Substrate integrated waveguide differential antenna for full duplex applications

Abstract

This paper presents a substrate-integrated waveguide (SIW) differential antenna for full duplex applications. The proposed antenna consists of two square SIW cavities named as outer and inner. The inner cavity is nested into the outer cavity. The outer cavity is differentially excited with a pair of coaxial feed lines, while the inner square patch is orthogonally excited with another pair of differential coaxial feed lines. This orthogonal feeding arrangement results in high isolation between the differential ports. The modified hybrid TE_130/310 mode of the outer cavity radiates through a pair of arc-shaped slots at 9.35 GHz, while the TM₀₁ mode of the inner square patch is responsible for the radiations at 8.65 GHz. The proposed antenna prototype is fabricated and measured for validation. Moreover, the designed antenna has a front-to-back ratio better than 22 dB and measured maximum gain values of 6.1 dBi and 7.6 dBi at 8.65 GHz (Port 2 ON) and 9.35 GHz (Port 1 ON), respectively.

Introduction

Differential antennas are preferred over single-ended ones due to their several advantages, such as low cross-polarization, ease of integration with differential circuits, excellent environmental noise immunity, high spectral efficiency, etc [1]. Several three-dimensional differential antennas have been studied in detail [2–5]. However, these antennas occupy a large footprint and have a relatively high profile. To integrate these antennas in the system, low-profile antennas are required. Several low-profile planar differential antennas are also investigated in the literature [6–9]. Nowadays, substrate-integrated waveguide (SIW) is a promising technology that allows the realization of low-profile high-gain differential antennas [10–12].

Moreover, modern wireless communication systems require compact, low-profile, and multiband antennas with high input port isolation. These multiband antennas are connected to multiple transceivers, which suffer from poor port-to-port isolation. A diplexer can be used to enhance the isolation between the ports [13]. However, this requires additional circuit elements, which increases the design complexity and results in a large transverse size. Thus, the full-duplex antenna systems, which do not require extra components for isolation enhancement, can provide an excellent solution to the limitations mentioned above [14–16].

In full-duplex communication systems, the simultaneous transmission and reception of signals takes place. This simultaneous transmission and reception of signals not only double the spectral efficiency but also increase the data throughput [17–19]. Two separate frequencies, one for transmission and another for reception of signals, are used in a dual-band full-duplex system. High isolation between the transmitter and receiver is one of the main requirements of any full-duplex systems. For acceptable performance of full‐duplex systems, at least 110 dB of inter-port isolation is essential [20]. This high isolation can be attained in different design stages of the wireless communication systems. Several methodologies are applied in the antenna-stage, analog-stage, and digital-stage to obtain high isolation in each stage. High inter-port isolation in the antenna stage alleviates the requirement for isolation in the subsequent stages.

Two separate operating frequencies are employed in a dual-band full-duplex system, one for the transmission and another for the reception of the RF (Radio Frequency)signals. For the faithful performance of full-duplex systems, high isolation between the transmitter and the receiver is one of the essential requirements. High isolation can be obtained with the usage of different methods such as polarization diversity [21], differential feeding arrangement [22], introduction of defected ground structure [23, 24] etc. between the transmitting and receiving antenna. These aforementioned methods have constraints of inadequate isolation, which is one of the essential requirements for full-duplex systems. Owing to the requirements of high port-to-port isolation with high spectral efficiency, the design of a compact full-duplex antenna, operating in two different frequency bands is a challenging task.

In this article, a full duplex differential antenna with high isolation between the differential ports is presented. The designed antenna consists of two square cavities, named as outer and inner. The inner cavity is one nested into the outer cavity. The modified hybrid TE_130/310 mode of the outer cavity is differentially excited by a pair of coaxial feed lines, which radiates through a pair of arc-shaped slots at 9.35 GHz. To obtain radiations at 8.65 GHz, a TM₀₁ mode of the square patch is differentially excited by another pair of coaxial feed lines. With the simultaneous orthogonal differential excitation of the outer cavity and square patch, high isolation between the differential ports is obtained.

The salient key features of the proposed full-duplex antenna are as follows:

1. 1. The low profile of the proposed full-duplex antenna provides ease of integration with the planar circuits.

2. 2. The combination of differential feeding arrangement along with the dual-polarized radiation characteristics provides high isolation (better than 52 dB) in both the operating bands of the proposed full-duplex antenna.

3. 3. Due to the usage of SIW technology, the designed antenna exhibits high gain in both operating bands.

4. 4. The proposed antenna also exhibits the advantage of independent tunability of each operating band. Thus, the proposed full-duplex antenna has the flexibility to redesign it within X-band frequency spectrum.

5. 5. The proposed antenna provides very good polarization purity, i.e., the designed antenna has very low cross-pol levels (<−30 dB) with respect to co-pol in the broadside direction.

The designed antenna can be used for X-band radar applications including dual-polarized synthetic aperture radar for weather monitoring, air traffic control, maritime vessel traffic control [25–27].

Antenna configuration

Figure 1 shows the proposed differential full duplex antenna. The differential antenna is designed on RT/duroid 5880 substrate with dielectric constant 2.2, loss tangent 0.0009, and thickness of 0.508 mm. In the present design, there are two SIW cavities, named as outer and inner. The inner cavity is nested into the outer cavity. The side walls of cavities are obtained by introducing a series of metallic vias of diameter d with separation p. The values of d and p are chosen in such a way that there is confinement of electromagnetic energy within the SIW cavities. The outer cavity is excited by the differential Port 1 through a pair of coaxial feed lines. The diff. Port 1 comprises of coax Port 1+ and Port 1−. A square ring slot of dimension b and width w _i is introduced on the top surface of the inner cavity to obtain a square patch radiator of dimensions a × a. This square patch radiator is differentially excited through another pair of coaxial feed lines, which is designated as differential Port 2. The differential Port 2 consists of coax Port 2+ and Port 2−.

Figure 1. (a) Geometry of the proposed differential full duplex antenna (b) Photograph of fabricated prototype.

The terminals of the differential Port 1 are placed along the x-axis, while the terminals of the differential Port 2 are placed along the y-axis to obtain the orthogonal placement of the feeds. This orthogonal placement of the feeds is responsible for the high isolation between the differential ports. The design parameter s₁ is the center-to-center distance between the terminals (Port 1+, Port 1−) of the differential Port 1, while s₂ is the center-to-center distance between the terminals of the differential Port 2 (Port 2+, Port 2−). A pair of arc shaped slots of radius R, and subtended angle 2θ are etched on the top surface of the outer cavity to facilitate radiations at 9.35 GHz under differential Port 1 excitation. While the square patch radiator is responsible for the radiations at 8.65 GHz, under Port 2 excitation.

Operational mechanism

Square SIW cavity

The resonant frequency for TE_mn0 mode of the cavity is

(1)\begin{equation}{f_{mn0}} = \frac{1}{{2\pi \sqrt {\mu \varepsilon } }}\sqrt {{{\left( {\frac{{m\pi }}{{{a_{eff}}}}} \right)}^2} + {{\left( {\frac{{n\pi }}{{{b_{eff}}}}} \right)}^2}} \end{equation}

where μ and ε are permeability and permittivity of the dielectric material, a _eff and b _eff are effective dimensions of the cavity [28]. For the square cavity, a _eff = b _eff, m ≠ n, several pairs of degenerate modes are excited simultaneously inside SIW cavity, e.g. TE₁₃₀ and TE₃₁₀ modes can form one degenerate modal pair and the corresponding electric field distributions inside the square cavity are shown in Fig. 2(a), where positive sign indicates the upward direction and negative sign indicates the downward direction of the fields. Since they are degenerate modes, their fields will superimpose each other and resulting in hybrid TE_130/310 mode and the corresponding field distribution shown in Fig. 2(a). When the inner square cavity is introduced inside the outer cavity, the resonant frequency of hybrid TE_130/310 mode will shift from 6.65 GHz to 9.55 GHz and fields of the modified hybrid TE_130/310 mode will be confined between the outer and inner cavities as shown in Fig. 2(b). With the coaxial differential feeding, Port 1+ is fed with positive signal and Port 1− with negative signal of equal amplitude. With this feeding arrangement, the E-fields inside SIW cavity will modify accordingly. The vector electric fields for this differential feeding arrangement is shown in Fig. 2(b).

Figure 2. (a) E-fields for TE₁₃₀ mode, TE₃₁₀ mode and hybrid TE_130/310 mode (b) E-fields inside the cavity for modified hybrid TE_130/310 mode and vector E-field at 8.98 GHz under Port 1 excitation.

Proposed geometry

The proposed differential full duplex antenna is realized by inserting a pair of arc-shaped slots inside the outer cavity and a square ring-shaped slot inside the inner cavity. These slots are inserted at the top-surfaces of the cavities. With the placement of the arc-shaped slots, the resonant frequency of the modified hybrid TE_130/310 mode shifts to the lower value i.e. from 9.55 GHz to 9.35 GHz. This lowering is due to the strong reactive loading effect of these slots. This phenomenon can be better explained by plotting the contour of the electric field distribution in the outer cavity with Port 1 excitation [Fig. 3(a)]. When the arc-shaped slots are energized by the Port 1, it perturbs the fields of the modified hybrid TE_130/310 mode and results in x-polarized radiations at 9.35 GHz. Furthermore, the locations and lengths of these arc-shaped slots are fine adjusted for the better radiation characteristics.

Figure 3. (a) Electric-field isolines for modified hybrid TE_130/310 mode under differential Port 1 excitation (b) Vector electric field at 8.65 GHz under Port 2 excitation.

On the other hand, the other radiating frequency of the differential full duplex antenna is obtained by inserting a square ring slot of dimensions (b, w _i) on the top surface of the inner cavity. This square ring slot separates the inner cavity from the inner conductor and forms a floating square microstrip patch of dimensions a × a. When this square microstrip patch is differentially fed with a pair of coaxial feed lines, designated as Port 2+ and Port 2−, it will excite the odd order resonant modes such as TM₀₁, TM₂₁, TM₀₃, etc. and the remaining even order modes such as TM₀₂, TM₁₂, etc. will be suppressed. According to the cavity model of microstrip patch antenna [29], the resonant frequency f_mn of TM_mn mode of the differentially excited patch is given as

(2)\begin{equation}{f_{mn}} = \frac{c}{{2\sqrt {{\varepsilon _{eff}}} }}\sqrt {{{\left( {\frac{m}{{{W_P}}}} \right)}^2} + {{\left( {\frac{n}{{{L_P}}}} \right)}^2}} \end{equation}

where c represents the speed of light, ε _eff is the effective dielectric constant of the substrate, W _p and L _p indicates the patch width and patch length, respectively. For the square microstrip patch, W _P = L _P. The dimensions of the square microstrip patch a × a are chosen in such a way that it operates at 8.65 GHz. The electric field distribution of the square microstrip patch under differential Port 2 excitation in TM₀₁ mode results in y-polarized electromagnetic wave [Fig. 3(b)]. Figure 3 clearly demonstrates the dual polarized behavior of the proposed full-duplex antenna.

To minimize the mutual coupling of the fields between the outer cavity and square microstrip patch, the patch is differentially excited through differential Port 2, which is orthogonally placed w.r.t. to differential Port 1 of the outer cavity. Thus, high isolation is obtained between the differential ports, which facilitates the full duplex characteristics of the designed antenna.

Parametric analysis

To investigate the effect of different varying parameters on the radiating frequencies of the differential full duplex antenna, a parametric study is performed using Ansys HFSS 2020. The variations of the radiating frequencies for different slot and patch dimensions are presented in this section. During the parametric analysis, only one design parameter is varied, while the others are kept constant. By varying the angle θ of arc-shaped slots, only the first radiating frequencies of the differential full duplex antenna can be changed, while the other radiating frequency remains unchanged. The first radiating frequency can be independently tuned in the frequency range from 9.1–9.45 GHz by varying the value of θ from 70° to 50°mm, as shown in Fig. 4(a). The second radiating frequency of the differential full duplex antenna can be changed by varying the dimension a of the square microstrip patch. It can be tuned from 8.3 to 8.8 GHz, by changing the value of a from 11.25 to 10.75 mm, as depicted in Fig. 4(b). It is clear from Fig. 4 that during these variations, one of the radiating frequency bands changes while the other remains unaltered. Thus, it can be concluded that both the frequency bands of the proposed differential full duplex antenna can be controlled independently. Due to the orthogonal arrangement of the differential ports, excellent isolation, better than 50 dB, is also maintained.

Figure 4. (a) Simulated S-parameters of the proposed differential full duplex antenna for different values of θ (b) Simulated S-parameters of the proposed differential full duplex antenna for different sizes of inner square patch.

To have better understanding about feed locations, parametric analysis for the design parameters s₁ and s₂ are also performed and their results are plotted. Figure 5 shows the simulated S-parameters for different values of s₁ and s₂ for differential Port 1 and Port 2, respectively. As the value of s₁ increases, impedance bandwidth enhancement is observed. However, the value of s₁ cannot be increased beyond s₁ = 33 mm due to the presence of arc-shaped slots. Thus, s₁ = 33 mm, provides optimum feed location which is nearest to the radiating arc-shaped slots. Figure 5(b) shows simulated S-parameters for different values of s₂. It can be observed from the figure that designed antenna has good impedance match for the center-to-center distance s₂ = 3.0 mm of the differential Port 2.

Figure 5. S-parameters of the proposed full-duplex antenna (a) |S_11dd|-parameters for different values of s₁ (b) |S_22dd|-parameters for different values of s_2.

The design parameters of the full duplex antenna are: L = 56 mm, W = 56 mm, W _c = 49 mm, W _{c_inner} = 23 mm, R = 19 mm, a = 11 mm, b = 12 mm, s₁ = 33 mm, s₂ = 3 mm, d = 1 mm, p = 2 mm, w _i = 0.5 mm, w _o = 0.8 mm, θ = 60.

Design guidelines

Based on the aforementioned simulation studies, the design guidelines can be summarized as:

1. 1) Calculate the initial dimensions (L _c × W _c) of the outer cavity using (1) for the degenerate modes TE₁₃₀ and TE₃₁₀ modal pair.

2. 2) Place the nested inner cavity inside the outer cavity such that frequency of modified hybrid TE_130/310 mode lies in the X-band (8–12 GHz).

3. 3) Apply the differential coaxial feed lines (Port 1+ and Port 1−) to the outer cavity.

4. 4) Place a pair of arc-shaped slots in the outer cavity such it radiates in X-band. Tune the slot dimensions and location for better radiation characteristics.

5. 5) Insert a square ring slot of dimension b and width w _i in the nested inner cavity to obtain the floating microstrip patch of dimension a × a. Tune the dimensions such that TM₀₁ mode of the square patch lies in the X-band.

6. 6) Apply the differential coax feed lines (Port 2+ and Port 2−) to the microstrip patch along the orthogonal axes w.r.t. differential Port 1 to minimize the mutual coupling of the electric fields of the modified outer cavity and microstrip patch.

7. 7) Finally, the cavity and/or slot dimensions can be fine-tuned to adjust the frequency bands.

Results and discussion

To validate the proposed concept, the designed differential full duplex antenna is fabricated on RT/Duriod 5880 substrate and tested. The photograph of the fabricated prototype is shown in Fig. 1(b). The S-parameters of the fabricated antenna are measured using Agilent 5071C VNA, whereas radiation patterns and gains are measured inside the anechoic chamber. The differential S-parameters are evaluated as follows [1]:

\begin{equation*}{S_{11dd}} = \frac{1}{2}\left( {{S_{{1^ + }{1^ + }}} - {S_{{1^ + }{1^ - }}} - {S_{{1^ - }{1^ + }}} + {S_{{1^ - }{1^ - }}}} \right)\end{equation*}

\begin{equation*}{S_{22dd}} = \frac{1}{2}\left( {{S_{{2^ + }{2^ + }}} - {S_{{2^ + }{2^ - }}} - {S_{{2^ - }{2^ + }}} + {S_{{2^ - }{2^ - }}}} \right)\end{equation*}

\begin{equation*}{S_{21dd}} = \frac{1}{2}\left( {{S_{{2^ + }{1^ + }}} - {S_{{2^ + }{1^ - }}} - {S_{{2^ - }{1^ + }}} + {S_{{2^ - }{1^ - }}}} \right)\end{equation*}

Figure 6(a) shows the simulated and measured S-parameters of the designed differential full duplex antenna. The measured results are found to be good agreement with the simulated results. The fabricated full duplex differential antenna radiates at 9.35 GHz (9.25–9.45 GHz) under Port 1 excitation and at 8.65 GHz (8.55–8.85 GHz) under differential Port 2 excitation. An isolation better than 52 dB is observed between the differential ports. In terms of bandwidth, our designed full duplex antenna has the narrow bandwidth, an inherent characteristic of SIW-cavity-based antennas. However, the bandwidth provided by our antenna is more than sufficient for radar applications including dual-polarized synthetic aperture radar. Though, there are various methods that can be adopted to improve the impedance bandwidth. From cavity-backed antenna theory, it is well known that impedance bandwidth is directly proportional to cavity depth [30, 31]. The impedance bandwidth of the proposed antenna can be improved by increasing substrate thickness.

Figure 6. (a) Simulated and measured S-parameters of the designed full duplex differential antenna (b) Experimental set-up for antenna radiation pattern measurements.

Figure 6(b) shows the setup for the measurement. To measure the far field radiation pattern and gain of the fabricated differential full duplex antenna, a wideband 180° coupler is utilized to generate differential signals. The hybrid coupler has the difference (Δ) and the summation (Σ) ports. The first one is connected to the signal source whereas the latter one is connected to 50Ω matched load. During the measurement of radiation pattern with Port 1 excitation, the outputs of the hybrid coupler are connected to Port 1− and Port 1+ of the differential antenna, while the other ports (Port 2− and Port 2+) are terminated with 50 matched Ω loads. In the similar fashion, with Port 2 excitation, the outputs of the hybrid coupler are connected to Port 2− and Port 2+.

The simulated and measured radiation pattern of the designed differential full duplex antenna at 9.3 GHz (under diff. Port 1 excitation) and 8.65 GHz (under diff. Port 2 excitation) in both the principal planes are plotted in Fig. 7. The designed antenna has broadside radiation characteristics with high front-to-back ratio of 22 dB in the operating bands. The peak realized gains of full duplex differential antenna are plotted in Fig. 8 and are 7.6 dBi (under diff. Port 1 excitation at 9.35 GHz) and 6.1 dBi (under diff. Port 2 excitation at 8.65 GHz), respectively. The radiation efficiencies better than 80% are also observed.

Figure 7. Simulated and measured far field radiation patterns at (a) 8.65 GHz (diff. Port 2 ON) (b) 9.35 GHz (diff. Port 1 ON).

Figure 8. Peak realized gain of the proposed full-duplex differential antenna.

The designed full-duplex antenna system is compared with other state-of-the-art works reported in the literature. The comparison is performed in terms of size, polarization, gain, and port-to-port isolation and is depicted in Table 1. It is clear from the table that the performance of the designed antenna is comparable with the other antennas w.r.t. size, operating frequency, port-to-port isolation, and gain. Moreover, the designed antenna has high gain and excellent isolation along with good polarization purity that makes it a potential candidate for the full duplex systems operating in X-band.

Table 1. Comparison with other full duplex antennas

λ_g is the wavelength at the lowest operating frequency,.

Conclusion

A low-profile differential, full-duplex antenna is designed in this paper. The proposed antenna is fed by two pairs of differential coaxial feed lines. It radiates at 9.35 and 8.65 GHz, along with an isolation better than 52 dB between the differential ports. The operating bands of the designed differential antenna can be independently adjusted as per the requirements. The designed differential antenna eliminates the requirements of additional circuitry that provides high isolation between the differential ports. The proposed antenna provides excellent polarization purity, i.e., the designed antenna has very low cross-pol levels (<−30 dB) with respect to co-pol in the broadside direction. The designed antenna can be used for X-band radar applications, including dual-polarized synthetic aperture radar for weather monitoring, air traffic control, and maritime vessel traffic control.