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A Note on the Impedance Variation with

Feed Position of a Rectangular

Microstrip Patch Antenna

T. Samaras

A. Koulog lou

and

J . N

Sahalos

Radio comm unication s Laboratory, Departme nt of Phys ics. Aristotle University

of

Thessaloniki

GR-54124 Thessa lonik i , Gre ece

Tel: t30 231 0 998161; Fax: t3023 10 9980 69; E-mail : saha los@auth .gr

Abstract

The variation with fee d position of th e input impedance of a rectangular patch

antenna

is investigated theoretically. Two differ-

ent f e e d types are examined: an

inset

microstrip

line,

a n d a coaxial probe. Th e Finite-Difference Time-Domain (FDTD) tech-

nique

is

u s e d for

the

calculations.

Numerical

results are com pared with published

measurements

a nd o the r theoretical mod-

e l s .

Keywords: Impe dance ; microstrip antennas; antenna f e e ds ; coaxial

coupler

1. In t roduc t ion

icrostnp-patch antennas have been widely used in applica-

tions where lo w-profile antennas are needed. The input

impedance of a microship antenna depends

on

its geometrical

shape and dim ensions, the physical pro perties of the materials

involved, and the feed type. The transmiss ion-line model [ l ] pre-

dicts that the input resistance of a pro he-fed rectangular patch

antenna is proportional to the cosine-square d

(

cos2) of the normal-

ized feed-point distance from the patch edge. This depen dence can

also be seen in [Z], where measured values of resistance were

compare d with calculations based on modal analysis for different

positions of the probe feed. Using the Method of Moments (M UM ),

similar res ults were obtaine d in

[3]

for three width laspect ratios of

the rectan gular patch. However, recent work [4] has shown that

when the patch is fed with an inset microstrip line, the resistance

90

n/I.

dependen ce becomes proportional to the fourth power of the cosine

cos4 ), although no theoretical justification was given there for

this result. This short note presents numerical calculations that

confirm the different behaviors of the

two

feed types, as shown in

[41.

2. Method

The results were obtained with

SEMCAD

(Schmid and

Partner Engineering

AG,

Zeughausstrasse

43 8004

Zurich, Swit-

zerland), which implements the Finite-D ifference Time-Domain

(FDTD) technique

[ 5 ]

The antenn a modeled was the one measured

in

[4] ;

the dimensions are shown in Figure 1. The distance s was

chosen to he equal to the width of th e microstrip feed 3.8 mm). In

both configuration s, the impedance was calculated as function of

IEEEAnfennas and Propagation Magazine

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14cm

W=5.94cm

Figure l a. The dimensions of the antennas examined: micro-

stri p inset feed.

w = 5 . ~ c m I

~

. . . . . . . . . . . . . . . . . . . . . .

I

. . .

1

. .

. . . . .

..

....

. . . . . .

. .

Figure lh. The dimensioiis of the antennas examined: probe

feed.

y o , where yo varied

fiom

zero (at the edge

of

the patch) to

L 2

= 20.2 mm

(at the center of the patch). The substrate was

1.27

mm thick, and had dielectric properties of ta n6

=

0.0019 and

r = 2.42 .

The antenna was d esigned to work at 2.3 GHz.

A non-uniform grid was used in the model. The maximum

cell size (near the boundaries of the computational domain) was

2 mm Af6 .5) . The Gaussian pulse that excited the antenna was

cehtered at 2.3 CHz, hut had harmonics up to 4

GHz.

Therefore,

iEEEAnfenn as and Propagation Magazine

Vol.

46 No. 2 April 2004

the maximum ccil size corresponded to about

1/26

of thc minimum

wavelength. The computational domain was truncated with a six-

cells-thick perfectly matched layer

(PML).

3. Results

and

Discuss ion

The variation of the input impedance with the feed position

is

shown in Figure 2 for the two feed types. The values of the red s-

tancc for the microstrip inset feed followed the

cos4

variation, and

those for the probe fecd followed the cos2 variation

[I],

normal-

ized at 2y0/L

= 0.25.

The results for the resistance compared well

with the measurements of

[4]

specially for feed positions close to

the center of the patch. The agreement of the reactance values was

also good.

One would expect that symmetrical excitation of the patch

( y o = L I Z ) should lead to zero resistance, since

11

mode can he

excited. However, experimental [4] and numerical rcsults (this

letter) showed that the input still resistance takes

some

value in

this casc, since the symmetty of the patch is destroydd by the inset.

Thus, a mode

is

produced that makes the patch radiate. The trans-

. .

........

-

.

0 0 2 0.4

0.6

0.8 1

normalised d istance, 2y /L

Figur e 2a. The variation of th e inpu t impedance with feed posi-

tlon for the antenn a with a microstrip inset feed:

0 FDTD,

X C2) ; A

Measured

141 R C2) ; A

Measured

141

a);

Transmission-line model, [ 6 ]

R

C l ) .

- c o s ~ ( A Y ~ / L ) ; - - -

C O S * ~ ~ ~ / L ) ;FDTD,

R R);

Figure 2b. The variation of the inp ut impedaiicc with feed posi-

tion for th e antenn a with a pro be feed (legend as in Figure 2a).

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Table 1 Results for the microstrip-fed antenna.

0.50

0.65

Ta b le 2. Results fo r the probe-fed antenna.

mission-line model [6] fails to predict this phenomenon (Table

I .

Mor eover, the values it gives follow the cos2 law (F igure 2).

In

the c, ae of the probe feed at the center of the patch, the sym metry

was respected and the input impedan ce was zero (Table 2).

The results confirmed that the variation of the input imped-

in ce with the feed position is different, depending on the feed type.

4.

References

1. C.

A.

Balanis, Advanced Engineering Elecfromagnefics,New

York, John Wiley Sons, 1989.

2. W.

F:Richards, Y. T.

Lo,

and D. D. Harrison, “An Improved

Theory for Microship Antennas and Applications,”

IEEE Trans-

actions on Antennas and Propagation, AP-29, 1 1981, pp. 38-46.

3. J. R. M osig, R. C. Hall, and

F.

E. Gardiol, “Numerical Analysis

of M icrostrip Patch Anten nas,” in J. R. James and P. S Hall (eds.),

Handbook of Micro sfrip Antennas,

London, P eter Peregrinus Ltd.,

1989.

4. L.

I.

Basilio, M. A. K hayat, J. T. W illiams and

S.

A. Long, “The

Dependence of the Input Impedance on Feed Position of Probe and

Microstrip Line-Fed

Patch Antennas,” IEEE Transacfions on

Antennas and P ropagafion,AP-49, 1,200 1, pp, 45-47.

6. A. Taflove, CompufafionalElecfrodynamics: The Finife-Differ-

ence Time-Domain Method,

Norwood, MA, Artech House, 1995.

7. R. A. Sainati,

CAD OfMicrostrip Antennasfor Wireless Appli-

cations, Norwood, MA, Artech House Inc., 1996.

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for

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Notebook. Please send your suggestions to Tom Milligan and

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46 o. 2, April 2004