Abdallah et al. - 2009 - Quasi-Unidirectional Radiation Pattern of Monopole Coupled Loop Antenna

Quasi-Unidirectional Radiation Pattern of Monopole

Coupled Loop Antenna

Mohamed Abdallah,Franck Colombel,Gilles Le Ray,and Mohamed Himdi

Abstract—A monopole coupled loop antenna (MCLA)is studied and modi?ed.The modi?cation includes reducing the size of the antenna and its radiating properties in order to obtain quasi-uni-directional radiation pattern for mobile communications.An anal-ysis of the surface current distribution has been performed to ex-plain this phenomenon.The theoretical results have been validated with measurements.

Index Terms—Inductance loading,loop monopole antenna,quasi-unidirectional radiation patterns.

I.I NTRODUCTION

W

ITH the development of wireless communication ser-vices,such as mobile applications for locating vehicles

or broadcast reception for handy devices,the need for small an-tennas is ever demanding.It is well known that a self-inductor loaded monopole antenna is a suitable solution for reducing the size of the antenna [1],[2].This technique is also applicable to the monopole coupled loop antenna (MCLA)[3].In addition to this,we have to improve the signal reception.Some mobile ap-plications also require to avoid a null in the main direction of the radiation pattern,which in turn provides a quasi-unidirectional pattern.In [4]and [5],three-dimensional (3D)loop antennas provide isotropic and unidirectional radiation patterns,respec-tively.

In ?rst part of this letter,we will consider the theoretical as-pects of self-inductance and investigate its affect on the reso-nant frequency of a MCLA.As per the physical aspects of the antenna,we carefully studied them in order to explain the evolu-tion of the radiation pattern,in the main direction,versus self-in-ductance value.The ?rst part of this study has been carried out using an in?nite ground plane.

In the second part,we present the best size reduction and quasi-unidirectional pattern that we achieved.For a practical case,we propose a modi?ed MCLA on a limited ground plane (300

mm 300mm).The experimental and theoretical results we obtained are in good agreement.

II.MCLA L OADED BY S ELF -I NDUCTANCE

To achieve our ?rst part (decrease the resonant frequency),we modi?ed the MCLA [3]by loading the short circuit of the half-Manuscript received March 17,2009;revised April 22,2009and May 04,2009.First published June 10,2009;current version published July 21,2009.The authors are with the Institute of Electronics and Telecommunications at Rennes,Universitéde Rennes 1,Campus de Beaulieu,35042Rennes,France (e-mail:mohamed.abdallah@univ-rennes1.fr).

Digital Object Identi?er

10.1109/LAWP.2009.2024966

Fig.1.Geometry of the proposed

antenna.

Fig.2.Simulated return loss for the proposed antenna with different values of self-inductance.

loop on the left side with self-inductance (Fig.1).The ground plane is an in?nite

one.

and are respectively the radii of the half-loop and the arc monopole

lines,

and are their

widths.is the angle between the arc monopole extremities.The antenna

has been printed on a Neltec NY9300circuit board

(

,mm).The main dimensions of the antenna are as fol-

lows:

mm,

mm,

mm,

mm,

mm,

mm,.

The inductor component used is an ATC 0805WL modeled with a parallel RLC device.Simulations have been performed with CST Microwave Studio where the equivalent RLC circuit model has been introduced as lumped elements.

In Fig.2,we present the return loss of the loaded MCLA versus frequency for various self-inductance values.The self-inductance values vary from 0nH (equivalent to short circuit)to an in?nite value (equivalent to an open circuit).

In Fig.2,for each inductance,we can notice two resonance frequencies.The ?rst one,linked to the arc monopole,remains constant and very close to 475MHz,whatever may be the self-inductor values.The second one,linked to the half loop radiator associated to the self inductor,decreases from 455to 209MHz.We can also notice that,for these resonances,the antenna be-comes more and more mismatched when the inductance value

1536-1225/$26.00?2009IEEE

Fig.3.Normalized radiation patterns of the short-circuited MCLA at the res-onant frequency (455

MHz).

Fig.4.Schematic of surface currents distribution on radiator elements of short-circuited MCLA at 455MHz.

increases.As mentioned in [3],the antenna return loss at these resonant frequencies could be matched by modifying the length of the arc monopole.

To complete this calculation,we have computed and repre-sented the normalized radiation patterns of the short-circuited MCLA (Fig.3).

For

the

-plane [Fig.3(a)],we obtain a dissymmetric shape of radiation pattern due to the arc monopole in?uence [3].In

the

-plane [Fig.3(b)],the radiation pattern is quasi-omnidirec-tional.It can be noted that the general behavior of the antenna is very close to that of a monopole,except for the dissymmetry

of the radiation pattern in

the

-plane.The schematic surface currents drawn on the radiator ele-ments in Fig.4will fully back up this analysis.

Near the feed point and the short-circuit areas,the surface currents are maximum.It explains that the maximum radiations

are obtained for

the

in

the -plane

when and for

the

in

the -plane

when or 270(Fig.3).In Fig.4,the surface currents become null in

the -direction,which in turn con?rms the deep null of the radiation pattern [Fig.3(a)].Regarding the short-circuited half-loop dimensions,we

noticed

Fig.5.Normalized radiation patterns.(a),(b):MCLA loaded by a 68-nH inductance at 380MHz.(c),(d):MCLA loaded by a 390-nH inductance at 270MHz.(e),(f):Open-circuited MCLA at 210

MHz.

Fig.6.Schematic of surface currents distribution on radiator elements of the MCLA loaded by a 390-nH inductance at 270MHz.

that the operating resonant mode is a half-wavelength one.It explains why there is a current null in the mid-part of the half-loop.

Normalized radiation patterns of the MCLA loaded by 68-nH inductance,390-nH inductance,and in?nite self-inductance (open-circuited)respectively are shown in Fig.5,which com-pletes the results provided in Fig.2.

As depicted in Fig.5,increasing the value of the inductor will

reduce the deep null in the radiation pattern of

the

-plane,

Fig.7.Geometry of the modi?ed open-circuited

MCLA.

Fig.8.(a)Simulated and measured return losses for the modi?ed open-circuit MCLA.(b)Simulated and measured antenna input impedances.(–––)mea-sured,(——)simulated at 450MHz.

which will make the pattern of the antenna a quasi-unidirec-tional in this plane.This phenomenon can be explained with the help of Fig.6,where surface currents represent an inductor value of 390nH.Also in Fig.6,we can notice the change in the position of the current null by comparing it to the null lo-cation in Fig.4.We notice a maximum current located near the short circuit on the right side of the half-loop and a current null located on the left part of the half-loop as seen in Fig.6.We carefully checked the position of this null and noted that it is located at eight-tenths of the total half-loop length starting from the right side (maximum of current).A short calculation shows that this length is close to a quarter-wavelength at 270MHz,and it con?rms the correlation between the maximum and the null location.The inductance modi?es the equivalent antenna’s elec-trical length,and we can deduce the operating wavelength lower than half a wavelength.In the extreme case where the self-induc-tance is in?nite,equivalent to an open circuit,the open-circuited MCLA operating wavelength is worth up to a quarter-wave-

length.

Fig.9.Normalized radiation patterns.(–––)measured,(——)simulated at 450MHz.

In order to complete our analysis we should realize that there

is no signi?cant modi?cation

of

in

the -plane,which re-mains a quasi-unidirectional pattern independent of polariza-tion.

III.M ODIFIED O PEN -C IRCUIT MCLA,T HEORY ,

AND M EASUREMENTS

In this second part,we keep the case of the modi?ed MCLA with an in?nite self-inductance.We call this antenna a modi?ed open-circuit MCLA.As mentioned in the previous paragraph and in [3],we could modify the arc monopole length to enhance the return losses at the resonant frequencies.

Since our interested operating frequency is at 450MHz,we optimized the arc monopole length in order to achieve impedance matching as well as size reduction.Fig.7shows the modi?ed open-circuited MCLA design.For practical reasons,the antenna has set perpendicularly above a limited square ground plane (300

mm 300

mm 4mm).

The main dimensions of the antenna are listed as

follows:

mm,

mm,

mm,

mm,

mm,

mm,.The reduced ratio of

the modi?ed antenna is 55.5%compared to the initial MCLA [1].

Fig.8(a)and (b)show a good agreement between theoretical and measured return losses and input impedance.

Fig.9shows the measured and simulated normalized radia-tion patterns of the modi?ed open-circuit MCLA.

As expected,the radiation pattern is quasi-unidirectional,and there is a good agreement between theory and measurement.At 448MHz,the measured and the theoretical gain are 3.8and 3.9dBi,respectively,and the comparison between them is also good.

https://www.360docs.net/doc/9211100252.html,parative view of antenna dimensions on ground plane.

Despite the reduced dimensions of the modi?ed open-circuit MCLA,compared to the initial one,the gain is higher than that obtained with the earlier MCLA (2.5dBi mentioned in [3]).This difference in gain is due to the difference in the dimension of the ground plane (300

mm 300mm)(Fig.10)

In Fig.10(a),the modi?ed open-circuit MCLA is small com-pared to ground plane and provides radiation pattern with low

back-side radiation

(

dB).In Fig.10(b),the size of the the MCLA from [3]compared to the ground plane is quite equiv-alent,and the back-side radiation is also equivalent to the ra-

diation in the main direction in

the

-plane.This assumption has been veri?ed another time by calculating the theoretical gain with in?nite ground plane for both cases.In the ?rst case (mod-i?ed open-circuit MCLA)and in the second case (MCLA [3]),the directivities are 4and 6.7dBi,respectively,and are propor-tional to antenna dimensions.

IV .C ONCLUSION

While the inductor element is well known to decrease the an-tenna resonant frequency,based on MCLA,a theoretical study has been performed to demonstrate the modi?cation of the radi-ating behavior of MCLA loaded with an inductor.Thus,a mod-i?ed open-circuit MCLA has been proposed.This antenna pro-vides a quasi-unidirectional radiation pattern and allows for a 55.5%reduction in the size of the antenna compared to the ini-tial MCLA [3].The measured gain is high as 3.8dBi.

R EFERENCES

[1]Z.N.Chen and Y.W.M.Chia,“Broadband monopole antenna with

parasitic planar element,”Microw.Opt.Technol.Lett.,vol.27,no.3,pp.209–210,Sep.2000.

[2]C.W.Harrison,“Monopole with inductive loading,”IEEE Trans.An-tennas Propag.,vol.AP-11,no.4,pp.394–400,Jul.1963.

[3]M.Abdallah,F.Colombel,G.L.Ray,and M.Himdi,“Novel printed

monopole coupled loop antenna,”IEEE Antennas Wireless Propag.Lett.,vol.7,pp.221–224,2008.

[4]S.Krishnan,L.-W.Li,and M.-S.Leong,“A V-shaped structure for

improving the directional properties of the loop antenna,”IEEE Trans.Antennas Propag.,vol.53,no.6,pp.2114–2117,Jun.2005.

[5]A.Mehdipour,H.Aliakbarian,and J.Rashed-Mohassel,“A novel elec-trically small spherical wire antenna with almost isotropic radiation pattern,”IEEE Antennas Wireless Propag.Lett.,vol.7,pp.396–399,2008.