The Decoupling Methods For Increasing The Isolation Between Two

Missouri University of Science and Technology Missouri University of Science and Technology

Scholars' Mine Scholars' Mine

Electrical and Computer Engineering Faculty

Research & Creative Works

Electrical and Computer Engineering

01 May 2019

The Decoupling Methods For Increasing The Isolation Between The Decoupling Methods For Increasing The Isolation Between

Two Antennas Two Antennas

Min Li

Lijun Jiang

Missouri University of Science and Technology

, [email protected]

Follow this and additional works at: https://scholarsmine.mst.edu/ele_comeng_facwork

Part of the Electrical and Computer Engineering Commons

Recommended Citation Recommended Citation

M. Li and L. Jiang, "The Decoupling Methods For Increasing The Isolation Between Two Antennas,"

2019

IEEE MTT-S International Wireless Symposium, IWS 2019 - Proceedings

, article no. 8803978, The Institute

of Engineering and Technology, May 2019.

The de<nitive version is available at https://doi.org/10.1109/IEEE-IWS.2019.8803978

This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been

accepted for inclusion in Electrical and Computer Engineering Faculty Research & Creative Works by an authorized

administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including

reproduction for redistribution requires the permission of the copyright holder. For more information, please

contact [email protected].

The Decoupling Methods for Increasing the Isolation

between Two Antennas

Min Li and Lijun Jiang

Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong

[[email protected], [email protected]]

Abstract—This paper presented the parasitic structure

decoupling method and other related approaches to increase the

isolation between two closely coupled antennas. The design

parameters of the resultant decoupling structures based on the

proposed methods can be precisely derived instead of fitted to

achieve high antenna isolations. A two-element monopole array is

employed as a benchmark to demonstrate the effectiveness of the

proposed methods. The simulation results show that good

impedance matching and high port isolation could be achieved

simultaneously based on the proposed methods.

Keywords—Isolation, decoupling methods, MIMO antenna,

network parameters, impedance matching

NTRODUCTION

For a compact MIMO system, the mutual coupling (MC)

among antennas could degrade the system performance.

Various decoupling techniques (DTs) including the decoupling

network (DN) [1], artificial metamaterial (MM) [2],

neutralization line (NL) [3], defected ground structure (DGS)

[4], parasitic structure (PS) [5], etc., have been developed to

reduce the MC. However, most DNs suffer from narrow

isolation bandwidths. The decoupling designs employing MMs

usually require large antenna separations. Complex

optimization processes were adopted for the NL, DGS and PS

to realize high antenna isolations.

In this paper, we present derivation based methods to increase

the isolation between two antennas. These methods are realized

by adopting different decoupling structures, i.e., the PS, DN, NL

and shorting structure (SS). The design parameters of the

resultant decoupling structure can be directly derived to

achieve high antenna isolations. These decoupling structures

have different formations but similar design procedure:

utilizing the reactive component to manipulate and eliminate

the MC between antennas.

II. M

ETHODOLOGY

In the following discussion, a theoretical network model is

developed first by using the parasitic structure (PS). The high

antenna isolation is achieved by deriving the network

parameters and manipulating its condition for the minimum

coupling. Notably, this network model can also be applied to

the other methods. But certain modifications are required.

A. Isolation Enhancement by the Parasitic Strucure (PS)

The network model of the proposed method for using the

parasitic structure assisted by reactive component is shown in

Fig. 1. To decouple the coupling between Ant 1 and Ant 2, a

parasitic structure (PS) is inserted in between them. Two

additional transmission lines (TLs) with the same characteristic

impedance Z

(normally equal to 50 Ω) and the electrical

lengths

and

are added to the antennas’ feed lines. The PS

is connected with a TL (Z

) and terminated by a reactive

load Z

. Hence, the problem is changed to find appropriate

parameter values of

and Z

to achieve the high

antenna isolation.

For a two-element antenna system with one PS in the middle,

as shown in Fig. 1, the 3 by 3 matrix [S

] at the reference plane

describes the relationship between two antennas and the

effect of the added PS. After adding the additional TLs with the

same characteristic impedances Z

and different electrical

lengths

{

}

, [1,3]

∈∈θ

to their feed lines, a new scattering

matrix [S

] is obtained at the reference plane t

The voltage-current relations of the new 3-port network can

be expressed by its impedance matrix [Z

] as

= ⋅

222

ttt

VZI

(1)

where [Z

] can be obtained by applying an S-to-Z

transformation to [S

]. If the PS is terminated by a reactive

component with an impedance Z

, the voltage-current

relationship across this reactive load can be describe by

313

VZI=− ⋅

. (2)

Substituting (2) into (1) yields a new 2×2 impedance matrix for

the input ports of the two objective antennas only, i.e.,

= ⋅

333

ttt

VZI

(3)

For a perfect isolation, the following decoupling condition shall

be satisfied in (3):

Z =

. (4)

Solving (4) yields Z

(

f= θ)

(5)

where

(

θ)

is a complex function with the input phase-delay

vector θ=[

]. For MIMO antennas, higher radiation

efficiency is always preferred. Hence, the Z

must be purely

imaginary to avoid any unwanted ohmic loss, i.e.,

Re{ } Re{ ( } 0

Ζ f==θ)

. (6)

Hence, the phase-delay vector θ is derived from (6) directly.

Then they are used to compute the purely imaginary Z

using

(5). The reactance Z

can be consequently realized by

appropriate inductor or capacitor.

In addition, implanting the resultant Z

into [Z

] in (3) yields

the input impedances of both decoupled antennas. They are

used to design the matching circuits with little impact to the

resultant high isolation.

Authorized licensed use limited to: Missouri University of Science and Technology. Downloaded on February 23,2024 at 17:31:47 UTC from IEEE Xplore. Restrictions apply.

Fig. 1. The proposed method using parasitic structure.

B. Other Decoupling Methods

To decouple Ants 1 and 2, we could connect a decoupling

network (DN) to the antennas’ feed lines, or connect a

neutralization line (NL) to the antenna radiators, or even use

the shorting structure (SS) to connect the radiators to the

ground. For DN and NL, a TL is needed in between two

antennas with a reactive lumped component installed in the

middle of it. The SS could be realized by using a TL with a

reactive component in serial to the ground. Two additional TLs

and matching networks are further added to the antennas. The

lengths of TLs and reactance of the lumped component can be

derived similarly by imposing the isolation condition to the

network analysis.

III.

ESIGN

XAMPLE

Fig. 2(a) shows a 2.4-GHz array consisting of two symmetric

inverted-L monopole antennas (Ants 1 and 2) with the edge

separation distance d

=18 mm=0.22

, where

is the guided

wavelength at 2.4 GHz. The antennas are fed through two 50-

Ω microstrip lines with the width

=1.8 mm on a 74 mm

mm (

) Rogers substrate, RO 4350. The detailed

dimensions are given in Table I. The simulated S-parameters of

the coupled array are presented in Fig. 2(b). It can be seen that

the antennas have the poor isolation of around 10 dB at the

frequency of 2.4 GHz.

To improve the isolation, the PS decoupling scheme is

presented together with other three methods, as shown in Fig.

3. Based on the methodology proposed in section II, the

required lengths of TLs and reactances of lumped components

are calculated to realize the high antenna isolations. The L-

section matching circuits are adopted at the antenna inputs for

the impedance matching. The derived design parameters are

given in Table I.

Fig. 4 shows the simulated S-parameters of the decoupled

arrays. It can be seen that the decoupled antennas all

demonstrate good matching conditions with S

<-10 dB at

2.4 GHz. Moreover, by using the proposed methods, the

simulated isolations of the decoupled arrays in cases 1, 2, 3 and

4 are substantially improved to around 35 dB, 28 dB, 32 dB and

29 dB, respectively at 2.4 GHz.

(a) (b)

Fig. 2. (a) The configuration and (b) the simulated S-parameters of the two-

element coupled array. ( metal in front and metal in bottom)

ABLE

I P

ARAMETER

ALUES

Common

parameters

Ls Lg W

Wf Ws d

17 10.2 80 52 2.2 1.8 74 18

Method #1 Method #2 Method #3 Method #4

/ w

26.5/2.2 1.5/1.8 2.2/2.3 2.0/1.5

2.23/2.23

/0.38

2.18/2.18

/3.75

2.38/2.38

/1.38

2.68/2.68

/0.77

-j22.1(3 pF) -j663(0.1 pF) j30.2 (2 nH) -j132(0.5pF)

Matching

circuits

Unit for

: rad, and unit for other length variables: mm

(a) (b)

Fig. 3. (a) Decoupling case using PS. (b) Decoupling case using DN. (c)

Decoupling case using NL. (d) Decoupling case using SS. ( metal in front

and metal in bottom)

Authorized licensed use limited to: Missouri University of Science and Technology. Downloaded on February 23,2024 at 17:31:47 UTC from IEEE Xplore. Restrictions apply.

(a) (b)

Fig. 4. Simulated S-parameters of the decoupled cases shown in Fig. 3: (a)

using PS, (b) using DN, (c) using NL, and (d) using SS.

IV.

ONCLUSIONS

This paper presented the parasitic structure decoupling

method and other methods for two closely spaced antennas.

The decoupling design parameters could be rigorously derived.

A two-element monopole array was analyzed and decoupled

for the demonstration purpose. By using the proposed four

methods, the decoupled antennas could have very high port

isolations over 25 dB.

CKNOWLEDGEMENT

This work was supported in part by the Research Grants

Council of Hong Kong GRF 17209918, AOARD FA2386-17-

1-0010, NSFC 61271158, and HKU Seed Fund 201711159228.

EFERENCES

[1] S. C. Chen, Y. S. Wang and S. J. Chung, "A decoupling technique for

increasing the port isolation between two strongly coupled antennas,"

IEEE Trans. Antennas Propag., vol. 56, no. 12, pp. 3650-3658, Dec.

2008.

[2] H.S. Farahani, M. Veysi, M. Kamyab and A. Tadjalli, “Mutual coupling

reduction in patch antenna arrays using a UC-EBG superstrate,” IEEE

Antennas Wireless Propag. Lett., vol. 9, no. , pp. 57-59, 2010.

[3] Chebihi, C. Luxey, A. Diallo, P. Le Thuc and R. Staraj, "A novel isolation

technique for closely spaced PIFAs for UMTS mobile phones," IEEE

Antennas Wireless Propag. Lett., vol.7, pp. 665-668, 2008.

[4] S. Zhang, S. N. Khan, and S. He, ‘‘Reducing mutual coupling for an

extremely closely-packed tunable dual-element PIFA array through a

resonant slot antenna formed in-between,’’ IEEE Trans. Antennas

Propag., vol. 58, no. 8, pp. 2771---2776, Aug. 2010.

[5] M. Li, B. G. Zhong and S. W. Cheung, "Isolation Enhancement for

MIMO Patch Antennas Using Near-Field Resonators as Coupling-Mode

Transducers," IEEE Trans. Antennas Propag. (Early Access)

Authorized licensed use limited to: Missouri University of Science and Technology. Downloaded on February 23,2024 at 17:31:47 UTC from IEEE Xplore. Restrictions apply.