# A Wideband Feed Network for Vivaldi Antenna Arrays

Authors: Natasha Antoinette Hall; Johann Wilhelm Odendaal; Johan Joubert
Journal: International Journal of Antennas and Propagation
 (2022)

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## Abstract

A wideband feed network for a Vivaldi antenna array is presented. A limitation of current wideband antenna arrays is the bandwidth of either the feed network or the antenna element. The proposed antenna array consists of four wideband Vivaldi antennas fed with an improved wideband feed network to extend the useable bandwidth of the array. The proposed feed network consists of coplanar waveguide-to-slotline-to-microstrip line transitions. The feed network has a single coplanar waveguide input and four microstrip line output ports. The feed network achieved uniform amplitude and phase balance and an impedance bandwidth of 160% from 1 GHz to 9 GHz. The feed network was used in a uniform linear antenna array to feed four Vivaldi antenna elements. The Vivaldi antenna array achieved stable radiation patterns from 1.3 GHz to 8 GHz, resulting in a useable bandwidth of 144%. The antenna array has a minimum gain of 8 dBi and a maximum of 13.8 dBi within the frequency band. Results for a prototype Vivaldi antenna array, measured in a compact antenna test range, are presented and compared to simulated results from CST Studio Suite.


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## Body

## 1. Introduction

Vivaldi antennas are typically used in wideband applications, viz., ground penetrating radars, wideband radar imaging, radio astronomy [1], and multiband communication systems (5G). The inherent bandwidth limitation of corporate feed networks limits the use of Vivaldi antenna elements in wideband arrays. Current feed networks for arrays include a self-matching feed network with a simple structure, but generally exhibit a narrow bandwidth. Using this feed network to feed rectangular patch elements, a relative impedance bandwidth of 10.7% was achieved [2]. A number of papers presented combinations of microstrip-to-slotline transitions to realize wideband feed networks [3–5]. These feed networks were combined with different radiation elements to realize wideband arrays, e.g., patch elements to achieve a bandwidth of 5.7% [3], UWB monopoles with a bandwidth of approximately 89% [4], and also Vivaldi antenna elements with a bandwidth of 84% [5]. Other similar wideband antenna arrays include the use of multiple substrate layered feed networks such as substrate-integrated waveguide (SIW) [6, 7] and an electromagnetic band gap (EBG) power divider [8] used to feed different patch elements. These antenna arrays achieved bandwidths between 20% and 50%.Individual Vivaldi antennas are generally capable of achieving bandwidths in excess of 100% [9], while current implementations of wideband arrays only demonstrate bandwidths of 80% to 90%. This paper presents a wideband corporate feed network with more than 160% impedance bandwidth from 1 GHz to 9 GHz. The wideband properties of microstrip-to-slotline transitions are exploited to design a four element corporate feed network. The proposed coplanar waveguide (CPW) fed network consists of a CPW-to-slotline transition and two slotline-to-microstrip line transitions to realize a uniform linear antenna array of four Vivaldi antenna elements. The Vivaldi antenna array achieved stable radiation patterns from 1.3 GHz to 8 GHz, resulting in a useable bandwidth of 144%.The design and performance of the feed network and a single Vivaldi antenna element are presented in Section2. Section 3 presents the measured and simulated results for the final antenna array, with concluding remarks in Section 4.


## 2. Feed Network and Vivaldi Element

### 2.1. Wideband Feed Network

The top and bottom sides of the proposed feed network are shown in Figure1 and consist of a CPW-to-slotline transition and two slotline-to-microstrip line transitions. The feed network was implemented on Rogers RO4003C substrate with a height of h = 1.524 mm and was simulated in CST Studio Suite [10]. Port 1 of the feed network consists of a CPW line with a characteristic impedance of Z0CPW = 50 Ω that transitions into two slotlines, each with a characteristic impedance of Z0SL = 119 Ω. Slotline-to-microstrip line transitions are employed to realize four microstrip ports with characteristic impedances of Z0MSL = 50 Ω.Figure 1
The top and bottom sides of the proposed feed network. (a) Top view. (b) Bottom view.
(a)(b)In order to realize a uniform, equally spaced Vivaldi antenna array structure, and to mitigate grating lobes in the radiation pattern, the distance between the antenna elements were chosen asd = 0.65λ0 with λ0 the free space wavelength at a frequency of 5.5 GHz. The 180° phase difference between ports 2 and 3, and ports 4 and 5, was mitigated by changing the polarity/orientation of the two inner Vivaldi elements. To ensure that the distance between the conductor sides of the Vivaldi radiating elements are equal, small offsets were introduced in the distances between the microstrip ports resulting in d1, d2, and d3 in Figure 1. The different slotline-to-microstrip transitions were optimized for maximum input impedance bandwidth. To realize uniform and in-phase array excitations, the physical lengths of the microstrip line section must be equal. This was obtained by implementing different radiuses for the two different microstrip line sections. The dimensions for the final feed network are given in Table 1 and a prototype of the feed network is shown in Figure 2.Table 1
Parameters for the final feed network.
SymbolDescriptionValue (mm)d1Distance between ports 2 and 335.61d2Distance between ports 3 and 434d3Distance between ports 4 and 532.39Lf1Length of vertical microstrip line14LgLength of CPW line16LstLength of vertical slotline21.7LsubSubstrate length51.95R1Radius of one microstrip line turn section8.75R2Radius of the other microstrip line turn section6RsRadius of circular slot stub5WmWidth of microstrip line3.53WsWidth of slotline0.5WsubSubstrate width154.56Figure 2
The top and bottom sides of the prototype feed network. (a) Top plane. (b) Bottom plane.
(a)(b)TheS-parameters of the prototype feed network were measured with a HP8510C vector network analyzer. The simulated and measured reflection coefficients for port 1 are shown in Figure 3, with an impedance bandwidth from 1 GHz to 9 GHz. The simulated and measured reflection coefficients agree reasonably well with the differences probably due to manufacturing accuracy, especially the width and radial curves of the CPW and slotlines. The magnitudes and phases of the transmission coefficients for the feed network are presented in Figures 4 and 5, respectively. The magnitudes of the different transmission parameters are approximately the same with some losses visible for the frequencies above 6 GHz. This is probably due to the large radial stubs required to increase the bandwidth. The measured and simulated phases at ports 2 and 3 are almost the same with a 180° difference between the 2 ports. The transmission parameters for ports 4 and 5 are similar to that of ports 2 and 3.Figure 3
Measured and simulated reflection coefficients for port 1 of the feed network.Figure 4
Measured and simulated magnitudes of the transmission coefficients for the feed network.Figure 5
Measured and simulated phases of the transmission coefficients for the feed network.


### 2.2. Vivaldi Antenna Element

The exponentially tapered Vivaldi antenna element was implemented on RO4003 C substrate with a height ofh = 1.524 mm and is shown in Figure 6 with final dimensions in Table 2. The simulated and measured reflection coefficients for a Vivaldi antenna element are given in Figure 7. The impedance bandwidth, for a −10 dB reflection coefficient, of the single Vivaldi antenna was from 1.1 GHz to 9.5 GHz. Four Vivaldi elements were manufactured and all obtained similar results.Figure 6
Vivaldi antenna element.Table 2
Dimensions for the wideband Vivaldi antenna element.
SymbolDescriptionValue (mm)DcDiameter of circular slot stub10.78HeWidth of Vivaldi aperture82LfeLength of Vivaldi taper158.76RVivaldi taper rate15e − 3substrate_LSubstrate length209.76substrate_WSubstrate width135WmtWidth of the microstrip line at the port3.53Wmt1Width of the microstrip line at the transition1.22WsLWidth of slotline0.6Figure 7
The simulated and measured reflection coefficient for the Vivaldi antenna element.


## 2.1. Wideband Feed Network

The top and bottom sides of the proposed feed network are shown in Figure1 and consist of a CPW-to-slotline transition and two slotline-to-microstrip line transitions. The feed network was implemented on Rogers RO4003C substrate with a height of h = 1.524 mm and was simulated in CST Studio Suite [10]. Port 1 of the feed network consists of a CPW line with a characteristic impedance of Z0CPW = 50 Ω that transitions into two slotlines, each with a characteristic impedance of Z0SL = 119 Ω. Slotline-to-microstrip line transitions are employed to realize four microstrip ports with characteristic impedances of Z0MSL = 50 Ω.Figure 1
The top and bottom sides of the proposed feed network. (a) Top view. (b) Bottom view.
(a)(b)In order to realize a uniform, equally spaced Vivaldi antenna array structure, and to mitigate grating lobes in the radiation pattern, the distance between the antenna elements were chosen asd = 0.65λ0 with λ0 the free space wavelength at a frequency of 5.5 GHz. The 180° phase difference between ports 2 and 3, and ports 4 and 5, was mitigated by changing the polarity/orientation of the two inner Vivaldi elements. To ensure that the distance between the conductor sides of the Vivaldi radiating elements are equal, small offsets were introduced in the distances between the microstrip ports resulting in d1, d2, and d3 in Figure 1. The different slotline-to-microstrip transitions were optimized for maximum input impedance bandwidth. To realize uniform and in-phase array excitations, the physical lengths of the microstrip line section must be equal. This was obtained by implementing different radiuses for the two different microstrip line sections. The dimensions for the final feed network are given in Table 1 and a prototype of the feed network is shown in Figure 2.Table 1
Parameters for the final feed network.
SymbolDescriptionValue (mm)d1Distance between ports 2 and 335.61d2Distance between ports 3 and 434d3Distance between ports 4 and 532.39Lf1Length of vertical microstrip line14LgLength of CPW line16LstLength of vertical slotline21.7LsubSubstrate length51.95R1Radius of one microstrip line turn section8.75R2Radius of the other microstrip line turn section6RsRadius of circular slot stub5WmWidth of microstrip line3.53WsWidth of slotline0.5WsubSubstrate width154.56Figure 2
The top and bottom sides of the prototype feed network. (a) Top plane. (b) Bottom plane.
(a)(b)TheS-parameters of the prototype feed network were measured with a HP8510C vector network analyzer. The simulated and measured reflection coefficients for port 1 are shown in Figure 3, with an impedance bandwidth from 1 GHz to 9 GHz. The simulated and measured reflection coefficients agree reasonably well with the differences probably due to manufacturing accuracy, especially the width and radial curves of the CPW and slotlines. The magnitudes and phases of the transmission coefficients for the feed network are presented in Figures 4 and 5, respectively. The magnitudes of the different transmission parameters are approximately the same with some losses visible for the frequencies above 6 GHz. This is probably due to the large radial stubs required to increase the bandwidth. The measured and simulated phases at ports 2 and 3 are almost the same with a 180° difference between the 2 ports. The transmission parameters for ports 4 and 5 are similar to that of ports 2 and 3.Figure 3
Measured and simulated reflection coefficients for port 1 of the feed network.Figure 4
Measured and simulated magnitudes of the transmission coefficients for the feed network.Figure 5
Measured and simulated phases of the transmission coefficients for the feed network.


## 2.2. Vivaldi Antenna Element

The exponentially tapered Vivaldi antenna element was implemented on RO4003 C substrate with a height ofh = 1.524 mm and is shown in Figure 6 with final dimensions in Table 2. The simulated and measured reflection coefficients for a Vivaldi antenna element are given in Figure 7. The impedance bandwidth, for a −10 dB reflection coefficient, of the single Vivaldi antenna was from 1.1 GHz to 9.5 GHz. Four Vivaldi elements were manufactured and all obtained similar results.Figure 6
Vivaldi antenna element.Table 2
Dimensions for the wideband Vivaldi antenna element.
SymbolDescriptionValue (mm)DcDiameter of circular slot stub10.78HeWidth of Vivaldi aperture82LfeLength of Vivaldi taper158.76RVivaldi taper rate15e − 3substrate_LSubstrate length209.76substrate_WSubstrate width135WmtWidth of the microstrip line at the port3.53Wmt1Width of the microstrip line at the transition1.22WsLWidth of slotline0.6Figure 7
The simulated and measured reflection coefficient for the Vivaldi antenna element.


## 3. Linear Vivaldi Antenna Array

The assembled uniform linear Vivaldi array mounted on the pedestal of a compact antenna test range is shown in Figure8. The measured and simulated S-parameter results for the complete antenna array are shown in Figure 9. The impedance bandwidth is from 1.3 GHz to 9 GHz with a reflection coefficient response below −10 dB with some minor exceptions. The VSWR for the entire frequency range was below 2.5 : 1. The simulated and measured reflection coefficients agree reasonably well except for some frequencies close to 4 GHz. The simulated values for the reflection coefficient are significantly lower than the measured values. Possible reasons for these differences can be attributed to small manufacturing or assembling differences between the prototype array and the ideal simulation model and/or interaction between the array and the measurement environment. The realized gain of the array on boresight was above 8 dBi between 1.5 GHz and 8 GHz, with a maximum of 13.8 dBi at 6.3 GHz, as shown in Figure 10. Above 8 GHz, the boresight gain decreases significantly due to a deterioration of the main beam of the individual Vivaldi antennas. The simulated total efficiency of the antenna array is shown in Figure 11. The total efficiency of the array is above 83% within the useable frequency range of 1.3 GHz to 8 GHz.Figure 8
Images of the proposed prototype antenna array. (a) Prototype antenna array in compact antenna range. (b) Prototype antenna array.
(a)(b)Figure 9
The simulated and measured reflection coefficient for the antenna array.Figure 10
The simulated and measured gain for the antenna array.Figure 11
The simulated total efficiency of the antenna array.TheE- and H-plane radiation patterns at discrete frequencies are presented in Figures 12–14. The measured and simulated results were similar with the cross-polarization for all frequencies below −10 dB in the main beam. Stable radiation patterns with well-defined main beams were obtained for frequencies up to 8 GHz. Although the feed network achieved an impedance bandwidth of 160%, the useable bandwidth of the array with stable radiation patterns is limited to a 144% bandwidth from 1.3 GHz to 8 GHz.Figure 12
The radiation pattern at 2 GHz.Figure 13
The radiation pattern at 5.5 GHz.Figure 14
The radiation pattern at 8 GHz.Table3 compares the performance of the proposed wideband Vivaldi antenna array with similar wideband arrays in the literature [4–8]. The proposed array in this paper achieved a wider useable bandwidth as well as a higher maximum gain compared to [4, 5], which has a similar single layer feed network structure. The array structures in [6–8] utilize multilayer feed networks such as a substrate-integrated waveguide (SIW) or an electromagnetic band gap (EBG) structure power divider. The advantage of these array structures is that they are more compact than the proposed array, however, the bandwidths achieved are less than that of the proposed antenna array.Table 3
Comparison of different four element wideband arrays.
Frequency band (GHz)Useable bandwidth (%)Maximum gain (dBi)UWB monopole [4]2.35–6.188.7610.5Vivaldi array [5]3.4–8.383.8012.6Compact wideband array [6]8.9–10.920.210.6SIW fed array [7]11.2–13.619.3517.1Magneto-electric-like array [8]24.5–404812.65Proposed antenna array1.3–8144.0913.8


## 4. Conclusion

Individual Vivaldi antennas are generally capable of achieving bandwidths in excess of 100%. A prototype four element uniform linear array of Vivaldi antennas fed with a wideband corporate feed network was presented. The wideband properties of microstrip-to-slotline transitions were exploited to design a four element corporate feed network consisting of a CPW-to-slotline transition and two slotline-to-microstrip line transitions. The Vivaldi antenna array achieved an impedance bandwidth with VSWR below 2.5 : 1, as well as stable radiation patterns from 1.3 GHz to 8 GHz, resulting in a useable bandwidth of 144%. The proposed antenna array achieved a greater bandwidth and higher gain compared to similar wideband antenna arrays from literature.



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*Source: 1496851-2022-09-10.xml*