where the wire radius is 0.002&, which is very small compared to the free space wavelength ,lo, allowing the thin wire approximation to be used.

To allow deformation of the loop to various shapes with ease, the geometry of the loop wire (wire axis) was defined by a superquadratic function in parametric form [5]:

x ( 4 ) = 4 ( 9 ) c o s 9, 0 5 9 5271 (1)

IEE Proc.-Microw. Antennus Propug.. VoL 149, No. 3, J i m 2002 169

Y

X

-F

X

Fig. 1 Printed loop antenna loaded with two reactive components

where a and b are the semi-axes in the x and y directions, respectively, and v is a 'squareness parameter' which controls the behaviour of the loop radius of curvature. Obviously, numerous different loop configurations can be modelled solely by varying the values of the shape parameters u, b and v as illustrated in Fig. 1.

In general, two reactive components are needed to achieve beam steering in the x-z plane [6]: one (XLI) at i#~ = 7r/2 and the other (XL2) at 4 = 3n/2 (refer to Fig. 1). Extensive numerical simulation revealed that the reactance of one of the two components (e.g. XL2) should keep as large as possible (2 lOkD at least). By changing the reactance of the other component (Le. XLl), beam steering in the x-z plane can be achieved.

2.2 Solution for current distribution For generality, consider a printed wire antenna loaded with L lumped impedance components Z,, Z,, Z,, ..., Zl ..., and ZL, located arbitrarily at s = sl, s2, s3, . . . , SI . . . , and s = sL (where s is the antenna length co-ordinate). Under the thin wire approximation and an assumption of infinite dimension for the dielectric substrate and perfectly con- ducting groundplane, the current distribution Z(s) on the wire is related to the loaded impedance Z, as an electric field integral equation (EFIE) [5, 61:

L

CZ(S)Z ,d (S - S I ) - E;m(s) I = 1

where 6(s-sl) is a delta-function, ko = 2n/& is the free-space wavenumber, E:" is the tangential component of the impressed electric field, II and n, are the Sommerfeld-type integrals, which are given in [5].

170

The EFIE (4) can be solved easily using a so-called parametric method of moments (P-MOM), and the radiation field directly obtained using the stationary phase integration approach based on the solved current distribu- tion as described in [5].

3 Numerical results and analysis

3.1 Beam steering feature To demonstrate the beam steering feature of the printed loop antenna, some calculated results for a substrate with dielectric constant E, = 2.1 and height h = 0.11& are presented. The values of u, b and v are chosen to be a = b = O. l& and v = 20, which correspond to a corner- rounded square loop with a perimeter of 0.7784&. In the computation, XL2 is fixed at -30kR. It should be mentioned that the beam steering is not sensitive to the exact value of XL2 provided it remains above 10 kR.

The calculated 1 E<b I patterns in the x-z plane are shown in Fig. 2 for three different reactive components: (a) XLl = 0, (b) XL1= -j30 kR (large capacitive reactance) and (c) XL1= j 1 130 R (large inductive reactance). These patterns indicate that the maximum direction is shifted approximately to 0 = -24" at XLl = -j30 kQ and O = +20° at XL1 =jl130R. The variation of the beam steering angle (about the z axis) with XLI is shown in Fig. 3. It is observed that the radiation pattern can shft continuously from 8= -22", at XL1= -5000R, to 0 = + 45", at XL1= 2000 R. It should be mentioned, however, that as XL1 increases beyond 2000R2, the side lobe and crosspolarisation levels will begin to degrade.

To examine the input impedance characteristics during beam steering, the variation of the input impedance and VSWR is illustrated in Fig. 4 as a function of the reactance XLI. It is found that VSWR remains approximately constant at 1.7 for a capacitive load. The input VSWR increases rapidly when the inductive reactance goes above J500 R.

3.2 Loop shape effects The variation of beam steering property for various loop shapes can be investigated simply by changing the shape parameters a, b and v. The capability of beam steering may be described by defining a maximum beam steering angle. From Fig. 3, it can be seen that there are two possible options for the definition: the maximum angle shifted toward the negative 0 direction or toward the positive 0 direction. Considering the poor VSWR behaviour and uncertainty of XL1 for a maximum positive 0 direction beam steering, the beam steering angle at XL1 = XL2 = -j30 kQ is defined as the maximum beam steering angle 0,.

Another parameter that must be considered in antenna design is the input VSWR. Hence, for each antenna shape, the VSWR is always kept to a minimum by adjusting the perimeter of loop.

Fig. 5 shows the maximum beam steering angle e,,, and input VSWR as a function of the squareness parameter v (a/b= 1). It is observed that there is little variation for O,, and VSWR as v varies from 1 to 20. This implies that the loop curvature (e.g. square or circle) has negligible effect on its beam steering performance. In addition studies of the effect that v has on far-field radiation patterns show that, for v > 5, there is no significant variation relative to the results shown in Fig. 2 were v = 20.

Fig. 6 illustrates the variations in 0,, and VSWR against the aspect ratio a/b (v = 20). It is found that the maximum beam steering angle slightly increases as a/b reduces, but

IEE Proc.-Microw. Antennus Propaq., Vol. 149, No. 3, June 20112

€#, measured o E@ computed

a=oo 0 Eo, computed [ L f , , , measured 1

-90" goo+ x 0 0.2 0.4 0.6 0.8 1.0

i n = v

0 a

0.2 0.4 0.6 0.8 1.0

Z

t o = o o

0 0.2 0.4 0.6 0.8 1.0 Z

t O=O"

-90" 0 0.2 0.4 0.6 0.8 1.0 b

i Q=OO

0 0.2 0.4 0.6 0.8 1.0 z t

0=0"

0 0.20.40.60.8 1.0

Fig. 2 Comparison of measured and computed radiation patterns ,for a printed loop antenna loaded with dflerent reactive loads

a XL1= 0, XL2 = -j30 kfi b XLI = XL2 = -j30 k 0 c XLI =jl130R, XL2 = -j30 kR

C

(a=b=0.1?.(, ~ ~ 2 0 , h=0.11?q, t+=2.1)

45

i . ; m

ai -0

-5000 -4000 -3000 -2000 -1000 0 1000 2000 XLI, R

Fig. 3 Calculated beam steering angle (0) variation with reactance XLI ( X U = -30 kQ, a = b = 0. I&, v = 20, h = 0. I I&, s,=2.l)

VSWR increases too, in general for a VSWR of 2:1, Q,, is about 24".

3.3 Dielectric substrate effects There are two parameters for a dielectric substrate which influence the antenna properties: the dielectric constant E,

and substrate height 11. In the analysis given here, these parameters are fixed at E, = 2.1 and h = 0.1 I&. Now, the authors will vary E , and h, respectively, to evaluate their effects on the beam steering characteristics of a reactively loaded printed loop antenna. Here, the aspect ratio and squareness parameter of the printed loop shape are fixed at ulb = 1 and v = 20. However, as a general principle, the value of u and/or b can be adjusted for a minimum VSWR.

75

50

a

5 25

E

._ c o

!?i -0

a ._ - 3 a

-25c000 1 -50' ' I ' ' ' I ' I ' I ' I I ' 1

-5000 -4000 -3000 -2000 -1000 0 1000 2000 XL1, n

Fig. 4 calculated input impedance and VSWR variation with reactance XLI ( X U = -30 kS2, a = h = 0.l& v = 20, h = 0.11&, E, = 2. I)

IEE ProccMicrow. Antennus Propqi., Vol. 149, No. 3, June 2002 171

3 0 - I I U I 1 1 1 1 I I B I I 1 0 1

- - m a-0 0 0.---.--.---.--.~~-(

-0- maximum beam steering angle 0, 20 - ai - D

m D

- E - I!-. 1 '1-1-1-1-1-1-11

0 I I I t ' I I n ' I I t '

10 -

- -

D $ 20

4

- 3

- a :

- > - 2 - 2

1

- ai m m D ._

10 -

0

3

2

5 ffl >

0.5 0.8 1.1 1.4 1.7 2.0

aspect ratio alb

Fig. 6 Muximum beuni steering cmgle Q,,, and minimum input VS W R us a function of uspect rutio aib ( X L I = XL2 = -30 kQ, 1) = 20, I? = 0. I ri,, Er = 2.1)

Figs. 7 and 8 illustrate the variations of the maximum beam steering angle Bn7 and input VSWR, respectively, as a function of the dielectric constant E, and substrate height h. It can be seen that both a hgher permittivity and a thicker height tend to enhance beam steering capability. It is also noted that a larger e,,, usually results in a higher VSWR. Hence, a compromise must be made in practice between beam steering angle and input impedance characteristics.

90 4 1 ' 1 ' 1 ' - -0- maximum beam steering angle 0, - -1- minimum input VSWR

60 -

a: -0 /. - a /a - >

- 2

-

I , I , I , I , 1 1 2 3 4 5 6

dielectric constant cy

Fig. 7 Maximum beum steering angle On, and minimum input V S W R as u junction of dielectric constant E, (XLI = XL2 = -30 kQ, d b = 1, v = 20, h = 0. I1 41)

9 0 1 I , I I I , I I , , 4

._

0

-0- maximum beam steering angle 0, I -.- minimum input VSWR If I ' /

3

a:

> 5 l2 1 0 0.05 0.10 0.15 0.20 0.25

substrate height, Lo

Fig. 8 Muximum beam steering angle e,,, and minimum input VS W R as u function vf substrute height h ( X L l = XL2 = -30 kQ, uib = I,v = 20, C, = 2.1)

3.4 Beam steering mechanism To obtain a physical insight into the beam steering mechanism associated with this antenna type, the calculated current distributions on the printed loop are presented in Fig. 9 for three cases: (u) XL1= XL2 = 0 R (a normal loop without any reactive loading, given as a reference), (6 ) XLl = 0 R, XL2 = -j30 k R and (c) XLl = XL2 = -j30 kR. Fig. 9u shows the near field current distributions associated with a standard rectangular loop, here it can be seen the current in arm AB is approximately 180" out of phase with that on arm CD, whch is spaced approximately 0.5 wavelengths, reinforcement, while those on A D and BC are essentially in phase yielding broadside radiation. In Fig. 9b, it can be seen that the current distribution for case (6) looks similar to that for case (a), especially on sides AB and CD (also see Fig. l), t h s is the reason why the printed loop antenna loaded with XL1= 0 and XL2 = -j30 kR radiates maximum power at broadside in the same manner as B

noma1 printed loop antenna. When XL1= XL2 = -j30 kR (case (c)), the current phase on the side AB is about 90' ahead of that on the opposite side CD, which results in a radiation pattern which is tilted toward side AB, while arms A D and BC are excited in phase. Hence, steering of the beam occurs in the zx plane only, see Fig. 2. For the case when XLI = 1 k R and XL2 = -30 k 0 , one inductive load, the current on AB has an approximate 90" phase lag with respect to that on CD, as a result, the radiation pattern is steered in the opposite direction.

4 Experimental verification

To demonstrate the beam steering characteristics of reactively loaded printed antennas, an experiment has been carried out at 1 GHz. The dielectric substrate used is PTFE with E,. = 2.1 and 12 = 33 mm (0.1 l&). The dimensions of the dielectric substrate and groundplane (aluminium) are 300" x 300". For ease of fabrication, a loop with the shape parameter u = b = 30 mm (0.13Lo) and v = 20 was constructed using a microstrip line of width w = 2.4" (0.008&) and printed first on an RT/duroid 5880 substrate (~,.=2.2, h=0.254mm) with its back ground plane removed, t h s was then pasted on the PTFE substrate. The printed antenna is excited by a quarter-wavelength folded balun.

Three cases were examined: (a) XL1= 0, XL2 = -j30 kS1, (b) XL2=XL1 =-j30kR and (c) XL1 =jl130S1, XL2 = -j30 kR. The authors used a 2"-wide gap to 'model the high capacitive reactance (-j30kR), and an EPCOS SIMID08B series inductor to provide the inductive

IEE Proc.-Microw. Antennas P~op~<j . , Vol. 149, No. 3, June 2002 172

reactance j 1 130 R (equivalent inductance L = 180 nH, the actual measured inductor impedance at lGHz is 50 + j 1 130 R, this actual value was used in subsequent numerical simulations).

The measured VSWRs for these, plus two other cases, are plotted in Fig. 10. Agreement can be seen between the calculated and experimental results. The input VSWR is less than 2 for the capacitive loads around 1 GHz.

"dl magnitude Phase ::: 15

60 a

._ a 10

E ai U

C m

2

D

-0

i 0

r a

-60

5 measured: - case (a)

case (b) case (c) case (d) case (e)

- . - .

.... ~ ....

-120

-180 0 0 0.25 0.50 0.75 1 .oo

normalised antenna length a 2 0 ' h j magnitude :::

60 a .E ai U

C D ._ a 10

E" .c Q

-60 I I I I I I I

0.8 0.9 1 .o 1.1 1.2 frequency, GHz

5

-120 Fig. 10 Input VSWR of a printed loop antenna loaded with dgerent reactive components (a = b = 30 mm, v = 20, h 4 33 mm,

a XL1= XL2 = 0 (a normal loop) F, = 2.1)

b XLl = 0, XL2 = -30 kR c XLl = XL2 = -30 kR d XL 1 = - 1600 R, XL2 = -30 kR e XLI = 1130R. XL2=-30!&

-180 0 0 0.25 0.50 0.75 1.00

120 20 The radiation patterns for the three cases cited here,

measured at 1 GHz, are shown in Fig. 2 with computed results for comparison. Again, good agreement is seen between the measured and computed results. As expected, the radiation pattern for XLI = 0 and XL2= -30 kQ is similar to that of a normal printed loop. When XLI is capacitive, the beam steers toward the negative x direction. On the contrary, the maximum direction turns to the positive x direction for an inductive load. This confirms that the radiation pattern of a printed loop antenna can be controlled by adjusting reactive load, XLl, Fig. 1. It is also noted that there is a lower crosspolarisation level ( N 20 dB) for increased capacitive loading, while the cross-polarisation component 'will increase with the inductance.

60 m U

a E 15

-60

5 -120

0 -180 0 0.25 0.50 0.75 1.00

C normalised antenna length

5 Conclusion

The concept of reactive loading is introduced into printed loop antennas for pattern control. A unified method of moment formulation has been presented for analysis of parametrically defined printed loop antennas loaded with arbitrary lumped impedance. The capability of beam steering has been investigated for differently deformed loops printed on various dielectric substrates. It was shown that continuous beam steering can be achieved just by

173

Fig. 9 Calculated current distributions on a printed loop antenna loaded with dSfferent reactive load (a = b = 0. I&,v = 20, h = 0. I l l ( , , E , = 2.1) a XLI = XL2 = 0 (a normal loop) b XL1= 0, XL2 = -30 kR c XLI = XL2 = -30 kR

IEE Proc.-Microw. Antennas Propag.. Vol. 149, No. 3, Jww 2002

adjusting a lumped reactive load placed in one arm of the

The effects of the loop shape and dielectric substrate parameters on the beam steering performances have been evaluated. It was found that the maximum beam steering angle with a minimum input VSWR is insensitive to the loop shapes, but is considerably dependent on the property of dielectric substrate. Usually, a loop antenna printed on a dielectric substrate with higher dielectric constant and/or thicker substrate height has a greater beam steering capability. In addition, the authors also note that a bigger beam steering angle is generally associated with a higher VSWR. Hence, a compromise must be made in practice between the maximum steering angle and minimum VSWR.

An experimental verification has been performed at 1 GHz for a printed square loop loaded with different reactive components. It is demonstrated that a maximum beam steering angle of 24” can be obtained with a lower crosspolarisation level and a minimum VSWR of 1.7. In addition, discrete switching between inductive and capaci- tive values would lead to electronically discrete steering, whde analogue control would lead to continuous beam steering. Such antennas may be useful in many microwave and millimetre wave wireless applications.

loop. 6 Acknowledgment

This work was sponsored by the UK Engineering and Physical Research Council under grant reference GR/ M8865.

References

NAKANO, H., KERNER, S.R., and ALEXOPOULOS, N.G.: ‘The moment method solution for printed wire antennas of arbitrary configuration’, IEEE Trans Antennas Propuy., Dec. 1988, AP-36, (12),

HEJASE, H.A.N.: ‘Analysis of a printed wire loop antenna’, IEEE Trans. Microw. Theory Tech., Feb. 1994, 42, (2), pp. 227-233 COOK, G.G., and M A M A S , S.K.: ‘Efficient moment method for analysing printed wire loop antennas’, IEE Proc, Microw. Antenna7 Propug., October 1997, 144, pp. 364366 L1, R.L., and NAKANO, H.: ‘Numerical analysis of arbitrarily shaped probe-exited single-ann printed wire antennas’, IEEE Trans. Antenna; Propay., September 1998, 46, (6), pp. 1307-1317 LI, R.L., N1, G., and NAKANO, H.: ‘Numerical analysis of printed superquadric wire loop antennas’, IEEE Trans. Magn., September

FUJIMOTO, K., JAMES, J.R., and HENDERSON, A,: ‘Small antennas’ (Research Studies Press, John Wiley and Sons, 1987), pp. 37.- 59, 160-163 LI, R.L., FUSCO, V.F., and CAHILL, R.: ‘Pattern control using a reactively loaded loop antenna’. Proceedings of 1 1 th Intemational Conference on Antennas and propagation, Manchester, April 2001, pp. 766769

pp, 1667-1674

1998, 34, (5), pp. 2787-2790

174 IEE Proc.-MicroM;. Antennas Propug.., Vol. 149, No. 3, June 2002