SIW Sub-Array Antenna With High Isolation Offering Dual-Polarized Monopulse Patterns

A high isolation antenna is presented which offers dual-polarization monopulse beam patterns, making the design suitable for polarization diverse or full-duplex (FD) systems. The single-layer structure is defined by a network of 2-D sub-arrays oriented in a cross-shape configuration using series-fed slots in substrate integrated waveguide (SIW) technology. Also, the proposed design exploits dual-differential feeding to achieve a −10 dB impedance matching bandwidth (BW) from 23.2 GHz to 25.4 GHz, and measured isolation values are more than 70 dB (peaking to about 90 dB) over this same operating range. Realized gain values are 13 dBi (and above) while cross-polarization values are less than 35 dB below the beam maximum. The novelty in the design can be defined by the inclusion of a simple SIW isolation element at the center of the antenna structure. This provides enhanced isolation and enables co-location for the vertical and horizontal SIW sub-array branches, and this simple and low-profile arrangement supports efficient dual-linear polarized radiation. To the best knowledge of the authors, no similar 2-D planar antenna design has been previously reported offering high isolation in the K-band and which can offer sum and difference beam patterns in the far-field (FF). Some applications include monopulse tracking setups for communications as well as in-band FD systems which require dual-polarization at microwave and millimeter-wave frequencies.


I. INTRODUCTION
T HE MICROWAVE and millimeter-wave frequency bands can offer high data capacity throughputs and Gigabit transmission speeds for modern 5G communications, monopulse radar tracking systems, and direction of arrival estimation [1], [2], [3], [4], [5], [6].Additionally, 5G offers lower latencies and high data rates suitable for Internet of Things (IoT) devices and automotive radar with communications, for example, where information can be analyzed on flight [7], [8].For these modern and complex scenarios, monopulse and dual-polarization antennas as well as fullduplex (FD) setups can offer significant advancements when compared to more standard single-polarization, half-duplex, and simplex versions.This has made polarization-diverse systems and in-band FD antennas a topic of interest.
Such diversity and FD concepts are well established in the literature and support simultaneous transmission and reception (STAR) by signal orthogonality, however, some technical challenges still remain [1], [2], [3], [9], [10], [11], [12].In particular, isolation and self-interference (SI) are problems that must be overcome and mitigation techniques can include the appropriate signal processing [13], [14].Classic approaches to enhance isolation include the use of circulators or spatially separated transmit and receive antennas.These strategies not only complicate the antenna design, but can also make the terminal bulky, expensive, and require an increased size for the physical antenna.In addition, in-band full-duplex (IBFD) STAR systems are generally more preferable as they can support efficient spectrum use, however, this can require a complicated antenna system demanding high levels of SI reduction.
A technique recently reported to achieve high isolation was to employ dual-polarization and co-located antennas as well as single-or dual-differential feeding [15], [16], [17], [18].Those systems are based on a 0 • and 180 • phase difference at the antenna ports (APs).This approach not only reduces SI and improves the isolation, but decreases the crosspolarization levels in the far-field (FF) and this enhances signal quality.To achieve such phasing and excitation for these multi-port antennas, external or internal feed systems are required as in [18], [19], [20], [21], [22].
One such antenna using a multi-layer configuration was proposed in [19].The structure was matched from 27.6 to 29.5 GHz with isolation levels up to 60 dB.In [20] an ultra-wideband antenna with monocones and bent loops was proposed.Using an external coupler system, the reported antenna matching bandwidth (BW) was 98% with isolation over 50 dB.Another Ka-band design was reported in [22] where a dual-polarized antenna with a lens system was proposed.The antenna consisted of a circulator-based feeding network, a dual-polarized horn, and a spherical lens.Isolation values were 30 to 50 dB.In [23] a W-band reflectarray was reported with isolation levels of almost 70 dB.Similar to [19], [23] also employed a multi-layer pillbox transition system for feeding.
Following these trends to offer new antenna systems with high isolation for FD and to meet the needs for 5G point-topoint communication systems as well as monopulse radar, whilst generating sum ( ) and difference ( ) patterns [6], a number of monopulse antennas have been recently reported.For example, a conical 4-port monopulse antenna system was proposed in [24].The antenna was printed on a conical conformal PCB with an integrated feeding circuit for sum and difference operation.The antenna was well matched from 9.0 GHz to 11.2 GHz, and depending on the excited port, the beam-pattern and polarization control was established.Similarly, a 2-port 2×2 multilayer type microstrip patch array with monopulse characteristics was proposed in [25].The antenna shared bandwidth was from 9.3 GHz to 9.5 GHz with isolation levels below 40 dB.Recently, another monopulse design was reported in [26] using substrate integrated waveguide (SIW) technology in a 2×2 triple-layer antenna system.The antenna shared BW was 2% for all ports for a center frequency of 9.8 GHz and a reported coupling of around −30 dB.
In [27], a dual-layer 2×2 patch antenna-array was also proposed where two hybrid couplers were connected to the radiating elements to produce a sum and difference beam pattern.While the design offers isolation of around 58 dB it is only capable of producing two orthogonal difference beam patterns and a single-sum pattern.The structure also offers a 2% BW which is likely related to antenna manufacturing and feeding, in particular, a probe-like connection impinging the PCB.In another work, [28], a dual-port design was presented where an SIW configuration and cross-shaped dipoles were implemented to produce sum and difference patterns and circular polarization.The reported isolation levels were around 20 dB and the radiating BW was reduced to 2% due to the reported axial ratio challenges.
In an effort to advance on these previous developments and propose a simple and single-layer monopulse antenna design suitable for FD scenarios, communications, and tracking systems, etc., which offers dual-linear polarization, high isolation (reaching values of 95 dB), and broadband impedance matching, we report on an alternative and more advanced configuration (see Fig. 1) when compared to the classic monopulse antenna presented earlier in [29], for example.That work employed a continuous 1-D array of series-fed microstrip patches for (single) linearly-polarized radiation.
Our newly proposed 2-D antenna (see Fig. 1), which offers dual-polarization, uses a co-located arrangement of four series-fed slot arrays in SIW technology.Moreover, our SIW structure is realized by the appropriate placement of sub-arrays orientated in a cross-shaped network.Also, the series-fed slots employ tapering for improved radiation and matching.A square isolation structure, and with missing slot elements, is positioned at the center of the antenna to enable co-location; i.e., the square via wall as shown in see Fig. 1(a).A general comparison to the more a classic structure, which is defined by two-sided feeding of a 1-D slot array and our proposed design, is illustrated in Fig. 2(a) and (b), respectively.With our newly developed 2-D structure, and also by the appropriate dual-differential or dual-common feeding (see Table 1 and Fig. 3), dual-linear polarization sum or difference beam patterns; i.e., or (or both) are respectively made possible in the far-field (FF) by the four-port 2-D antenna system.Such diversity in the possible beam patterns is not realizable with earlier designs like those in [29], and, as in Fig. 2(a).
As outlined in Table 2, our reported SIW structure is compared to existing K-band antenna systems as found in the literature.As it can be observed, our proposed design offers the highest levels of isolation with the lowest crosspolarization levels (−30 dB or more) for the monopulse sum and difference patterns, and, over the largest BW (≈10%).To the best knowledge of the authors no similar antenna configuration, with similar metrics and possible beam patterns with different polarization states has been reported previously.

II. DESIGN CONSIDERATIONS AND SIMULATIONS
The proposed planar antenna consists of a simple and compact arrangement of sub-array SIW slots (series-fed), positioned in a cross-shape orientation as shown in Fig. 1.The SIW lines can be driven with an internal feeding circuit or connected to a conventional 50-microstrip transition for external feeding (see Figs. 1 and 3).In addition, tapering is used when designing the slot widths for each subarray to improve matching and decrease the sidelobe levels (SLLs).By this configuration, the horizontal and vertical SIW branches offer controlled leakage and high isolation is achieved over the operating BW of the antenna.

A. DIFFERENT POLARIZATION STATES AND FEEDING
The PCB material choice was Taconic TLY-5 with a relative permittivity of 2.2 and a thickness of 0.51 mm.Also, the general design procedure for the proposed 2-D SIW slot  antenna is outlined in Fig. 4. Should it be required, antenna gain can also be improved by increasing the number of slots and this follows conventional SIW series-fed slot array design.Also, the antenna structure was designed and simulated in CST microwave studio and the results are in Figs. 5 to 8.
The antenna configurations for the different polarization states can be seen in Fig. 3 with responses compiled in Table 1, and this expands on our preliminary findings in [30] which only reported sum pattern simulations.In addition, the structure proposed in this paper exploits a newly recessed ground plane (see Fig. 1(a)), which is an advancement from [30], in that such a reduced ground plane configuration can minimize surface wave excitations and improve antenna radiation.Moreover, when the newly reported antenna herein; i.e., in this paper, is excited considering dual-common feeding as applied to antenna ports 1 (AP1) and 2 (AP2), or antenna ports 3 (AP3) and 4 (AP4) (see Fig. 1), a differencelike FF beam pattern ( ) can be realized in both principal planes.To illustrate the possible FF beam patterns, 3D views are reported in Fig. 5 considering the possible feeding arrangements; i.e., differential and common.
This feeding setup can realize a dual-linearly polarized antenna which can be useful for monopulse.The Fparameters (or active S-parameters) can be observed in Fig. 6 where the antenna is well matched from 23.2 GHz to 25.3 GHz and the isolation reaches 40 dB at 23.5 GHz.The antenna beam patterns are reported in Fig. 8 and the crosspolarization levels are 30 dB below (or more).The realized gain of the antenna reaches 13 dBi while the simulated total efficiency is more than 85% and 90% when connectors are respectively included and excluded in CST.Here total efficiency is defined as the ratio of the radiated power to the stimulated power driven at the antenna ports, taking into account reflection losses and material losses.
When dual-differential feeding is applied, dual-linearly polarized radiation is made possible with a FF sum pattern ( ).The F-parameters can be observed in Fig. 6 with −10 dB matching from 23.2 GHz to 25.3 GHz and with isolation levels of more than 85 dB.Also, the simulated electric field within the substrate is shown in Fig. 7a) and it can be observed that minimal leakage occurs to the vertical SIW transmission line branches.Like this dual-differential feeding case, leakage of the fields is also minimal (see Fig. 7b)) for common/differential feeding.Basically, good isolation of the branches can be observed for both possible cases.Also, the sum patterns are reported in Fig. 8b) where the cross-polarization levels can be observed to be about 30 dB (or more).

B. APPROACH TO ACHIEVE DUAL-POLARIZATION
To enable dual-polarization, the center radiating slots elements for the vertical and horizontal SIW branches were removed and isolation vias were added, mainly, to enable array co-location whilst maintaining symmetry and defining the sub-arrays.This isolation element also makes each horizontal and vertical SIW antennas sparse [31], [32], [33] when compared to the more conventional series-fed array as defined in [29] (realized by series-fed microstrip patches), for example.The adopted square via ring arrangement (see inset Fig. 1a)) supports high isolation and dual-polarization as in [34] for an S-band antenna.This an important design feature, which in our understanding has not been employed previously for SIW slot arrays at millimeter-wave frequencies.
If a more conventional design and feeding approach was implemented for the series-fed arrays, as in [29], all the SIW sections would need to maintain the center radiating slots and no center square via ring would be included.More importantly, without this simple isolation element (see Fig. 1a), the vertical and horizontal placement of the SIW branches would not be possible, as well as, dual-polarization.We also carried out studies without this square via wall, and results suggest that the isolation can decrease to about 6 dB (all results not reported for brevity), making the design not ideal for polarization diversity and FD scenarios due to the noted SI.Also, due to the absence of the via ring, the beam pattern (polarization) purity was reduced as well as general antenna performance due to the increased SIW branch coupling.For these important reasons, the optimized and fabricated structure included the noted square via ring for polarization purity and enhanced isolation.

C. FURTHER DESIGN CONSIDERATIONS
To further discuss our design choices for the proposed SIW structure, we should mention that additional studies were completed in the optmization stages and prior to fabrication.For example, when the antenna was simulated with practical connectors (SuperSMA [35]), we noticed that minor ripples were observed in the beam pattern.
Further simulations indicated that parasitic surface-wave fields were generated by the connector and transition (similar to [36]).To minimize these effects, the majority of the ground planes were removed (whilst maintaining the SIW transmission lines and the square PCB shape, see Fig. 1).This slightly reduced antenna gain by about 0.4 dBi and increased the SLL by at most 1.2 dB.Other design choices increased the SLL by more than 1.5 dB; i.e., SLLs were −6 (or more) dB which we deemed not acceptable.
It should also be mentioned that the front-to-back ratio decreased when the ground plane was reduced.In particular, front-to-back ratios were about 20 dB and 12 dB for a complete and reduced ground plane, respectively, considering the same and employed substrate size.However, the peak realized gain was still relatively stable as noted and for both cases.Regardless of these findings and given that one of our main design motivations was to reduce sidelobes, the main SLL for the final and optimized design with connectors and reduced ground plane was −7 dB (see Fig.  in the worst case and more than −12 dB at 25 GHz (at best) as further described next.

D. SIDELOBE LEVEL MITIGATION
Due to the noted missing elements at the center of the structure, because of the added square via ring for dualpolarization, a higher SLL is observed in the FF than when compared to a more standard array.This SLL issue is more problematic for the dual-differential feeding case.For example, SLLs are about −7 dB (or lower) as shown in Fig. 8(b).While it is indeed possible to design a dual-polarized slot [37], the required dual-layer configuration will contribute to a higher cost, potential alignment errors, and other Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.1).The peak realized gain is also shown (right axis).Analogous results are observed when feeding AP3 and AP4.manufacturing tolerances that could be problematic when operating at microwave and millimeter-wave frequencies.
In an effort to generally reduce the SLLs, tapered radiation of the slots was introduced and it was noticed that this also helped to provide better matching than without.Furthermore, additional simulations were completed to investigate how the number of slots can help to minimize the SLL.In our prototype, there are 10 elements in total for the vertical and horizontal SIW branches (5 per each sub-array), and as mentioned, SLLs of about −7 dB (in the worst case) were observed at 24 GHz (see Fig. 8).When considering 20 elements per branch, the SLL improved to −8.5 dB, and finally, when considering 32 slot elements the SLL reached −10 dB at 24 GHz (note: lower values were observed for other frequencies).However, the matching impedance BW became narrower for the long arrays and since we are trying to make a competitive design when compared to other dualpolarization and monopulse antennas in the literature (see Table 2), we chose to manufacture a prototype for proofof-concept with 5 slots per sub-array (10 per branch, see Figs. 1 and 3).Basically, a compromise was chosen between SLL and the −10 dB impedance matching BW.
With all these details in mind, our overall antenna development process can be summarized by the following three critical steps: (1) design co-located arrays which are orthogonal, (2) enhance the isolation and inadvertently make the array sparse by adding a centre isolation element, and, (3) include tapering for SLL mitigation.This general methodology is further illustrated in Fig. 4.

III. ANTENNA MEASUREMENTS AND DISCUSSIONS
The radiation performance of the antenna was measured using a NSI-2000 near-field planar scanner.The measurements for common and differential feeding (as applied to AP1 and AP2) can be seen in Figs. 9 and 10.Similar results were observed for AP3 and AP4 for the other polarization states.The slightly higher cross-polarization levels (when compared to the simulations) are most likely related to some minor phase imbalances of the coupler, cable bending connections, and other practical tolerances.Regardless, the simulated and measured realized gain is in good agreement as observed in Fig. 9 with values about 14 dBi and 12 dBi for the sum pattern (differential feeding) and the difference patterns (common feeding).
The S-parameter measurements were completed using a Keysight PNA (N5234A).Measurement results are compared with simulations in Fig. 11.The passive reflection coefficient was first measured showing that the antenna is well-matched with a −10 dB impedance BW from 23.2 GHz to 25.4 GHz.To measure the coupling coefficients (or isolation) for the antenna system as outlined in Fig. 3, two external couplers (Krytar 4060265 [38]) were used for the common and differential feeding arrangements.It should also be mentioned that ideal couplers were defined in CST.Regardless, results are in general agreement.
When both couplers are connected for a common power split (difference beam pattern), the measured isolation is less than about 50 dB.When both couplers were configured for the dual-differential feeding case (sum patterns), the simulated response showed more than 80 dB of isolation, while the measurements only showed values of 65 dB (or more, approaching 90 dB near 24 GHz).Lastly, when the system was configured for common and differential feeding, the isolation was below 50 dB and reached about 65 dB at the center frequency.Those differences in simulations and measurements for the isolation responses are most likely related to the following factors: (i) the cable bending near the coupler, and (ii), nonideal magnitude and phase imbalances from the coupler.For example, measurements and further investigations (not reported for brevity) showed that the coupler imbalances can reach up to ± 10 • for frequencies where the antenna operates.Regardless of these practical effects, the general trend for the isolation responses for the feeding configurations are in agreement when comparing the simulations and measurements.

IV. CONCLUSION
A dual-polarized single-layer monopulse SIW slot antenna is presented.The planar design uses a newtwork of sub-arrays   1) and system ports (Fig. 3).For the simulations, an ideal coupler was considered while measurements employed a pair of 180 • hybrid couplers.and external couplers to provide dual-common and dualdifferential feeding which generates sum and difference beam patterns respectively in the FF.Also, the antenna is well matched at millimeter-wave frequencies making it suitable for 5G communication applications, IBFD scenarios, radar tracking setups, and monopulse.Measured peak isolation levels values are about 50 dB and 90 dB for the dual-common and dual-differential feeding arrangements, respectively.The simple design approach makes it attractive for low-cost integration within FD systems, diversity scenarios, and monopulse setups and where high isolation levels and broad BWs are required.

FIGURE 1 .
FIGURE 1. Proposed SIW-based planar antenna configuration defined by series-fed slots: a) top view, b) bottom view.The structure is defined by two pairs of sub-arrays, one for each polarization (i.e., the vertical or horizontal arrangements), with a square isolation structure and missing slot elements at the center to enable antenna co-location.

FIGURE 2 .
FIGURE 2. (a): Conventional implementation offering single-polarization only.(b): Proposed 2-D monopulse antenna using four sub-arrays which can offer dual-polarization and high branch isolation.

FIGURE 3 .
FIGURE 3. Possible feeding configurations for AP1+AP2 and AP3+AP4 defining the vertical (y-z) and horizontal (x-z) states of antenna operation, respectively (note: illustration not to scale).

FIGURE 4 .
FIGURE 4. Illustration of the developed design procedure.Step 1: design two series-fed arrays which are orthogonal.Step 2: introduce a center element to enhance the isolation between the arrays.This also separates the structure into sub-arrays.Step 3: optimize further by introducing slot width tapering for improved input matching, SLL reduction, and generally improved radiation performances.This tapering mitigates the high SLL due to center missing elements.

FIGURE 5 .
FIGURE 5. Simulated 3-D sum and difference beam patterns at 24 GHz (normalized) defining dual-linearly polarized radiation: a) sum and b) difference pattern by driving AP1 and AP2; c) sum and d) difference pattern by driving AP3 and AP4.

FIGURE 6 .
FIGURE 6. Simulated active S-parameters (or F-parameters from CST) for the different pattern types: sum pattern (dashed line) and difference pattern (solid line) by driving AP1 and AP2 (see Table1).The peak realized gain is also shown (right axis).Analogous results are observed when feeding AP3 and AP4.

FIGURE 8 .
FIGURE 8. Simulated FF patterns (normalized) in the y-z plane, with the applied phase shifts at AP1 and AP2: a) difference pattern and b) sum pattern.Analogous results were observed in the x-z plane for AP3 and AP4 with similar feeding.

FIGURE 9 .
FIGURE 9. Maximum realized gain for the proposed antenna prototype for polarization state 1 and 2. Simulations and measurements are in good agreement with about 13 dBi (or more) for common or differential feeding.

FIGURE 10 .
FIGURE 10.Measured FF patterns (normalized) in the y-z plane, with the applied phase shifts at AP1 and AP2: difference pattern and b) sum pattern.Similar results in the x-z plane for like feeding.

FIGURE 11 .
FIGURE 11.Simulated and measured reflection and coupling for the antenna itself(Fig.1) and system ports (Fig.3).For the simulations, an ideal coupler was considered while measurements employed a pair of 180 • hybrid couplers.