# Design of Sectoral Antennas Using a Metallic EBG Structure and Multiple Sources Feeding for Base Station Applications

Authors: M. Hajj; E. Rodes; D. Serhal; T. Monédière; B. Jecko
Journal: International Journal of Antennas and Propagation
 (2008)

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## Abstract

This work aims to study and design base station antennas with metallic electromagnetic band gap (EBG) materials able to create a sectoral radiation pattern presenting at least a60∘ beamwidth. The use of metallic structures offers a new approach to industrial partners seeking to reduce costs and facilitate design procedures. A new method allowing the improvement of both the directivity and the bandwidth by using a printed antenna array is studied.


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## Body

## 1. Introduction

In recent years, a new kind of compact directive
antenna has been developed using EBG materials. In these periodic structures,
the wave propagation is prohibited for precise frequency bands and defined
incidence angles [1, 2]. The insertion of a defect within the EBG crystal
periodicity can create an allowed band inside the forbidden band. It has been
shown that by exciting such a structure inside the allowed band, it is possible
to design directive antennas able to produce gains as high as 30 dB, thus
providing a quasiplanar alternative to the aperture antennas [2–4].More recently, some structures have been
proposed, where the EBG superstrate has been replaced by a metallic partially reflecting surface (PRS).
These materials have been used to design different types of antennas with
directive [3–6] or
omnidirectional [7, 8] radiation patterns. An example of a directive antenna
is given in Figure 1, where the PRS is constituted by a layer of square metallic rods.Figure 1
Example of directive EBG antenna.Our objective was to obtain a metallic EBG antenna able to create a
sectoral radiation pattern presenting a half power beamwidth of at least60° in
one plane. There are several types of antenna presenting such a pattern, (e.g.,
antenna arrays) but they require complicated feeding mechanisms. The structure of the sectoral EBG antenna is
presented in Section 1 along with the functioning principle. A first
design example with experimental results is given in Section 2. An
antenna in horizontal polarization designed for uplink UMTS application [1.92–1.98] GHz with a
multisource feeding is presented in Section 3. Finally, another antenna
working on the first UMTS band in vertical polarization is introduced and its
performances detailed.


## 2. Principle of Sectoral EBG Antenna

The
radiation pattern of an EBG antenna being created by a radiating aperture, one
only needs to modify the latter properties and shape
to obtain the desired form. A sectoral radiation pattern can be obtained by
using a rectangular radiating aperture. This can be achieved with an EBG
resonator antenna, provided that the energy propagation is impaired in one
direction of the azimuth plane. The use of vertical PEC (Figure2) to limit the
energy propagation along the Ox axis results in the desired effect, thus
creating the sought after radiation pattern.Figure 2
Sectoral EBG Antenna Structure.The functioning
of such a structure is quite similar to a classic rectangular waveguide working
on the TE01 mode, except for the energy transmitted through the EBG material.
The working frequency of the antenna can thus be obtained through the following
relation:(1)f=c2⋅h⋅(12+∠EBG2.π). Due to the
waveguide-like comportment of the antenna, the E field on the radiating
aperture shows no variation along the transverse axis (Figure 3). Along the
antenna length, an exponential attenuation can be observed as the antenna works
under the guide cutoff frequency.Figure 3
Field distribution inside the antenna.The radiation
angle can, therefore, be approximated through the following relation, according
to the general theory on rectangular radiating aperture:(2)θ=50.8*λl. The design of
the PRS will then determine the antenna maximum directivity and operating
bandwidth, as in the case of more classic directive EBG structures.


## 3. First Antenna Design and Experimental Validation

To illustrate
both the principles and the performances of the antennas presented in this
paper, a first antenna with 15 dBi directivity and a60° beamwidth has been
designed. The working frequency corresponds to the first two U-NII frequency
bands, that is, 5.15–5.35 GHz.According to the
antenna description, its width will directly determine the radiation angle
which has to meet the60° requirement. The application of the formula given in
the previous paragraph yields a value of 47.5 mm, calculated at the upper limit
of the frequency band intended for the antenna.With the antenna
width calculated above, 15 dBi directivity requires a 185 mm length for a
rectangular aperture with a uniform field distribution. By considering a 0.8
radiation efficiency factor for the aperture, which is the value usually
associated to the EBG resonator structures, a length of 230 mm is obtained. The
PRS will then be designed to ensure such a length for the radiating aperture.In order to
facilitate the realization of a prototype, an EBG structure composed by a 2 mm
thick FR4 substrate, with metallic lines 2.5 mm wide printed on it, has been
chosen. The reflection coefficient of this structure is0.91*exp (j*158°) at
5.3 GHz, which is compatible with the desired directivity. To obtain an
operating frequency consistent with the intended application, the height of the
antenna can be determined using the relation (1) which yields a value of
26.6 mm. The final structure, designed with the values previously calculated, is
presented in the Figure 4. The patch antenna used for the excitation of the
resonant cavity can be seen in the middle of the picture.Figure 4
Prototype of the sectoral antenna.The simulated
performances yielded by the antenna in term of directivity and radiation patterns are displayed in
Figures5, 6, and 7. A maximum directivity of 15 dB has been obtained, which is
consistent with the intended values. The radiation angle in the horizontal
plane is also correct, being higher than 60° on the whole frequency band.Figure 5
Directivity of the antenna (simulated).Figure 6
Radiation patterns at 5.3 GHz (E-plane).Figure 7
Radiation patterns at 5.3 GHz (H-plane).Once the
prototype realization completed, it has been measured in the anechoic chamber
of the XLIM Laboratory,
Limoges, France. The radiations patterns are presented in Figures6 and
7 and compared with the simulation results.As shown above,
the experimental results present a good agreement with the simulated ones,
particularly when considering the radiation patterns displayed in Figures6 and
7. The maximum
measured directivity is around 14.5 dB, which corresponds to a 0.5 dB
difference between simulation and experimentation. This discrepancy is caused
by the dielectric losses which have been underestimated during the simulation.The principle of
the sectoral antenna having been validated by these experimental results, more
complicated structure has been studied in order to improve the performances.


## 4. Structure Improvement for UMTS Application

### 4.1. Sectoral Antenna for a UMYS Base Station in Horizontal Polarization

#### 4.1.1. Presentation and Description

The new objective is to design an antenna still intended for wireless
telecommunication networks base stations but with improved performances. Our
sectoral metallic EBG monoband antenna must satisfy the uplink UMTS
specifications (1.92–1.98 GHz) [9] with an 18 dB gain and a radiation
beamwidth of at least 60° in the horizontal plane. This antenna uses only a unidimensional
metallic structures working in TE polarization, feed ports and metallic rods
must be in a horizontal distribution (Figure 8).Figure 8
Front, top, and 3D views of the sectoral EBG antenna, monosource case: only the patch in the center is fed, multisource case: three patches are fed simultaneously.The metallic EBG
structure is designed with 21 rods 10 mm wide in order to regulate the allowed
frequency band around 2 GHz. The rods period is 60.5 mm and their thickness is
10 mm.The simulated ground plane has the same size as the structure, that is,800*164mm2.
The antenna cavity height will be determined by the relation (1), where the
phases ∠EBG correspond to the reflection coefficient of EBG
structure.The EBG
structure is thus situated at 69 mm over the ground plane, which is aboutλ0/2.
Its operating frequency is around 1.96 GHz. The vertical walls used in Section 3
are replaced by metallic rods in order to reduce the antenna weight and to
facilitate the design.At the
beginning, the antenna directivity with only one feeding source was about 15 dBi
with a narrow operating bandwidth. There are several parameters which influence
the antenna performances, the filling ratio being one of the most important ones.


#### 4.1.2. Filling Ratio

The EBG structure is formed by metallic rods that can be
dimensioned by considering several parameters like the rods section (w), the rods period (p), in other terms by defining the
filling ratio τ, which directly
influences the antenna performances (directivity and bandwidth). w and p values are now fixed according to the value of the filling ratio,
the latter being defined by (3)(3)τ=wp.In our case, the
PRS structure is composed of 21 metallic rods. Several values ofτ are considered by modifying w and p. The following table (Table 1) shows the w and p variations related
by τ (Figure 9).Table 1
Different values ofw and p.
w (mm)p (mm)τ=0.082560.5τ=0.1651060.5τ=0.2471560.5Figure 9
w and p values following τ.Figure10 presents the directivity variations for different
values of the filling ratio.Figure 10
Directivity evolution for the three cases.The curves of figure10 show that increasing the
filling ratio results in an increased directivity increases and a reduced −3 dB
directivity bandwidth. We can see that this technique is, moreover,
insufficient to reach the desired 18 dBi and to obtain the suitable −3 dB
directivity bandwidth. The multisource technique will, therefore, be used to
achieve good results in
terms of both directivity and bandwidth.


#### 4.1.3. Performances Enhancement: Principle of Multisource Technique

The principle of the multiple sources feeding is based on the arrays
theory, which can produce an increased directivity by the summation of various
elements contributions [10, 11]. Furthermore, the use of multisource technique
enables us to increase the directivity and the −3 dB
directivity bandwidth simultaneously.To observe the phenomena, the electric field distribution in a horizontal
plane situated in the resonant cavity at the center operating frequency (1.96 GHz) is given for
each case (Figures11 and 12).Figure 11
Distribution of the tangential electric field module|Ex|.Figure 12
Distribution in the multisource case.The multisource structure presents three sources interspaced by1.5λ0 in order to obtain a good apertures interlacement, that is, to obtain quasiconstant
field amplitude along the antenna length. Moreover, this dimension of the
antenna is extended in order to avoid spillover losses and high side lobes.


#### 4.1.4. Directivity

The simulation
results of the metallic EBG antenna directivity versus frequency are shown in
Figure13, for the monosource, multisource, and patch only configurations.Figure 13
Directivity evolution for the two cases.The use of the
multisource technique enabled us to increase at the same time the gain and the−3 dB directivity bandwidth. It is thus noted that
this technique can give very interesting results. Between the two extreme
cases, the directivity is increased by 3.2 dBi (from 15 to 18.2 dBi) and the −3 dB directivity bandwidth raises from 8.4% to 13%.


#### 4.1.5. Simulated Results of the Final Structure

Figure14
represents the radiation pattern in the vertical (H) and horizontal (E) planes
of the antenna.Figure 14
Radiation patterns at 1.96 GHz.The antenna is
directive in the H-plane and sectoral in the E-plane (63°).  The maximum directivity reaches 18.2 dBi. The
patterns have low side lobes, so we can estimate that the structure is well
dimensioned.The return loss
shown in Figure15 is satisfying, as it remains lower than −10 dB on the whole
band.Figure 15
Return loss.


### 4.2. Antenna in Vertical Polarization

This new antenna
uses only a one-dimensional structure in TM polarization; the feed ports and
the metallic rods must be in a vertical distribution (Figure16).Figure 16
Front, top, and 3D views of the sectoral EBG antenna.The metallic EBG
structure in TM polarization is made of 3 square rods 10 mm wide, the
periodicity beingp=60.5 mm. The four metallic rods, 10 mm wide, on the side of
the antenna are used in place of a PEC. The simulated ground plane has the same
size as the structure, that is, 1200*164mm2. The cavity height will
be determined by the following relation (4), where the phases correspond to the
reflection coefficient of the walls defining the cavity: (4)h=12⋅c⋅(1/2+∠EBG/360)f2−(c2/4⋅l2)⋅(∠Walls/180)2, with l the antenna width.The EBG
structure is thus situated at 80 mm over the ground plane, which is aboutλ0/2.
Its operating frequency is around 1.95 GHz.The influence of the different parameters such as the multisource
technique and the filling ratio is the same as for the last antenna. This
antenna offers a significant directivity reaching more than 18 dBi and a very
good sectoral cover in the azimuth plane.Figure17 represents the radiation patterns at 1.97 GHz; we obtain a
radiation beamwidth in the H plane of 65° and a very directive pattern in the E
plane. Maximum directivity reaches 18.3 dBi. The side lobes are correct, being
lower than −16 dB compared to the main lobe in the E plane.Figure 17
Radiation patterns at 1.97 GHz.The return loss showed in Figure18 is very correct.Figure 18
Return loss.


## 4.1. Sectoral Antenna for a UMYS Base Station in Horizontal Polarization

### 4.1.1. Presentation and Description

The new objective is to design an antenna still intended for wireless
telecommunication networks base stations but with improved performances. Our
sectoral metallic EBG monoband antenna must satisfy the uplink UMTS
specifications (1.92–1.98 GHz) [9] with an 18 dB gain and a radiation
beamwidth of at least 60° in the horizontal plane. This antenna uses only a unidimensional
metallic structures working in TE polarization, feed ports and metallic rods
must be in a horizontal distribution (Figure 8).Figure 8
Front, top, and 3D views of the sectoral EBG antenna, monosource case: only the patch in the center is fed, multisource case: three patches are fed simultaneously.The metallic EBG
structure is designed with 21 rods 10 mm wide in order to regulate the allowed
frequency band around 2 GHz. The rods period is 60.5 mm and their thickness is
10 mm.The simulated ground plane has the same size as the structure, that is,800*164mm2.
The antenna cavity height will be determined by the relation (1), where the
phases ∠EBG correspond to the reflection coefficient of EBG
structure.The EBG
structure is thus situated at 69 mm over the ground plane, which is aboutλ0/2.
Its operating frequency is around 1.96 GHz. The vertical walls used in Section 3
are replaced by metallic rods in order to reduce the antenna weight and to
facilitate the design.At the
beginning, the antenna directivity with only one feeding source was about 15 dBi
with a narrow operating bandwidth. There are several parameters which influence
the antenna performances, the filling ratio being one of the most important ones.


### 4.1.2. Filling Ratio

The EBG structure is formed by metallic rods that can be
dimensioned by considering several parameters like the rods section (w), the rods period (p), in other terms by defining the
filling ratio τ, which directly
influences the antenna performances (directivity and bandwidth). w and p values are now fixed according to the value of the filling ratio,
the latter being defined by (3)(3)τ=wp.In our case, the
PRS structure is composed of 21 metallic rods. Several values ofτ are considered by modifying w and p. The following table (Table 1) shows the w and p variations related
by τ (Figure 9).Table 1
Different values ofw and p.
w (mm)p (mm)τ=0.082560.5τ=0.1651060.5τ=0.2471560.5Figure 9
w and p values following τ.Figure10 presents the directivity variations for different
values of the filling ratio.Figure 10
Directivity evolution for the three cases.The curves of figure10 show that increasing the
filling ratio results in an increased directivity increases and a reduced −3 dB
directivity bandwidth. We can see that this technique is, moreover,
insufficient to reach the desired 18 dBi and to obtain the suitable −3 dB
directivity bandwidth. The multisource technique will, therefore, be used to
achieve good results in
terms of both directivity and bandwidth.


### 4.1.3. Performances Enhancement: Principle of Multisource Technique

The principle of the multiple sources feeding is based on the arrays
theory, which can produce an increased directivity by the summation of various
elements contributions [10, 11]. Furthermore, the use of multisource technique
enables us to increase the directivity and the −3 dB
directivity bandwidth simultaneously.To observe the phenomena, the electric field distribution in a horizontal
plane situated in the resonant cavity at the center operating frequency (1.96 GHz) is given for
each case (Figures11 and 12).Figure 11
Distribution of the tangential electric field module|Ex|.Figure 12
Distribution in the multisource case.The multisource structure presents three sources interspaced by1.5λ0 in order to obtain a good apertures interlacement, that is, to obtain quasiconstant
field amplitude along the antenna length. Moreover, this dimension of the
antenna is extended in order to avoid spillover losses and high side lobes.


### 4.1.4. Directivity

The simulation
results of the metallic EBG antenna directivity versus frequency are shown in
Figure13, for the monosource, multisource, and patch only configurations.Figure 13
Directivity evolution for the two cases.The use of the
multisource technique enabled us to increase at the same time the gain and the−3 dB directivity bandwidth. It is thus noted that
this technique can give very interesting results. Between the two extreme
cases, the directivity is increased by 3.2 dBi (from 15 to 18.2 dBi) and the −3 dB directivity bandwidth raises from 8.4% to 13%.


### 4.1.5. Simulated Results of the Final Structure

Figure14
represents the radiation pattern in the vertical (H) and horizontal (E) planes
of the antenna.Figure 14
Radiation patterns at 1.96 GHz.The antenna is
directive in the H-plane and sectoral in the E-plane (63°).  The maximum directivity reaches 18.2 dBi. The
patterns have low side lobes, so we can estimate that the structure is well
dimensioned.The return loss
shown in Figure15 is satisfying, as it remains lower than −10 dB on the whole
band.Figure 15
Return loss.


## 4.1.1. Presentation and Description

The new objective is to design an antenna still intended for wireless
telecommunication networks base stations but with improved performances. Our
sectoral metallic EBG monoband antenna must satisfy the uplink UMTS
specifications (1.92–1.98 GHz) [9] with an 18 dB gain and a radiation
beamwidth of at least 60° in the horizontal plane. This antenna uses only a unidimensional
metallic structures working in TE polarization, feed ports and metallic rods
must be in a horizontal distribution (Figure 8).Figure 8
Front, top, and 3D views of the sectoral EBG antenna, monosource case: only the patch in the center is fed, multisource case: three patches are fed simultaneously.The metallic EBG
structure is designed with 21 rods 10 mm wide in order to regulate the allowed
frequency band around 2 GHz. The rods period is 60.5 mm and their thickness is
10 mm.The simulated ground plane has the same size as the structure, that is,800*164mm2.
The antenna cavity height will be determined by the relation (1), where the
phases ∠EBG correspond to the reflection coefficient of EBG
structure.The EBG
structure is thus situated at 69 mm over the ground plane, which is aboutλ0/2.
Its operating frequency is around 1.96 GHz. The vertical walls used in Section 3
are replaced by metallic rods in order to reduce the antenna weight and to
facilitate the design.At the
beginning, the antenna directivity with only one feeding source was about 15 dBi
with a narrow operating bandwidth. There are several parameters which influence
the antenna performances, the filling ratio being one of the most important ones.


## 4.1.2. Filling Ratio

The EBG structure is formed by metallic rods that can be
dimensioned by considering several parameters like the rods section (w), the rods period (p), in other terms by defining the
filling ratio τ, which directly
influences the antenna performances (directivity and bandwidth). w and p values are now fixed according to the value of the filling ratio,
the latter being defined by (3)(3)τ=wp.In our case, the
PRS structure is composed of 21 metallic rods. Several values ofτ are considered by modifying w and p. The following table (Table 1) shows the w and p variations related
by τ (Figure 9).Table 1
Different values ofw and p.
w (mm)p (mm)τ=0.082560.5τ=0.1651060.5τ=0.2471560.5Figure 9
w and p values following τ.Figure10 presents the directivity variations for different
values of the filling ratio.Figure 10
Directivity evolution for the three cases.The curves of figure10 show that increasing the
filling ratio results in an increased directivity increases and a reduced −3 dB
directivity bandwidth. We can see that this technique is, moreover,
insufficient to reach the desired 18 dBi and to obtain the suitable −3 dB
directivity bandwidth. The multisource technique will, therefore, be used to
achieve good results in
terms of both directivity and bandwidth.


## 4.1.3. Performances Enhancement: Principle of Multisource Technique

The principle of the multiple sources feeding is based on the arrays
theory, which can produce an increased directivity by the summation of various
elements contributions [10, 11]. Furthermore, the use of multisource technique
enables us to increase the directivity and the −3 dB
directivity bandwidth simultaneously.To observe the phenomena, the electric field distribution in a horizontal
plane situated in the resonant cavity at the center operating frequency (1.96 GHz) is given for
each case (Figures11 and 12).Figure 11
Distribution of the tangential electric field module|Ex|.Figure 12
Distribution in the multisource case.The multisource structure presents three sources interspaced by1.5λ0 in order to obtain a good apertures interlacement, that is, to obtain quasiconstant
field amplitude along the antenna length. Moreover, this dimension of the
antenna is extended in order to avoid spillover losses and high side lobes.


## 4.1.4. Directivity

The simulation
results of the metallic EBG antenna directivity versus frequency are shown in
Figure13, for the monosource, multisource, and patch only configurations.Figure 13
Directivity evolution for the two cases.The use of the
multisource technique enabled us to increase at the same time the gain and the−3 dB directivity bandwidth. It is thus noted that
this technique can give very interesting results. Between the two extreme
cases, the directivity is increased by 3.2 dBi (from 15 to 18.2 dBi) and the −3 dB directivity bandwidth raises from 8.4% to 13%.


## 4.1.5. Simulated Results of the Final Structure

Figure14
represents the radiation pattern in the vertical (H) and horizontal (E) planes
of the antenna.Figure 14
Radiation patterns at 1.96 GHz.The antenna is
directive in the H-plane and sectoral in the E-plane (63°).  The maximum directivity reaches 18.2 dBi. The
patterns have low side lobes, so we can estimate that the structure is well
dimensioned.The return loss
shown in Figure15 is satisfying, as it remains lower than −10 dB on the whole
band.Figure 15
Return loss.


## 4.2. Antenna in Vertical Polarization

This new antenna
uses only a one-dimensional structure in TM polarization; the feed ports and
the metallic rods must be in a vertical distribution (Figure16).Figure 16
Front, top, and 3D views of the sectoral EBG antenna.The metallic EBG
structure in TM polarization is made of 3 square rods 10 mm wide, the
periodicity beingp=60.5 mm. The four metallic rods, 10 mm wide, on the side of
the antenna are used in place of a PEC. The simulated ground plane has the same
size as the structure, that is, 1200*164mm2. The cavity height will
be determined by the following relation (4), where the phases correspond to the
reflection coefficient of the walls defining the cavity: (4)h=12⋅c⋅(1/2+∠EBG/360)f2−(c2/4⋅l2)⋅(∠Walls/180)2, with l the antenna width.The EBG
structure is thus situated at 80 mm over the ground plane, which is aboutλ0/2.
Its operating frequency is around 1.95 GHz.The influence of the different parameters such as the multisource
technique and the filling ratio is the same as for the last antenna. This
antenna offers a significant directivity reaching more than 18 dBi and a very
good sectoral cover in the azimuth plane.Figure17 represents the radiation patterns at 1.97 GHz; we obtain a
radiation beamwidth in the H plane of 65° and a very directive pattern in the E
plane. Maximum directivity reaches 18.3 dBi. The side lobes are correct, being
lower than −16 dB compared to the main lobe in the E plane.Figure 17
Radiation patterns at 1.97 GHz.The return loss showed in Figure18 is very correct.Figure 18
Return loss.


## 5. Conclusion

We showed in this paper how EBG antennas
are able to radiate with a sectoral pattern, both in vertical and horizontal
polarizations. The half-power beamwidth obtained is greater than60° in the
azimuthal plane and can be increased to more than 90° but with a lower
directivity.Moreover, the use of the multisource technique enabled us
to increase the gain and the bandwidth at the same time, reaching the
interesting performances of 18 dBi directivity with a 13% bandwidth, while
conserving a60° beamwidth. It is also possible, by combining the last two
structures independently designed, to obtain an antenna operating in both polarizations
with a sectoral pattern in azimuth. Such
an antenna can be used to radiate in circular polarization [12].



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*Source: 359053-2009-02-26.xml*