A Wideband Feed Network for Vivaldi Antenna Arrays

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. e proposed antenna array consists of four wideband Vivaldi antennas fed with an improved wideband feed network to extend the useable bandwidth of the array. e proposed feed network consists of coplanar waveguide-to-slotline-to-microstrip line transitions. e feed network has a single coplanar waveguide input and four microstrip line output ports. e feed network achieved uniform amplitude and phase balance and an impedance bandwidth of 160% from 1GHz to 9GHz. e feed network was used in a uniform linear antenna array to feed four Vivaldi antenna elements. e Vivaldi antenna array achieved stable radiation patterns from 1.3GHz to 8GHz, resulting in a useable bandwidth of 144%. e 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.


Introduction
Vivaldi antennas are typically used in wideband applications, viz., ground penetrating radars, wideband radar imaging, radio astronomy [1], and multiband communication systems (5G). e 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][4][5]. ese feed networks were combined with di erent 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 di erent patch elements. ese 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%. is paper presents a wideband corporate feed network with more than 160% impedance bandwidth from 1 GHz to 9 GHz. e wideband properties of microstrip-to-slotline transitions are exploited to design a four element corporate feed network. e 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. e Vivaldi antenna array achieved stable radiation patterns from 1.3 GHz to 8 GHz, resulting in a useable bandwidth of 144%. e design and performance of the feed network and a single Vivaldi antenna element are presented in Section 2. Section 3 presents the measured and simulated results for the nal antenna array, with concluding remarks in Section 4.

Wideband Feed Network.
e top and bottom sides of the proposed feed network are shown in Figure 1 and consist of a CPW-to-slotline transition and two slotline-to-microstrip line transitions. e 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 Z 0CPW � 50 Ω that transitions into two slotlines, each with a characteristic impedance of Z 0SL � 119 Ω. Slotline-to-microstrip line transitions are employed to realize four microstrip ports with characteristic impedances of Z 0MSL � 50 Ω.
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 as d � 0.65λ 0 with λ 0 the free space wavelength at a frequency of 5.5 GHz. e 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 d 1 , d 2 , and d 3 in Figure 1. e different slotlineto-microstrip transitions were optimized for maximum input impedance bandwidth. To realize uniform and inphase array excitations, the physical lengths of the microstrip line section must be equal. is was obtained by implementing different radiuses for the two different microstrip line sections. e dimensions for the final feed network are given in Table 1 and a prototype of the feed network is shown in Figure 2.
e S-parameters of the prototype feed network were measured with a HP8510C vector network analyzer. e simulated and measured reflection coefficients for port 1 are shown in Figure 3, with an impedance bandwidth from 1 GHz to 9 GHz. e 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. e magnitudes and phases of the transmission coefficients for the feed network are presented in Figures 4 and 5, respectively. e magnitudes of the different transmission parameters are approximately the same with some losses visible for the frequencies above 6 GHz. is is probably due to the large radial stubs required to increase the bandwidth. e measured and simulated phases at ports 2 and 3 are almost the same with a 180°difference between the 2 ports. e transmission parameters for ports 4 and 5 are similar to that of ports 2 and 3.

Vivaldi Antenna Element.
e exponentially tapered Vivaldi antenna element was implemented on RO4003 C substrate with a height of h � 1.524 mm and is shown in  Table 2. e simulated and measured reflection coefficients for a Vivaldi antenna element are given in Figure 7. e 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.

Linear Vivaldi Antenna Array
e assembled uniform linear Vivaldi array mounted on the pedestal of a compact antenna test range is shown in Figure 8. e measured and simulated S-parameter results for the complete antenna array are shown in Figure 9. e impedance bandwidth is from 1.3 GHz to 9 GHz with a reflection coefficient response below −10 dB with some minor exceptions.
e VSWR for the entire frequency range was below 2.5 : 1. e simulated and measured reflection coefficients agree reasonably well except for some frequencies close to 4 GHz. e 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. e 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. e simulated total efficiency of the antenna array is shown in Figure 11. e total efficiency of the array is above 83% within the useable frequency range of 1.3 GHz to 8 GHz.
e E-and H-plane radiation patterns at discrete frequencies are presented in Figures 12-14. e 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. Table 3 compares the performance of the proposed wideband Vivaldi antenna array with similar wideband arrays in the literature [4][5][6][7][8].
e 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. e array structures in [6][7][8] utilize multilayer feed networks such as a substrate-integrated waveguide (SIW) or an electromagnetic band gap (EBG) structure power divider. e 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.     1 0 Frequency (GHz) S12 Measured S13 Measured S14 Measured S15 Measured S12 Simulated S13 Simulated S14 Simulated S15 Simulated   Frequency (GHz) 6 7 8 9 1 0 S12 Measured S13 Measured S12 Simulated S13 Simulated

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. e 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. e 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%. e proposed antenna array achieved a greater bandwidth and higher gain compared to similar wideband antenna arrays from literature.

Data Availability
Data are available on request from the corresponding author: wimpie@up.ac.za.  International Journal of Antennas and Propagation 9