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SIMULATION OF MICROSTRIP SMALL ANTENNAS

H Y Wang*,S Taylor., J Simkin*, M Oakley., C Emson’ and M J Lancaster**

*Ve ctor Fields Limited, United K nigdom * * University of Birmingham, United K ingdom

INTRODUCTION

Micro strip patch ante nnas are being increasingly used incommunication and radar systems because ’ hey havemany advantages over conventional antennas, such as

being lightweight, compact and conformal. Recently,there has been increased interest in minimising

microstrip patch antennas for some specific applications,such as mobile comm unications and monolithic

microwave integrated circuits. There are a number ofapproaches to reduce the size of patch antennas: (i) Usehigh permittivity substrate [11. (ii) Introduce shorting

pins at the edge of a patch to modify the boundary

conditions for the patch to be resonant with its size lessthan half waveguide wavelength [2]. (iii) Increase

electrical length by optimisi ng the shap e of a patch . (iv)

Combination of these approaches.

H-shape patch and meander line an tenna s fed by a

‘microstrip line through an rectangular slot or an H-

shaped aperture have recently been proposed andinvestigated using a cavity model in conjunction with a

full-wave simulator based on th e method of moment [3,

41. The microstrip antennas were miniaturised byoptimising their shapes to increase electric length. The

dimensions of these meander antennas are between one

quarter and one tenth of waveguide wavelength while

their radiation patternsare

basically the sam e as those ofconventional rectangular patches. As the cavity model isan approximate approach it is pertinent to use a moresophisticated echnique to achieve higher accuracy.

In this paper, a meander slot anten na and a meander lineantenna were investigated using a 3-D FDTD simulator[SI nd a microwave network analyser. The meander slotantenna is a single layer structure fed by an open-end of

a stripline whereas the meander line antenna is a multi-layer structure fed by an open-end of a microstrip. Thebasic principle of these meander antennas is developedand illustrated through a tran smissio n line theory andsimulated electric and magnetic field distributio ns in the

meander slot or line. The return loss, resonant

frequency, bandwidth, radiation patterns and radiationefficiency of these meander antennas with various

meander sections have been presented and compared

with measured results. The agreements between thesimulated and measured results are good.

3-D FDTD SIMULATOR

The algorithm in the 3-D FDTD simulator is based on atheory [6] proposed by K. S . Yee in 1966. It discretizesand solves time-dependent Maxwell’s curl equations in .

both time and space do mains. The space containing thestructure of interest is divided into a number of small

element ‘cells’ where electric field and magnetic fieldsare interleav ed both in spac e and time. This permits thespace and time derivatives in Maxw ell’s equations to be

approximated by central difference operations withsecond-order accuracy. For problems with open regions,

the super absorbing boundary condition [5] is employed

to simulate the radiation condition in free space.Furthermore, the simulator is based on a modified

conformal method so that it allows greater flexibility ofthe cell shape in comparison with conventional stair-case cell [7]. Hence it is well suited for handling

antennas with complex structures. In relation tosimulation efficiency and memory requirements, it only

requires O(N) ultiplication to update N grid points. Asthe FDTD method has been well documented inreference [8], the details of its theory and

implementation are not discussed here.

PRINCIPLEOF MEANDER ANTENNAS

Fig.1 shows the configurationsof

the meander slot andmeander line ant ennas .with three meander sections. For

the meander slot antenna, the slot can actually be

considered as a slot evolved from a conventional

rectangular slot with a half waveguide wavelength on aconducting plane. The simulated y-component of

electric fields obtained from the FDTD simulator is

illustrated in Fig.2(a). It can be seen from the electricfield distribution that the maximum value is at the

middle of the meander slot while at the two ends of themeander slot the value is very weak (short circuit).

Furthermore, because the y-component of the electricfields on the five segments in the x-direction of themeander slot has the same direction, they behave like a

rectangular slot in the x-direction consisting of five

‘short slots’. On the other hand, the x-component of theelectric fields on the each pair of the three symmetrical

segments in the y-direction are opp osite and henc e they

produce little radiation in the desired direction normal tothe con ducting plane. +

11th International Conference on Antennas and Propagation, 17-20 April 2001 , Conference Publication No. 4800 EE 2001

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Fig.2(b) shows the simulated y-component of magnetic

fields that represents the electric current along the x-

direction on the meander line. Similar to the meander

slot antenna, the total length of the meander line is abouta half waveguide wavelength. The currents on the five

segments in the x-direction of the meander line have thesame direction and hence they behave as a number of

radiation elements ‘short dipoles’ constructing a ‘longdipole’ in x-direction. The direction of currents on any

two symmetrically the segments in the y-direction areopposite, which have much lower contribution to the

desired radiation fields.

It is obvious that the resonant frequency of the meanderslot antenna drops with the increment of the meander

sections. However, with the increment of the meander

sections, the length of the ‘short slots’ or ‘short dipoles’

becomes shorter, and the mutual coupling and comer

effects become stronger. Under such a circumstance,most of the electromagnetic energy is constrained in the

vicinity of the meander slot, especially around corner,and conductor loss becomes severe. This will result in

lower radiation efficiency and higher cross-polarisation.Measured resonant frequency of meander line antennas

with various meander sections is illustrated in Fig.3,

where the overall size of these meander line antennas is

5.0 x 8.0 mm2 and the dielectric constant of the

substrate is 1 0.8. It is shown that the resonant frequency

of these meander line antennas drops substantially from2.75 GHz to 1.50 GHz with the increment of the

meander sections from two to seven.

RESULTSA N D DISCUSSIONS

A copper meander slot antenna shown in Fig.l(a) was

fabricated on RTDuroid substrates with thickness H1

H2 = 1.27 mm and relative dielectric constant E = E

= 10.8. The overall size of the meander slot is 5.5 x 5.5

mm2, and the slot width and the gap between two slots

ar e 0.5 mm. The distance between the centre of the

meander slot and the open-end S is adjusted to achieve

good match between the feed and the meander slot. Acopper meander line antenna illustrated in Fig.l(b) has

also been made on the same kind of RTDuroid

substrates with thickness HI=Hz = 1.27 mm. In orde r to

make direct comparisons between the meander slotantenna and the meander line antenna, the overall size,the line width and gap of the meander line antenna werechosen to be identical to the corespondent overall size,

slot width and gap of the meander slo t antenna.

The resonant and radiation properties of the meanderantennas have been simulated and measured using the 3-

D FDTD simulator, a full-wave simulator based the

method of moments [9] and a microwave networkanalyser. The simulated model in the FDTD simulator is

shown in Fig.4. There are two boxes in the model, the

smaller one is the near-field to far-field box used for the

calculation of radiation pattems while the larger with

absorbing boundary wall is used to simulate radiation

boundary condition. The antenna in the simulator based

on the method of moments is within a metal box inwhich the impedance of the top w all of the box is set to

be the impedance of free space (377 ohm).

Consequently, the results obtained from the FDTDsimulator are more accurate and reliable. Fig.5 shows

the simulated and measured return loss as a function offrequency for the meander slot and meander line

antennas. It can be seen that the resonant frequency of

the meander slot and meander line antennas is about2.65 GHz and 2.50 GHz, respectively, according to themeasured results. This indicates that the overall size of

the meander line antenna is slightly smaller than that of

the meander slot antenna for a given frequency. The

resonant frequency of an H-shaped patch with the same

overall size is abou t 4.50 GHz [3]. The 3 dB bandwidthof the meander slot and meander line antennas is around

2.0 % and 10 dB bandwidth is around 0.7 %. Thebandwidth of these antennas can be improved by using

substrates with a lower diele ctric constant.

The radiation patterns of the meander antennas have

also been simulated and measured. Fig.6 shows thesimulated 3-D radiatio n patterns of the meander

antennas obtained from the 3-D FDTD si,mulator. The

simulated radiation patterns are generally in goodagreement with the measured radiation patterns. In

comparison with the radiation intensity of the co-polarisation, the cross-polarisation is about 8 dB lower.

Based on the measured results, the radiation efficiency

of the meander slot antenna is about 4 5% - 8 % for thechosen substrates and dimensions of the meander slot.

This is relatively low in comparison with conventional

microstrip patch or slot antennas. How ever, the radiationefficiency of the meander slot antennas can be

significantly increased if substrates with low er dielectric

constant are used and high-temperature superconductorsare introduced for the fabrication of the antennas for the

applications wh ere radiation efficiency is crucial.

CONCLUSIONS

Meander slot and meander line antennas fed by astripline or microstrip open-end are proposed. Themeander antennas with three meander sections havebeen examined numerically using a FDTD simulator and

a simulator based on the method of moments andexperimentally using microwave network analyser. The

overall size of the meander slot antenna could bereduced to one tenth of waveguide wavelength, which ismuch smaller than that of conventional small antennas,

whereas the radiation pattern of the meander antennas is

generally the same as that of conventional patch

antennas.

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REFERENCES

Chaloupka, H., 1991, IEEE Trans. on Microwave

Theorv and Tech., vol. 39, 1513-1521Liu, Z. D. and Hall, P. S., 1997, IEEE Trans. on

Antennas and Prop agat., vol. 45, 1451-1458Lancaster, M. J . and Wang, H. Y., 1998,a

Tra ns. App1. SuDerconduct.. vol. 8, 168-177Wang, H. Y. and Lancaster, M. J., 1999,

Trans. on Antennas and Propagat., vol. 47,829 -836

CONCERT O user’s manual (Oxford, OX2 lJE,

England: Vector Fields lim ited)

Figl.(a) Meander slot antenna

Yee, K. S., 1966, IEEE Trans. on Antennas andPropaeat., vol. 14,302-30 7

Celuch-Marcysiak, M. and Gwarek, W. . 1995,IEEE Trans. on Microwave Theorv and Tech., vol,

Taflove, A., 1995, Computational Electromagnetics:The F inite-differe nce time-domain method, Artech

House, BostonSonnet user’s manual (Liverpool, NY 13088, USA:Sonnet Software Inc)

-3, pp.208 1-2089

H12-I

H2

1

Figl.(b) Meander line antenna

Fig2.(a) Electric fieldsof meander slot antenna Fig2.(b) Magnetic fields of meander line antenna

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0

h -5

3. -10v1

-15

E -20

38 -25

-30

-3s I 1 !

1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00

Frequency (GHz)

Fig3 Return loss against meander sections

h

%3v1

Ead

0

-5

-10

-15

-20

-25

2 5 2.6 2.7 2.8 2.9 3.0

Frequency (GHz)

Fig.S(a) Return loss of meander slot antenna

Fig.6(a) Pattern of meander slot antenna

Fig.4 Antenna model in 3-D FDTD simulator

0

-10

-20

1-30

2.1 2.2 2.3 2.4 2 5 2.6 2.7 2.8

Frequency (GHz)

Fig.S(b) Return loss of meander line antenna

Fig.6(b) Pattern of meander line antenna