Wideband Bidirectional Same Sense Endfire Circularly Polarized Antenna

In this paper, a wideband bidirectional endfire circularly polarized (CP) antenna is proposed by combining two endfire CP antennas with a single coaxial probe feed. The wideband characteristics are achieved with the help of a two-layered substrate separated by an air gap, and the excitation of CP waves in the endfire directions is yielded by the wideband magnetic dipoles and tapered electric dipoles. The proposed CP antenna has shown wide 10-dB impedance bandwidth (IBW) and 3-dB axial ratio bandwidth (ARBW) of 20.76% (5.05 - 6.22 GHz) and 16.10% (5.14 - 6.04 GHz), respectively. Besides showing an overall size of 0.733λL × 0.863λL× 0.058λL (λL is the wavelength corresponding to the lowest operating frequency) with wide operational bandwidth, the proposed CP antenna has also demonstrated a desirable peak gain of 4.45 dBic.

The bidirectional CP antennas [3], [7]− [14] have various advantages over the bidirectional LP antenna type, such as orientation insensitive, better weather penetration, and interference reduction in the multipath environment. The bidirectional CP antenna can be mainly divided into two categories namely, broadside bidirectional CP antenna [3], [7]− [10], and endfire bidirectional CP antenna [11]− [14]. In [7] and [8], bidirectional CP with the same sense is achieved by the two-polarization conversion surface (coupling strip as mentioned in [8]) at the top and bottom of the microstrip fed slot antenna. The operating bandwidths of these polarization conversions surface-based bidirectional same sense CP antenna are 14.3% (5.2 − 6.0 GHz) [7] and 5.8% (2.33 -2.47 GHz) [8]. Similarly, in [9], a polarization conversion surface is used on the top of the CP antenna to accomplish the same sense of circular polarization in both the top and bottom of the structure. Another way to realize the bidirectional CP antenna with the same sense is by using back-to-back slot coupled patches [10]. Even though good CP sense is achieved with the help of corner truncated patches, the operating bandwidth is only 0.81% (2.455 -2.475 GHz).
Due to the continuous airflow ventilation in the coal mine tunnel, the endfire bidirectional antenna is a better candidate than the broadside bidirectional antenna because it provides a minimal cross-sectional area against the airflow [1]. Several bidirectional same sense endfire CP antennas have been reported in [11]− [14]. In [11], an array of crossed dipole and composite right/left-handed transmission lines (CRLH-TL) are applied, and the crossed dipole based bidirectional endfire CP antenna has an overall dimension of 0.717λL × 0.446λL × 0.446λL (λL is the 2 wavelength corresponding to lowest operating frequency) and operating bandwidth is limited by 11.8%. However, the above-reported literature shows that the endfire CP antenna profile is very large (3-Dimensional structure) and inappropriate for many applications. The bidirectional endfire CP antenna can be constructed using a back-to-back combination of the unidirectional endfire CP antenna. The unidirectional endfire CP antenna is formed by the combination of vertically polarized (VP) magnetic dipole and horizontally polarized (HP) electric dipole with a feed line [16]− [18]. The back-to-back combination of this unidirectional endfire CP antenna is used to form a bidirectional endfire CP antenna with the same sense as reported in [13]− [15]. The overall bandwidth of the bidirectional endfire CP antenna with same sense is 3.45% in [13], 2.93% in [14] and 2.8% in [15]. Therefore, the overall operational bandwidth of the above-reported antennas is limited, and it will not be suitable for wideband applications.
In this paper, a wideband bidirectional endfire same sense CP antenna is proposed. The bandwidth of the bidirectional endfire CP antenna is enhanced by combining the wideband bidirectional VP magnetic dipole with HP wideband electric dipoles. The operational bandwidths of both bidirectional magnetic dipole and electric dipole are further improved by implementing a two-layered structure with an air spacer. The overall 3-dB ARBW of the proposed bidirectional endfire CP antenna is 16.10% with the dimension of 0.733λL × 0.863λL × 0.058λL. The proposed CP antenna is optimized using ANSYS Electronics Desktop (HFSS) and fabricated using the printed circuit board (PCB) method to verify the simulated predictions.
This manuscript is organized as follows. Firstly, the principle of operation for bidirectional endfire CP is investigated in section II, followed by studying the wideband VP magnetic dipole (unidirectional and bidirectional) in section III. Section IV describes the geometrical configuration of the proposed wideband bidirectional endfire CP antenna with experimental results. Finally, section V concludes the paper.

II. OPERATING PRINCIPLE OF BIDIRECTIONAL ENDFIRE CP ANTENNA WITH SAME SENSE
To generate a bidirectional endfire CP antenna with the same sense, an array of electric dipoles and magnetic dipoles are placed as shown in Fig. 1. Among two pairs of electromagnetic dipoles, one pair of the electric and magnetic dipoles is collocated along +x-direction, and the other pair is set along the -x-direction. The distance between the magnetic and electric dipole are 2dm and 2de, as illustrated in Fig. 1. The total electric current ( ̅ ) is the sum of ̅ and ̅ and it can be written as (1) Similarly, the total magnetic current ( ) is the sum of and , and it can be stated as where ̅ = − ̅ = and − = = . Therefore, magnetic vector potential ̅ ) and electric vector potential ) due to these current sources can be expressed as The total far-field electric field intensity along the +ydirection due to the above magnetic and electric vector potential can be specified as where B is a constant. The condition for CP sense along the +y direction is expressed below Here, the distance between the electric current source and magnetic current source is λ/4 to get 90 0 phase differences. As two magnetic currents can be formed by using the backto-back combination of half mode substrate integrated waveguide (HMSIW) [19]− [20], the position of the magnetic current source (dm) and electric current source (de) are set to be λ/8 and 3A 8 ⁄ , respectively, which play a vital role in obtaining the accurate dimension of the bidirectional magnetic dipole. Consequently, the modified magnitude condition is 3| | = | | , while phase condition will remain the same as equation (8). Notably, analogous conditions will be obtained along the −y-direction accordingly. To further achieve wideband behavior for the bidirectional CP antenna with the same sense, the wideband bidirectional magnetic dipole and electric dipole are required. The electric dipole with wideband characteristics has been reported in many papers, while there are limited papers available based on the wideband magnetic dipole. Therefore, a wideband magnetic dipole with unidirectional and bidirectional characteristic are investigated in subsequent sections.

A. Wideband Unidirectional Magnetic Dipole
The half mode substrate integrated waveguide (HMSIW) is used to form a vertically polarized (VP) unidirectional magnetic dipole. To reduce the complexity, shorting walls on the three sides are used to prevent leakage instead of vias [18], as depicted in Fig. 2. Here, a double-layered HMSIW is designed by implementing two FR4 substrates (ɛ " = 4.4, tan = 0.02) each with a thickness of 0.8 mm separated by an air gap of 2 mm, as shown in Fig. 2(a). The quasi TE1,0.5,0 is the dominant mode in the HMSIW [13], and it can be verified by observing the magnitude of electric field (V/m) distribution on the top surface of the two-layered cavity, as shown in Fig. 2(a). The equivalent dielectric constant of the two layered-cavity can be calculated by using equation (9) [21].
Here, the calculated equivalent dielectric constant is obtained as 1.523. To verify the antenna design, the equivalent dielectric constant (ɛ F ) is taken as 1.523, and the height of the substrate 'h' is the summation of h1, h2, and h3. Therefore, the single-layered magnetic dipole is constructed by taking a single dielectric constant of ɛ F = 1.523 and height h = 3.6 mm, as shown in Fig. 2(b). The reflection coefficient curve for both single and doublelayered unidirectional magnetic dipole is shown in Fig. 3(a). It reveals that both the structure provides a similar response with IBW of around 6%. However, lowering the profile of the similar unidirectional magnetic dipole delivers lesser IBW, and for the given thickness of 0.8 mm, it provides only 1.9% of IBW. The resonance frequency of the dominant mode of both single and double-layered with respect to width (W) of the cavity is plotted in Fig. 3(b). The resonance frequency of the dominant mode can also be calculated by using equation (10) [13], and Fig. 3(b) shows the frequency plot for different widths. It can be observed that all three curves are in good agreement with each other. Therefore, the above-mentioned equations are valid to calculate the dimension of the HMSIW based magnetic dipole.

B. WIDEBAND BIDIRECTIONAL MAGNETIC DIPOLE
The VP bidirectional magnetic dipole can be constructed by exciting the two magnetic current sources on the open aperture, as shown in Fig. 4(a). The back-to-back combination of the HMSIW constructs the two-element array of the magnetic dipole. The electric field distribution on the open aperture is illustrated in Fig. 4(b). As the direction of the electric field on the open aperture is the same (+ z-direction), and outward unit normal vectors on these apertures are in opposite directions, these magnetic current sources are in the opposite direction, as shown in Fig. 4(a). Here, the single feed mechanism is applied instead of using two separate feeds to excite the two magnetic dipoles. The feeding location and the gap (W1) within the shorting wall are decided by the good matching condition of the antenna element. The simulated reflection coefficient curve and the gain curve are depicted in Fig.  4(c). Here, it can be observed that the resonance frequency of this magnetic dipole is dependent on the length L1, width W, and the opening space of W1. The same phenomenon can be well observed in Fig. 2, as discussed above. The proposed two-layered magnetic dipole alone provides IBW of 11.7%, with a peak realized gain of 4.5 dBi. The simulated 2D radiation patterns plotted across the two principal planes (xy and yz plane) are shown in Figs. 5(a) and (b), respectively. It can be observed that the Eθ component is a co-polarized radiation pattern in the endfire direction. Along the θ = 0 0 , a null can be observed due to equal and opposite magnetic dipoles, which are symmetrically placed with respect to the center of the antenna. Consequently, these magnetic dipoles are combined with the two electric dipoles (collocated along the x-direction) with λ/4 feed length, and a good CP wave will be achieved with endfire direction. Since the x-directed electric dipole exhibits an Eϕ component as the co-polarized radiation pattern along the y-direction, the conditions to achieve CP wave will be satisfied when two orthogonal components Eθ (magnetic dipole) and Eϕ (electric dipole), will become equal in magnitude and have 90 0 phase difference. Therefore, to achieve a bidirectional endfire CP antenna, the magnetic dipole loaded with a pair of electric dipoles will be investigated in the next section.

A. ANTENNA DESIGN
The schematic diagram and fabricated structure of the proposed wideband endfire CP antenna are shown in Figs. 6 and 7, respectively. The proposed structure is printed on the top layer of substrate-1 and the bottom layer of substrate-2. Both substrates are made of FR4 dielectric sheet (ɛ T = 4.4, UVW X= 0.02) with a thickness of 0.8 mm. The bidirectional magnetic dipole is formed at the open aperture of the cavity, as discussed earlier in section III. This cavity can be considered as the back-to-back combination of the HMSIW. The two-layered back-to-back combination of the HMSIW is constructed by applying vias along the two side edges (along the y-direction) and along the centerline (along the x-direction), as shown in Fig. 6(a). Since the proposed antenna is a double-layered PCB (printed circuit board) structure, conventional vias formation cannot be implemented. Therefore, these vias are implemented using copper wire of diameter 0.6 mm, bypassing it through each hole and followed by proper soldering connection at both top and bottom layer, as shown in Fig. 7. Proper soldering is required to get a proper connection with printed copper metal and vias.   To get a bidirectional endfire CP wave, tapered electric dipoles are placed on both sides of the magnetic dipole but in the reverse direction. The straight feed line is used to connect the magnetic and electric dipoles. A 50 Ω coaxial connector is connected at the center to excite the wideband bidirectional endfire CP antenna. The opening space (W1) and overlapping edge at shorter width of the electric dipole of length (L4) are very significant parameters to yield good impedance matching as well as axial ratio bandwidth (ARBW).

B. RESULT AND ANALYSIS
The proposed wideband bidirectional endfire CP antenna with the same sense is optimized and analyzed using ANSYS Electronics Desktop (HFSS). To validate the simulated results, the prototype of the antenna model was fabricated using printed circuit board (PCB) technology, as shown in Fig. 7. The simulated reflection coefficient and axial ratio curve are shown in Figs. 8(a) and (b), respectively. The reflection coefficient is measured using the N5222A PNA network analyzer (Agilent Technologies), and the far-field parameters are measured in a standard Anechoic Chamber. It can be observed that the simulation results are in good agreement with the measured ones.   The simulated 10-dB bandwidth (or IBW) and 3-dB ARBW (or CP bandwidth) of the proposed CP antenna are 20.22% (4.89 − 5.99 GHz) and 17.95% (4.97 − 5.95 GHz), respectively, while its corresponding measured ones are 20.76% (5.05 − 6.22 GHz) and 16.10% (5.14 − 6.04 GHz), respectively. Fig. 9 shows the simulated and measured radiation patterns of the endfire CP antenna in two principal planes. It is observed that the right hand circularly polarized (RHCP) radiation is along the endfire directions. The simulated polarization purity along the endfire direction is better than 40 dB at the center frequency. The measured co-polarized radiation pattern is well matched with the simulated co-polarized radiation 6 pattern. The deviation in cross-polarization is observed due to the higher value of the axial ratio at the center frequency, as observed in the measured axial ratio diagram shown in Fig. 8(b). The simulated and measured gain plot of the endfire CP antenna is illustrated in Fig. 10(a). The proposed antenna provides a peak gain of 4.45 dBic in the endfire direction. Due to combined effect of fabrication intolerances and material imperfections, slight deviations are observed in the measured responses. The 3D radiation pattern is depicted in Fig. 10(b), and it confirms that the proposed CP antenna has bidirectional endfire characteristics. To better comprehend the wideband bidirectional endfire CP antenna, the effects of tuning the antenna parameters length L4 and the opening space W1 have been analyzed through the parametric analysis, as shown in Figs. 11 to 13. In Fig. 11(a), as L4 increases from 2.0 mm to 4.0 mm (with a step increment of 0.5 mm), a minimal variation in the resonance frequency is observed. In contrast, the axial ratio curve shifted towards the lower frequency, as shown in Fig.  11(b). Therefore, the length L4 can be used to tune the axial ratio without affecting the reflection coefficient curve. The effect of length L4 on the ratio of the magnitude of orthogonal electric field ( Y Z [ Z \ Y and phase difference ∠1 ] − ∠1^ of the proposed wideband bidirectional endfire CP antenna along the +y direction is depicted in Fig. 12. It can be observed that when L4 = 3.5 mm, a good AR performance is achieved. Similarly, the opening space W1 is another significant parameter to yield good impedance matching without affecting the axial ratio curve, as shown in Fig. 13. Here, when L4 = 3.5 mm and W1 =18.6 mm, good IBW and ARBW can be achieved. For the visualization of CP wave generated by the combination of orthogonal components (electric dipole and magnetic dipole), the electric field distribution on the open aperture and electric field distribution away from the antenna are presented in Figs. 14 and 15, respectively. The electric field and surface current distribution on the open aperture and tapered electric dipole, respectively, with different time instant (ωt), are depicted in Fig. 14. For time instant ωt = 0 0 , the magnitude of the electric field on the open aperture is high, and a small magnitude of surface current distribution on the electric dipole is observed. On the other hand, at ωt = 90 0 , a low electric field appears on the open aperture and the electric dipole has shown high surface current distribution. Therefore, it is confirmed that there is an approximately 90 0 phase difference between the electric and magnetic dipole. Furthermore, the sense of polarization can be identified by Fig. 15 for both endfire directions. At time instant ωt = 0 0 , the electric field is directed along the +z-direction at both ends. For ωt = 90 0 , the polarization of the electric field is along the +xdirection and −x-direction for +y and −y directed CP wave, respectively. Similarly, for ωt = 180 0 and 270 0 , they have followed the same trend. Therefore, the proposed wideband bidirectional endfire CP antenna with the same sense provides the RHCP polarization for both endfire directions. Table 1 shows the performance comparison between the proposed wideband bidirectional CP antenna and those (with the same sense) reported in [12]- [15]. Notably, limited literature is available for bidirectional endfire CP antenna with the same sense. In [12], the overall operating bandwidth is only 7.8% with the very large antenna profile. Therefore, it can be realized that the proposed antenna provides a wider IBW (at least six times higher) and 7 ARBW (more than four times) with a comparative antenna profile with respect to the other reported literature. Furthermore, it has also exhibited decent gain characteristics without any additional director or array implementation. Nevertheless, it is noteworthy that the gain of this proposed endfire CP antenna can be further enhanced by considering low loss substrate, implementing additional directors at the front of the antenna, or arranging it as an array but at the cost of a larger size.

V. CONCLUSION
A wideband bidirectional endfire CP antenna with the same sense has been successfully designed theoretically and experimentally validated with a fabricated prototype. The bidirectional magnetic dipole was realized by a back-toback combination of HMSIW, while the bidirectional magnetic dipoles and tapered electric dipoles are connected via a feed line to yield a desirable CP wave in the endfire direction. The significant enhancement of the proposed antenna in the overall ARBW (16.10%) with overlapped IBW of 20.76% was achieved by using two-layered configurations. The measured peak gain of this endfire CP antenna was 4.45 dBic. Due to the above-mentioned antenna performances, the proposed wideband bidirectional endfire CP antenna with the same sense can be used in long tunnels or coal mines to improve security where wideband characteristics are required.