A Miniaturized Series Fed Tri-Slot Coplanar Vivaldi Antenna for RADAR Application With Reduced Ground Plane Effect

In this article, a printed series fed tri-slot coplanar Vivaldi antenna is studied and investigated for RADAR application. The antenna consists of three exponentially tapered slots excited by a single microstrip line. The three slots are series fed simultaneously with a radial stub on the microstrip feed line for impedance matching. The proposed antenna covers a frequency of 7.8-11.8 GHz at − 10 dB impedance bandwidth. The antenna gain varies from 7.5 dB at the lowest operating frequency and increases to around 9.5 dB at the center frequency of 10 GHz. A key versatility of the proposed design is the ability to modify the ground plane size without affecting the antenna impedance and gain. This makes the antenna suitable to incorporate RADAR transceiver components without modifying the existing antenna design. The proposed antenna is fabricated and measured results show good agreement with simulated results.


I. INTRODUCTION
T HE FUTURE wireless communication standards such as beyond fifth-generation (B5G) technologies and sixthgeneration (6G) will operate at extremely high frequencies to meet the demand for high data rates and link reliability.
A key technology dedicated to defense applications such as RADAR is now being deployed in existing 5G and B5G networks, i.e., Phased array antennas.To obtain high directionality in phased arrays end-fire antennas are now explored over the broadside antennas.
Another potential area that is gaining significant attention using RADAR is in healthcare.Lately, contactless human activity detection is being performed using RADAR.The contactless human activity detection includes freezing of gait and vital sign monitoring, e.g., breathing and heart rate [1]- [4].
As RADAR systems deploy directional antennas, an important class of traveling wave, the end-fire antenna-Vivaldi antenna [5]- [7] has always remained an attractive solution for researchers.They are broadband, high gain, high efficiency and generate a stable radiation pattern.The antipodal Vivaldi antenna (AVA) [6] has mirror radiators on the top and bottom plane, generating a broader bandwidth than the conventional coplanar Vivaldi antenna (CVA) design [5], but it suffers from high cross-polarization.Due to the inherent unbalanced design of the CVA, the design was converted to a balanced CVA by [7] but at the cost of fabrication complexity with additional two layers.
Recently most researchers have presented various AVA designs [8]- [15] to enhance the bandwidth and the antenna gain.However, the proposed antennas are mainly designed to operate as stand alone antennas.The antenna is integrated with a PCB with a bigger ground plane to accommodate the components in practical scenarios.As a result, this degrades and/or requires changes to the antenna design [16].For gain and bandwidth improvement, the CVA with corrugated slots has been proposed in [17]- [19].Apart from that, no other method exists in the literature to improve the performance of the CVA.
In this article, we propose for the first time a concept of series fed exponentially tapered triple-slots on a common continuous ground plane applied to the CVA design.In previously published works, the traditional CVA bandwidth is widened by using a hollow cavity at the feed point.The proposed design introduces separate simultaneously excited slots to achieve the required bandwidth and additionally contributing to the antenna gain.To the extent of our knowledge, the proposed design has also achieved versatility as the ground plane could be extended along both length and width without affecting the impedance and gain of the proposed antenna.This makes it able to incorporate transceiver components without affecting the impedance matching and gain.

II. ANTENNA DESIGN
The optimization of the antenna is performed using CST MWS 2019.The antenna is fabricated on Rogers 4350B substrate having a dielectric constant of 3.48 and a loss tangent of 0.0037.The thickness of the dielectric substrate is taken as 0.8 mm.The proposed tri-slot antenna design consists of three separate radiators excited by a single microstrip feed line.This methodology adds to an improved gain of the proposed design.The inter-slot spacing is kept half wavelength at the center frequency, i.e., 10 GHz.As the radiation of the single slot is coupled to each other, this contributes to resonance altogether and the slots appear to each other as a two-element array.The proposed work has kept a common ground plane for all three slots.The total dimension of the proposed antenna is around 50×21 mm 2 which has 60% reduced volume as proposed in [19].The complete layout of the antenna is given in Figure 1.The SMA connector is mounted at the bottom side and the pin is soldered perpendicular to the microstrip line.The stub on the top layer is carefully optimized to achieve 50 impedance matching when all three slots are excited together.
The exponentially tapered slots are designed using the equation from [20].Y = ±se rx (1) where 's' represents the slot width at the feed point and 'x' the total antenna length from the feed point.The value of 'r' is important to determine the flare shape of the antenna, i.e., how the slot widens.The shape of the tapered slots in the proposed design is produced from the following values by optimizing in CST MWS.

III. PARAMETRIC STUDY
The performance of the proposed antenna is dependent on the excitation of the slots.Instead of placing a hollow cavity at the radiator feed to achieve the impedance bandwidth, a novel approach is adopted.Three slots are simultaneously excited to achieve the required specification.While antennas are always integrated on PCB boards, an extension to the RF ground plane may affect the antenna performance.Hence, a parametric study is performed to analyze the inclusion of slots and extend the proposed antenna's ground plane size.

A. EFFECT OF SLOTS ON IMPEDANCE MATCHING AND GAIN
The antenna is designed in three steps.In the first iteration, a single tapered slot is excited.The tapering is achieved from equation (1).The antenna resonates from 7.1 GHz to 8.5 GHz.Thus, the cutoff frequency of 7 GHz is established.
A second slot is placed near the feed point and tapering is achieved based on equation (2).Impressively, the bandwidth is extended from 7.5 GHz to 11 GHz.This can be seen in Figure 2a.The gain in the required band is around 7 dB.It is important to note that both the slots share a common ground plane.

TABLE 1. Performance metrics of proposed tri-slot antenna.
To improve the gain and impedance bandwidth for the RADAR application, a third slot is placed and designed using equation (3).With the placement of the third slot, a stub is also placed on the microstrip feed line to achieve impedance matching.It can be seen in Figure 2a that the impedance bandwidth is adjusted from 7.8-11.8GHz and the gain is enhanced by around 2.5 dB.Hence, the average gain achieved is approximately 8.5 dB in the required band.The design steps of the proposed antenna are given in Table 1.
Table 1 shows the performance metrics achieved by the proposed tri-slot antenna.The versatility in terms of the profile of the antenna can be seen in Table 1.For a miniaturized case, a two-slot antenna has a very low footprint with acceptable operating bandwidth and moderate gain, which can be considered in small form factor devices.On the other hand,  an extension in width of the antenna to include the third slot further enhances the gain.

B. EFFECT OF GROUND ON IMPEDANCE MATCHING
Contrary to the patch antenna, the ground plane of the CVA act as the main radiator.Energy is coupled from the microstrip line to the slot in the ground plane.Hence modification and/or extension to the ground plane can alter both impedance and gain of the CVA.The proposed antenna design can be considered as a ground-independent slot antenna.This feature has never been incorporated in any of the existing antenna designs.It is evident from Figure 3 that increasing the parameter 'E 4m ' along length 'L' and/or 'W 1 ' along the width of the proposed tri-slot design, the impedance matching of the antenna is not affected at all.The length extension is performed from the edge of the third slot.This is useful in the case where the antenna needs to be integrated with the transceiver and an extension to the ground plane is imperative to accommodate various components.
The surface current distribution of the proposed antenna is shown in Figure 4.The red region offers higher  current concentration areas.Each slot by itself acts as an independent radiator and contributes to various resonances.It can be seen in Figure 4a that the third slot is excited at the lower frequency band of 7.5 GHz while in Figure 4b, the middle slot generates the frequency band of 9.5 GHz.The second slot near point a resonance of 10.5 GHz.Thus, a operating bandwidth achieved.

IV. RESULTS AND
Figure 5 shows the prototype of the proposed antenna.The measurement of the proposed antenna has been carried out using the N5230C vector network analyzer (VNA).The simulated and measured S-parameters of the proposed antenna are shown in Figure 6.As seen in the figure, a good agreement between the simulated and measured results has been obtained.The simulated and measured gain of the proposed antenna is shown in Figure 7.The measured gain follows the simulated gain curve.Due to a minimum number of points, interpolation has been applied to smoothen the gain curve.The antenna mounting positions is illustrated in Figure 8. Further, Figure 9 shows the simulated and measured normalized gain pattern of the proposed antenna.The radiation patterns are along the end-fire direction.As discussed earlier, the directionality of the radiation pattern is due to the array behavior of the antenna due to inter-slot spacing of half-wavelength at the centre frequency of 10 GHz.Consequently, this results in an improvement in the overall gain.

V. CONCLUSION
This article presents the design, implementation and measurement of a tri-slot Vivaldi antenna for RADAR and RF sensing applications.The proposed antenna design is fed using a microstrip to slot line transition with a stub for impedance matching.The design is versatile and can be adapted for any form factor device.The ground plane extension does not affect the impedance and gain properties of the antenna.Additionally, the radiation pattern is also directional making it suitable to be used in Phased array applications.The antenna has a −10 dB impedance bandwidth of 4 GHz at the centre frequency of 10 GHz.The measured average gain of the antenna is around 8.5 dB in the required band.

FIGURE 2 .
FIGURE 2. Inclusion of slots in the proposed antenna (a) Simulated input reflection and (b) Simulated Gain.

3 .
Effect on impedance matching due to the extension of the ground plane along the width and length.

FIGURE 7 .
FIGURE 7. Gain of the proposed antenna.

FIGURE 8 .FIGURE 9 .
FIGURE 8. Measurement setup of the proposed antenna: a) vertical polarized and b) horizontal polarized.