# Ultra Wideband Planar Microstrip Array Antennas for C-Band Aircraft Weather Radar Applications

Authors: Abdellatif Slimani; Saad Dosse Bennani; Ali El Alami; Jaouad Terhzaz
Journal: International Journal of Antennas and Propagation
 (2017)

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## Abstract

A miniaturized ultra wideband (UWB) planar array antennas for C-band aircraft weather RADAR applications is presented. Firstly, the effect of the ground plane is studied. Later, the realization and experimental validation of the geometry that has an UWB characteristic are discussed. This array antennas is composed of a twenty-four radiating element that is etched onto FR-4 substrate with an overall size of162×100×1.58 mm3 and a dielectric constant of εr=4.4. The results show that this miniaturized array antennas gives us a bandwidth which is about 115% and a gain greater than 13 dB which are required in aircraft weather radar applications.


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## Body

## 1. Introduction

Weather system radar, also called weather surveillance radar (WSR), is a type of system that is used to locate precipitation and estimate its type (snow, rain, etc.). Nowadays, various frequency bands are assigned in this system, for example, S, C, X, and K bands. The band covered in this paper is C which included in the UWB range frequency allocated by the FCC (Federal Communications Commission) [1].Aircraft weather measurement radars specifically use antennas which have a high characteristic radiation that treat the atmosphere by transmitting and receiving radio waves, for discovering the weather condition (Figure1) [2–4]. They are very complex electromagnetic systems, which are generally composed of many different components which are as follows: power source, transmitter, antenna, duplexer, receiver, and screen.Figure 1
The aircraft weather radar.The optimum transmission of radio waves by the radar requires antennas that have good radiation performance. In this paper, our target is dedicated to optimize the radiation performance and the beam steering of antenna component [5–7]. For that, microstrip array antennas are selected [8–12].In this paper, we tried to work on UWB (ultra wideband) patch antennas. The term UWB commonly refers to systems that either have a large relative bandwidth [1]; this technology is known by a lot of advantages, especially in term capacity of channels and data transfer rate. The FCC is informed that any antenna has a bandwidth equal or greater than 500 MHz, and this bandwidth is included in the frequency range of 3.1 GHz to 10.6 GHz which is valid for UWB communication systems, which based on narrow pulses to transmit data at extremely low power [2, 13, 14].Firstly, our objective in this manuscript is the conception and realization of an array antennas with a high radiation performance and UWB characteristic, which is a continuation of another work [15]. The design of this array antennas is down using a methodology well detailed in the paper [16] and based on the two electromagnetic simulators HFSS and CST. Finally, we made a comparison between simulation and experimental result in terms of bandwidth and adaptation performances.This paper is divided into four principal parts, such as in section 2, the proposed array antennas geometry and technique design are presented. In section 3, we presented a discussion and comparison between simulation results. Finally, in section 4, a discussion and comparison between simulation results and experimental results were presented.


## 2. Methods and Materials

In this paper, a standard T-junction power divider (Figure2) is used to divide power equally to two principal parts of array antennas (left and right parts (Figure 3)) [17, 18].
(1)Z3=Z1⋅Z42.Figure 2
T-junction power divider.Figure 3
Geometry of the proposed UWB array antennas. (a) Array antennas with total ground plan, (b) array antennas with partial ground plan, and (c) the side view of the array antennas.
(a)
(b)
(c)In the designing of the feed array, we have to consider the reflection levels and electrical lengths of the bends. Removing a part of the area of metallization in the bend’s corner can reduce the reflection level of the bend. The percentage mitre is the cut-away fraction of the diagonal between the inner and outer corners of the unmitred bend (Figure4).Figure 4
Microstrip mitred bend.The optimum percentage mitre is given by [19].
(2)M=100xe%=52+65exp−1.35Wi/h%,where h is the thickness substrate, Wi/h≥0.25, and dielectric constant εr≤25.For our array antennas design (Figure5), we give e for each corner and we calculate the value of x by applying (2). Table 1 shows the results obtained.Figure 5
Power divider with mitred bend.Table 1
Parameter corners of the proposed array antennas.
Corner
e
i(mm)
x
i(mm)
7
0.67
0.4
8
3
1.7
9
1.1
0.6
10
2.9
1.65The characteristic impedances of microstrip lines which are used for feeding array antennas elements are given in Table2.Table 2
Microstrip line impedances.
Impedance
Value (Ω)
Impedance
Value (Ω)
Z
1
44
Z
7
87.1
Z
2
33.9
Z
8
72
Z
3
54
Z
9
80.6
Z
4
=
Z
5
40.4
Z
10
60The geometry of the proposed UWB array antennas is depicted in Figure3. Figure 3(a) represents array antennas with total ground plane, while Figure 3(b) represents array antennas with partial ground plane. The antennas are located on the x-y plane, and the normal direction is parallel to z-axis. There are prints on FR-4 epoxy substrate with a dielectric εr=4.4, thickness h=1.58mm, and a loss tangent δ=0.02. These array antennas are excited by a 50 Ω source power.Table3 shows the geometric parameters of our array antennas that have been calculated by the use of the relation cited in the paper [12].Table 3
Parameters of the proposed array antennas.
Parameter
Value (mm)
Parameter
Value (mm)
W
1
3.5
W
13
2.2
W
2
28.7
W
10.5
W
3
5.2
L
10
W
4
λ
g
4
W
s
u
b
162
W
5
2.5
L
s
u
b
100
W
6
=
W
7
4
W
g
82.6
W
10
1
L
g
21.2
W
12
1.2
d
x
9.5
W
11
1.5
d
1
12


## 3. Simulated Results and Discussion

In this part, we made a comparison between the simulation results found by HFSS and CST, before moving to experimental validation of the geometry that gives us the desired results.


### 3.1. T-Junction Power Divider

Figure6 shows the input impedance of the power divider as a function of the frequency, such that the black curve (continuous) represents its imaginary part, while the red curve (dotted) represents its real part. We can observe that the real part is equal to 50 Ω, and the imaginary part is equal to zero. So the input impedance of our divider is well modeled with the source impedance.Figure 6
Input impedance of T-junction power divider.


### 3.2. Effect of Ground Plane in the Performance of Array Antennas

#### 3.2.1. Return Loss

Figures7 and 8 show the comparison return loss simulation between the patch array antennas with partial and total ground plane. If we observe the evolution of the returns loss, we can see that for the partial ground plane case, we have an adaptation over the entire desired band, with a S11 less than −10 dB (Figure 8), which justifies that this array antennas is an UWB.Figure 7
Return loss comparison between HFSS and CST for the array antennas with total ground plane.Figure 8
Return loss comparison between HFSS and CST for the array antennas with partial ground plane.In other hand, for the total ground plane case (Figure7), we can observe that the return loss is not adapted over the whole band C. So, this array antennas is not an UWB on all C-band. This distortion of the return loss amounts to the mutual coupling result between the radiating elements and ground plane, such as the waves radiated by the radiating elements and guided by the substrate, towards the total ground plane, its return totally towards the radiating elements, which produces a mutual coupling between radiating elements, and makes a disruption and mismatch of the array antennas. For that, in UWB applications, we do not use array antennas with total ground plane.From the result shown in Figure8, we observed that the array antenna with partial ground plane has a bandwidth with UWB characteristic. The S11 is lower than −10 dB from 3.4 GHz to 9 GHz, which is about (115%), and it covers the standard of IEEE 802.15a (3.1–10.6 GHz) fixed by the FCC.For CST simulation, we can observe that this array antennas has three resonant frequencies which arefr1=6GHz,fr2=6.8GHz, and fr3=7.8GHz, and four resonant frequencies for HFSS are fr1=3.8GHz,fr2=5.2GHz,fr3=5.8GHz, and fr4=7.7GHz.


#### 3.2.2. Voltage Standing Wave Ratio (VSWR)

Figures9 and 10 show the simulated VSWR for both patch array antennas. The result simulations indicate that the VSWR of the array antennas with partial ground plane is less than 2, over the bandwidth range of 3.4 GHz to 9 GHz, which includes all C-band. However, the VSWR of the array antennas with total ground plane is greater than 2 in a lot of frequency of the desired band, which do not respect our objective, consisting of the UWB characteristic in all C-band.Figure 9
VSWR comparison between HFSS and CST for the array antennas with total ground plane.Figure 10
VSWR comparison between HFSS and CST for the array antennas with partial ground plane.Since our objective is to have an UWB array antennas which covers all C-band, the simulation results show that the array antennas with partial ground plane is suitable for our application.


### 3.3. Gain Versus Frequency

Figure11 shows the gain of our array antennas with partial ground plane in the UWB frequency range. In most of the frequencies between 3 GHz and 9 GHz, the gain increase is between 13 dB and 23 dB. Therefore, we can notice that our array antennas has a high gain in all of the band C.Figure 11
Gain versus frequency.


### 3.4. Far-Field Radiation Pattern

Figure12 shows the polar radiations of microstrip array antennas with partial ground plane in 2D, between HFSS and CST at the resonance frequency 6 GHz. Figure 12(a) shows the polar gain radiation pattern in E-plane (x–z), and Figure 12(b) shows the polar gain radiation pattern in H-plane (y–z).Figure 12
Comparison of 2D gain radiation pattern of the proposed array antennas. (a) E_plane in CST and HFSS and (b): H_plane in CST and HFSS.
(a)
(b)According to Figure12, we can observe that our array antennas has a bidirectional radiation pattern directed towards the desired directions (End-Fire).In the E-plane, we can see that it has six secondary lobes. It is the same for the H-plane, where there are six side lobes with a main lobe directed to the desired angle. The level of the main lobe gain can reach 23.Finally, the results of the UWB array antennas present the best performance in terms of adaptation, bandwidth, and gain. These performances are summarized in Table4.Table 4
Compared result between HFSS and CST.
Array antennas
Bandwidth%
Gain (dB)
HFSS
CST
HFSS
CST
[9]
72
71
4–6.61
4–5.87
[10]
71
73
7–13.46
7–13.14
[15]
115
115.1
13–20.7
11–21.2
[16]
115
115.1
13–20.7
11
–
21.2
This work
115.1
115.2
13–23
12.5–22


## 3.1. T-Junction Power Divider

Figure6 shows the input impedance of the power divider as a function of the frequency, such that the black curve (continuous) represents its imaginary part, while the red curve (dotted) represents its real part. We can observe that the real part is equal to 50 Ω, and the imaginary part is equal to zero. So the input impedance of our divider is well modeled with the source impedance.Figure 6
Input impedance of T-junction power divider.


## 3.2. Effect of Ground Plane in the Performance of Array Antennas

### 3.2.1. Return Loss

Figures7 and 8 show the comparison return loss simulation between the patch array antennas with partial and total ground plane. If we observe the evolution of the returns loss, we can see that for the partial ground plane case, we have an adaptation over the entire desired band, with a S11 less than −10 dB (Figure 8), which justifies that this array antennas is an UWB.Figure 7
Return loss comparison between HFSS and CST for the array antennas with total ground plane.Figure 8
Return loss comparison between HFSS and CST for the array antennas with partial ground plane.In other hand, for the total ground plane case (Figure7), we can observe that the return loss is not adapted over the whole band C. So, this array antennas is not an UWB on all C-band. This distortion of the return loss amounts to the mutual coupling result between the radiating elements and ground plane, such as the waves radiated by the radiating elements and guided by the substrate, towards the total ground plane, its return totally towards the radiating elements, which produces a mutual coupling between radiating elements, and makes a disruption and mismatch of the array antennas. For that, in UWB applications, we do not use array antennas with total ground plane.From the result shown in Figure8, we observed that the array antenna with partial ground plane has a bandwidth with UWB characteristic. The S11 is lower than −10 dB from 3.4 GHz to 9 GHz, which is about (115%), and it covers the standard of IEEE 802.15a (3.1–10.6 GHz) fixed by the FCC.For CST simulation, we can observe that this array antennas has three resonant frequencies which arefr1=6GHz,fr2=6.8GHz, and fr3=7.8GHz, and four resonant frequencies for HFSS are fr1=3.8GHz,fr2=5.2GHz,fr3=5.8GHz, and fr4=7.7GHz.


### 3.2.2. Voltage Standing Wave Ratio (VSWR)

Figures9 and 10 show the simulated VSWR for both patch array antennas. The result simulations indicate that the VSWR of the array antennas with partial ground plane is less than 2, over the bandwidth range of 3.4 GHz to 9 GHz, which includes all C-band. However, the VSWR of the array antennas with total ground plane is greater than 2 in a lot of frequency of the desired band, which do not respect our objective, consisting of the UWB characteristic in all C-band.Figure 9
VSWR comparison between HFSS and CST for the array antennas with total ground plane.Figure 10
VSWR comparison between HFSS and CST for the array antennas with partial ground plane.Since our objective is to have an UWB array antennas which covers all C-band, the simulation results show that the array antennas with partial ground plane is suitable for our application.


## 3.2.1. Return Loss

Figures7 and 8 show the comparison return loss simulation between the patch array antennas with partial and total ground plane. If we observe the evolution of the returns loss, we can see that for the partial ground plane case, we have an adaptation over the entire desired band, with a S11 less than −10 dB (Figure 8), which justifies that this array antennas is an UWB.Figure 7
Return loss comparison between HFSS and CST for the array antennas with total ground plane.Figure 8
Return loss comparison between HFSS and CST for the array antennas with partial ground plane.In other hand, for the total ground plane case (Figure7), we can observe that the return loss is not adapted over the whole band C. So, this array antennas is not an UWB on all C-band. This distortion of the return loss amounts to the mutual coupling result between the radiating elements and ground plane, such as the waves radiated by the radiating elements and guided by the substrate, towards the total ground plane, its return totally towards the radiating elements, which produces a mutual coupling between radiating elements, and makes a disruption and mismatch of the array antennas. For that, in UWB applications, we do not use array antennas with total ground plane.From the result shown in Figure8, we observed that the array antenna with partial ground plane has a bandwidth with UWB characteristic. The S11 is lower than −10 dB from 3.4 GHz to 9 GHz, which is about (115%), and it covers the standard of IEEE 802.15a (3.1–10.6 GHz) fixed by the FCC.For CST simulation, we can observe that this array antennas has three resonant frequencies which arefr1=6GHz,fr2=6.8GHz, and fr3=7.8GHz, and four resonant frequencies for HFSS are fr1=3.8GHz,fr2=5.2GHz,fr3=5.8GHz, and fr4=7.7GHz.


## 3.2.2. Voltage Standing Wave Ratio (VSWR)

Figures9 and 10 show the simulated VSWR for both patch array antennas. The result simulations indicate that the VSWR of the array antennas with partial ground plane is less than 2, over the bandwidth range of 3.4 GHz to 9 GHz, which includes all C-band. However, the VSWR of the array antennas with total ground plane is greater than 2 in a lot of frequency of the desired band, which do not respect our objective, consisting of the UWB characteristic in all C-band.Figure 9
VSWR comparison between HFSS and CST for the array antennas with total ground plane.Figure 10
VSWR comparison between HFSS and CST for the array antennas with partial ground plane.Since our objective is to have an UWB array antennas which covers all C-band, the simulation results show that the array antennas with partial ground plane is suitable for our application.


## 3.3. Gain Versus Frequency

Figure11 shows the gain of our array antennas with partial ground plane in the UWB frequency range. In most of the frequencies between 3 GHz and 9 GHz, the gain increase is between 13 dB and 23 dB. Therefore, we can notice that our array antennas has a high gain in all of the band C.Figure 11
Gain versus frequency.


## 3.4. Far-Field Radiation Pattern

Figure12 shows the polar radiations of microstrip array antennas with partial ground plane in 2D, between HFSS and CST at the resonance frequency 6 GHz. Figure 12(a) shows the polar gain radiation pattern in E-plane (x–z), and Figure 12(b) shows the polar gain radiation pattern in H-plane (y–z).Figure 12
Comparison of 2D gain radiation pattern of the proposed array antennas. (a) E_plane in CST and HFSS and (b): H_plane in CST and HFSS.
(a)
(b)According to Figure12, we can observe that our array antennas has a bidirectional radiation pattern directed towards the desired directions (End-Fire).In the E-plane, we can see that it has six secondary lobes. It is the same for the H-plane, where there are six side lobes with a main lobe directed to the desired angle. The level of the main lobe gain can reach 23.Finally, the results of the UWB array antennas present the best performance in terms of adaptation, bandwidth, and gain. These performances are summarized in Table4.Table 4
Compared result between HFSS and CST.
Array antennas
Bandwidth%
Gain (dB)
HFSS
CST
HFSS
CST
[9]
72
71
4–6.61
4–5.87
[10]
71
73
7–13.46
7–13.14
[15]
115
115.1
13–20.7
11–21.2
[16]
115
115.1
13–20.7
11
–
21.2
This work
115.1
115.2
13–23
12.5–22


## 4. Fabrication and Experimental Results

According to the simulation results, the array antennas with partial ground plane has good characteristics in terms of bandwidth (including all C-band) and radiation pattern. These results lead us directly to its experimental validation, where the prototype is connected to SMA-female connector. It is tested using VNA-network analyzer in collaboration between our laboratory of renewable energy and intelligent systems and the laboratory of electronic and communication. Its photos are shown in top and bottom view in Figure13 while Figure 14 represents its dimensions.Figure 13
Geometry of realized UWB array antennas. (a) Rectangular radiating elements in the top view of array antennas. (b) Partial ground plane in the bottom view of array antennas.
(a)
(b)Figure 14
Dimensions presentation of the realized UWB array antennas. (a) Length of array antennas. (b) Width of array antennas.
(a)
(b)


### 4.1. Return Loss

Figure15 shows the comparison between the simulated and measured results of the return loss. From the results, we can conclude that this array antennas is satisfactory, because we need an UWB array antennas which contains all C-band in our radar application. So from these results, we can observe that this array meets this requirement, such as S11 ≤ −10 dB from 3.4 GHz to 8.7 GHz with a bandwidth of 5.6 GHz with several resonance frequencies close to 3.7 GHz, 7 GHz, and 8.2 GHz. So the important thing in the impedance bandwidths of experimental result is able to cover the desired frequency band 4–8 GHz.Figure 15
Comparison between simulated and experimental return loss result of our array antennas.


### 4.2. VSWR

Figure16 represents a comparison between simulated and experimental VSWR results. We can note that the three results are clear and that their values are less than 2 on all band of 3.4 GHz to 9 GHz.Figure 16
Comparison between simulated and experimental VSWR result of our array antennas.Finally, the proposed UWB array antennas has a good characteristic in terms of several parameters. Table5 gives us a conclusion and comparison between simulated and experimental results obtained.Table 5
Comparison between measured and simulated results.
Bandwidth (%) at −6 dB
Bandwidth (%) at −10 dB
Resonant frequency
LevelS11 (dB)
Simulation result
HFSS
119
115
3.8 GHz
−
22
5.2 GHz
−
16
5.8 GHz
−
25
7.7 GHz
−
22
CST
119
115
6 GHz
−
26
6.8 GHz
−
16
7.8 GHz
−
21
Experimental result
117
110
3.7 GHz
−
22
7 GHz
−
22
8.2 GHz
−
30


## 4.1. Return Loss

Figure15 shows the comparison between the simulated and measured results of the return loss. From the results, we can conclude that this array antennas is satisfactory, because we need an UWB array antennas which contains all C-band in our radar application. So from these results, we can observe that this array meets this requirement, such as S11 ≤ −10 dB from 3.4 GHz to 8.7 GHz with a bandwidth of 5.6 GHz with several resonance frequencies close to 3.7 GHz, 7 GHz, and 8.2 GHz. So the important thing in the impedance bandwidths of experimental result is able to cover the desired frequency band 4–8 GHz.Figure 15
Comparison between simulated and experimental return loss result of our array antennas.


## 4.2. VSWR

Figure16 represents a comparison between simulated and experimental VSWR results. We can note that the three results are clear and that their values are less than 2 on all band of 3.4 GHz to 9 GHz.Figure 16
Comparison between simulated and experimental VSWR result of our array antennas.Finally, the proposed UWB array antennas has a good characteristic in terms of several parameters. Table5 gives us a conclusion and comparison between simulated and experimental results obtained.Table 5
Comparison between measured and simulated results.
Bandwidth (%) at −6 dB
Bandwidth (%) at −10 dB
Resonant frequency
LevelS11 (dB)
Simulation result
HFSS
119
115
3.8 GHz
−
22
5.2 GHz
−
16
5.8 GHz
−
25
7.7 GHz
−
22
CST
119
115
6 GHz
−
26
6.8 GHz
−
16
7.8 GHz
−
21
Experimental result
117
110
3.7 GHz
−
22
7 GHz
−
22
8.2 GHz
−
30


## 5. Conclusion

In this paper, the ultra wideband planar array antennas for C-band aircraft weather radar applications has been presented, which composed of a twenty-four radiating element.The simulation results show that the array antennas with partial ground plane has a best performance than the array antennas with total ground plane in terms of matching and bandwidth which allowed us frankly to validate it experimentally.The proportionality between the simulation and the measurement results with the high radiation performance over an ultra wide frequency range from about 4 GHz to higher than 8 GHz benefits this array antennas to be good candidates in C-band aircraft weather radar application.Our perspective is to draw up and plot the measurement radiation pattern, because in our country, there is no an anechoic chamber to make these measurements of the field.



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*Source: 2346068-2017-12-13.xml*