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IEE 1995

Electronics Letters Online No:

199SOS79

P. Lida, F. Temcamani, B. Delacressonniere, P. Pouvil and

J.L. autier (Equipe Micro Ondes, ENSEA, 95014 Cergy Pontoise,

France)

13 March 1995

References

1 GAUTIER, I.L.:

Etude des amplificateurs differentiels dans

la

gamme

microonde. PhD Thesis, Universite Paris

XI

Orsay, 1977

2 LIDA P.: Etude theorique et expkimentale de topologies de filtres

actifs microondes integrks

sur

Ardniure de Gallium . Universite

Paris

XI

Orsay, 1994

3

NICLAS,K.B.:

Active matching with common gate MESFETs,

IEEE Trans., 1985,MTT-33 6),pp. 492491

4 RUMELHARD, c., and CARNEZ, B.: A new biasing circuit for high

integration density GaAs MMICs. 15th EMC, 1985

Bandpass characteristics

of

new dual mode

microstrip square

loop

resonators

J.S. Hong and

M.J.

Lancaster

Indexing term s: Microstrip resonators, Microwave filte rs

A new type of dual-mode microstrip square loop resonator is

proposed for the design

of

compact microwave bandpass filters.

The associated characteristics of mode splitting are described.

A

novel bandpass filter, consisting of

such

a dual-mode resonator

having a

1.2

bandwidth at

1.52GHz

was designed and

fabricated. The measured fdter performance is presented.

Introduction; Miniaturised high performance narrowband micro-

wave bandpass fdters are highly desirable for the next generation

of satellite and mobile commmunications systems. To meet this

demand, it seems that the dual-mode microstrip filter is one of the

prospective candidates [l, 21. The dual-mode micros trip fdter is

composed of one or more dual-mode microstrip resonators which

are usually in the form of a ring, a disk or a square patch [l].

These microstrip resonators can

be

treated as waveguide cavities

with magnetic walls on the sides. The fields within the cavities can

be expanded by the TMzd modes, where

z

is perpendicular to the

ground plane. Thus, the resonators may also be referred to as 2-D

resonators since a resonance c n occur in either of two orthogonal

co-ordinates. The circuit surface area needed to accommodate

ring, disk and square patch resonators may be estimated by S,,

=

(X t)*, S

=

(1.84k&~)~ nd S,,

=

(kg,J2)2, respectively, where

h

s the guided wavelength at resonant frequencyf, in the associ-

ated resonators [3, 41. I t is felt that these sizes are still too large for

the design of high performance multipole fdters, especially at

lower microwave frequency bands, such as L, S or C bands, which

are used by many satellite and mobile communications systems.

Therefore, it is desirable to develop new types of dual-mode

microstrip resonators not only

for

offering alternative designs but

also for miniaturising filters.

In this Letter, we present for the first time the investigation of a

new dual-mode microstrip square loop resonator for the design of

compact microwave bandpass fdters. The characteristics of mode

splitting are described.

A

novel dual-mode bandpass ffiter com-

posed of a proposed dual-mode microstrip square loop resonator

was designed and fabricated. The

measured

filter performance is

also presented.

Dual-mode microstrip square loop resonator: The proposed dual-

mode microstrip square loop resonator is shown in Fig.1. A

square loop consisting of four identical arms forms a basic resona-

tor. A small triangle patch is attached to

an

inner comer of the

loop for exciting and coupling a pair of degenerate modes (having

the same propagation constant) to form a dual-mode resonator.

When d = 0 no perturbation is added and only a single mode is

excited. The electric field pattern of the excited resonant mode was

computed using a fullwave EM simulator [5] and is plotted

in

Fig.

2u,where the two poles located in the middle of the left and right

ELECTRONICS LETTERS 25th

M a y

1995 Vol

31

....

._

dielectric substrate

@

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

....

conductor

port2

Fig. 1 Dual-mode microstrip square loop resonator

arms and the two zeros located in the middle of the top and bot-

tom arms can be distinguished. This mode corresponds to the

TWIwmode in a square patch resonator. If the excitation port is

changed to po rt

2

the field pattern is rotated by 90 for the asso-

ciated degenerate mode, which corresponds to the TMzo l o ode in

a square patch resonator. When d 0

no

matter what the excita-

tion port is, both the degenerate modes are excited and coupled to

each other. The field pattern in this case is shown in Fig.

26

where the two poles and the two zeros have each moved to one of

the four corners. The degree of coupling modes depends on the

size of

d,

which in return controls the mode splitting.

c

Fig.

2

Electric field pattern

a

Singlemode(d = 0)

b Dual-mode (d t 0

1.50 1.55 1.60 1.65

1.70 1.75

frequency ,GHz

Fig.

3 Characteristics

of

made splitting (square loop: a

=

20mm and

b

= 16mm)

-d=2mm

d =

3mm

d

= 4mm

~~~~

Fig. 3 shows the frequency characteristics of mode splitting for

different values of d . As d is increasing from 2 to 4mm, resonance

frequency splitting increases almost linearly from 25 to 115MHz

It should be mentioned that, without the triangle patch (d = 0 ,

neither splitting of the resonance frequency

nor

bandpass response

is observed.

No

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