Wideband U-slot microstrip patch antenna array

K.L.Lau, K.M.Lukand K.F.Lee

Abstract: The design and measured results of a wideband antenna array of four U-slot rectangular patches are presented. The U-slot patches are proximity coupled by a microstrip feed line terminating with novel n-shaped stubs. By using a foam layer of thickness t - 5.5mm as the supporting substrate, an impedance bandwidth (SWR 5 2) of 27% ranging from 3.4GHz to 4.5GHz is achieved. It has an average gain of 13dBi and a cross-polarisation of less than -20dB.

1 Introduction

It is well known that microstrip antennas have very narrow impedance bandwidth, typically a few per cent. One of the methods of widening the bandwidth is to cut a U-shaped slot on the patch of the coaxially fed rectangular patch antenna [l]. The other method is to use a U-slot patch proximity coupled by microstrip feed line terminated with a novel n-shaped stub [2]. For a single element of this type of antenna, it has an impedance bandwidth of -20%, co- polarisation gain of 7.5dBi and cross-polarisation of -20dB across the passband.

In this paper, we report the measured results of a planar four-element array of such patches. This antenna can achieve an impedance bandwidth of -27%, co-polarisation gain of -13dBi and cross-polarisation of less than -20dB. A matching network is required for impedance matching. The array has good pattern characteristics.

dielectr substrat

microstriD feedline terminated by novel x - shaped stub z (H=O")

(0=90" g=o e=90° g = 90")

E plane (zx - p lane (zy - plane)

Fig. 1 Geometry of untenna element

0 IEE, 2001 ZEE Proceedings online no. 20010220 DO? 10.1@49iipmap:20010220 Paper frst received 13th March and in revised form 1 lth September 2000 K.L. Lau and K.M. Luk are with the Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S A R , People's Republic of China K.F. Lee is with the Department of Electrical Engkeiing, University of Mississippi, MS 38677-1848, USA

2 Description of antenna array

The geometry and detailed dimensions of a single antenna element [2] are shown in Figs. 1-3. Fig. 1 shows that this antenna element is a sandwich of three dielectric layers.

100 P line \

-1 1-4.75 Fig. 2 Detciiled ditnensions ofjbeche of mitennu element

I, 40 ,I

Fig. 3 Detailed dmen.tions of U-slot patch of untennu element

The top and bottom dielectric layers are made of the same material with relative permittivity of 2.33 and thick- ness of 1.524mm, which is separated by -5.5" (with tolerance 5mm I t I 5.5"). The U-slot rectangular patch is located at the lower side of the top dielectric layer, which

41 IEE Proc.-Mirr.ow. Antennas Propag.. Vol. 148, No. 1. Fehrnosy 2001

acts as a cover. The novel n-shaped stub and the feedline are printed on the upper side of the bottom dielectric layer. The patch is electromagnetically coupled by a microstrip feed line terminated with novel n-shaped stub.

The middle dielectric layer is made of foam with relative permittivity approximately equal to 1 and thickness equal to the separation between the top and bottom layers. The foam is used as the supporting material to reduce the exci- tation of surface waves between the other two layers.

dielectric substrate

microstrip feedlik - z (U = cp) terminated by novel n-shaped stub

'.. . y (0=90~l$=90~)

i..

a

Fig. 4 Geometry of antenna array

, 50 S2 line

5

upper set of antenna elements

100 Q line

upper set of antenna elements

- 60.57 Fig. 5 Detailed dknemiom of feedline and U-slot putches of antenna array

The geometry and detailed dimensions of the planar array are shown in Figs. 4 and 5. This array consists of four antenna elements (in Figs. 1-3) connected by a match- ing network. The matching network is printed on the upper side of the bottom dielectric layer. The distance between the centres of the patches d is 0.8& Although the diagrams in Figs. 4 and 5 are symmetrical about z , the feed is asym- metric, because the upper and lower set of antenna ele- ments (shown in Fig. 5 ) are pointing to directions that differ by 180 degrees. Therefore, x and y must not be equal but 180 degree different in electrical length. Thus the feed of this planar array is asymmetric.

42

I

3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 frequency, GHz

Standing wave ratio (SWR) ngainst3equency of untenna array Fig. 6

(0=270")

180 (N=180")

(0=9V)

where $=On

Fig. 7 Radiation patterns. 3.8GHz E-plane (,a-plane) (- co-polarisation, ~ ~ ~ X-polarisation)

(8=90°)

21 0-1 50 180 (0=180")

where @=goa Fig. 8 Rudiationpcitterns: 3.8GHz H-plane (zy-plane) (- co-polarisation, - - - X-polarisation)

3 Measured results

The standing wave ratio (SWR) of the antenna array, measured by an HP8510C network analyser, is shown in Fig. 6. The passband bandwidth (SWR 5 2) is in the fre- quency range 3.424.48GHz, corresponding to an imped- ance bandwidth of -27%. Figs. &9 show the radiation patterns of the antenna array at several frequency points over the operating frequency range. The co-polarisation pattems are stable across the passband. At the broadside direction of the array, the ratio of co-polarisation to cross- polarisation in the E-plane is 26dB at 3.8GHz, 24dB at 4GHz, 23dB at 4.2GHz and 23dB at 4.4GHz. The corre-

IEE Pioc -Microw Antenncis Propug.. Vol 148, No. I , Fehritary 2001

sponding values in the H-plane are 25dB, 23dB, 20dB and 24dB, respectively. We can also find from these figures that the half-power beamwidth is 35" in the E-plane and 37" in the H-plane at 3.8GHz, 36" in the E-plane and 34" in the H-plane at 4.0GHz, 35" in the E-plane and 32" in the H- plane at 4.2GHz, and 33" in the E-plane and 30" in the H- plane at 4.4GHz. Fig. 5 shows co-polarisation gain of the antenna array, which is about 13 dBi from 3.74.4GHz.

21 0

where $=On Fig.9 Radiation parterm: 4.OGHz E-plane (xz-plane) (- co-polarisation, - - - X-polarisation)

150

(8=270") 2i

(A = 270")

(tt=OO)

180 (8=l8O0)

(8=90")

(0=9O0)

where b=o" Fig. 1 1 Rudiution pattern,r: 4.2GHz E-plane (xz-plane) '(- co-polarisation, ~. - - X-polarisation)

I E E Proc.-Microw. Antennas Propug.. Vol. 148, No. I , Februnry 2001

(O=o") 0

(A=270°) 270 90 (R=90")

180 (I)=180°) where $=Oo

Fig. 12 Radiaiion putterm: 4.2GHz H-plane (zy-plane); (- ~ co-polarisation, - - - X-polarisation)

90 (H=90°)

180 (8=180")

where $=O"

Fig. 13 Radiation patterns: 4.4GHz E-plane (xz-plane); (- co-polarisation, - - - X-polarisation)

180 (H=180°)

where $=goo Fig. 14 Rndiatiunptrtterns: 4.4GHz H-plane (z.v-plane); (- co-polarisation, - - - X-polarisation)

: $ ( - - = ._ , F i 6

4 2

3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1

frequency, GHz

Fig. 15 Gain againstfiequency for antenna cirruy

43

4 Conclusions 6 References

We have successfully designed a wideband antenna array,

patches. The antenna array has a bandwidth of 27%, a gain of 13dBi and a cross-polarisation of less than -20dB.

1 MAK, C.L., LUK, K.M., and LEE, K.F.: 'Proximity-coupled U-slot patch antenna', Hecectron. Left., 1998, 34, (8), pp. 715-716

and LEE, R.Q.: 'Experimental and simulation studies of the coaxially fed U-slot rectangular patch antenna', IEE Proc., Microw. Antennm Propg., 1997, 144, (S), pp. 354358

Of four Of proximity coup1ed '-'lot 2 LEE, K,F,, LUK, K,M,, TONG, K,F,, SHUM, S,M,, H U m H , T,,

5 Acknowledgment

This project is supported by the CERG project: 9040210.

44 IEE Proc-Microvv. Antennm Prupr~x.. Vol 148, No. I , Fe'ehrziury 2001