A Concept of Advanced Design Governed by Theoretically Predicted Current Distributions on the Ground Plane Beneath an Aperture-Fed Microstrip Antenna

This work addresses a concern of aperture coupled microstrip patch which is commonly overlooked. Such feeding configuration is typically believed to be immune to high cross-polar (XP) radiations, which indeed is a paradox. It actually offers considerably low XP only across H-plane, but concerning high XP over its diagonal or skewed planes (azimuth $\approx 45^{\circ }$ -70°). An aperture feed, therefore, hardly reveals any advantageous feature in terms of the overall XP discrimination. Such a major shortcoming of aperture-fed microstrip, to the best of our knowledge, has been addressed and successfully resolved in this article for the first time. It explores a way of mitigating near field issues based on theoretical analysis and has proposed a simple strategic approach to reform the same for a rectangular patch. A representative design, theoretical justification, and experimental studies with an S-band prototype have been presented. XP suppression by 11dB has been experimentally achieved in the diagonal (D-) plane with no considerable changes in its H- or E-plane. That eventually attains an overall XP discrimination by nearly 27dB from the perspective of 3D radiation scenario. The proposed technique hardly affects the co-polar radiations or gain of its traditional design. Moreover, this is satisfactorily functioning for a $2\times 2$ sub-array with a remarkable co-to-cross isolation by about 34dB over the entire radiation planes.

T HE MICROSTRIP antenna community started realizing the significance of cross polarized (XP) radiation and its accurate estimation when Hansen [1], Huynh et al. [2], and Lee et al. [3] initiated the analytical studies. Their analyses identified the orthogonal modes as the XP sources and systematically quantified them as functions of azimuth angle ϕ indicating the minimum occurring at ϕ ≈ 0 0 , maximum around ϕ ≈ ±45 0 and moderate values near ϕ ≈ 90 0 . As a result, the cross polar discrimination (XPD) bears a minimum value over the skewed region spanning over ϕ ≈ 45 • -70 • .
Very recently, a few investigations have tried to address the D-plane issues [18], [19], [20], [21]. The work in [18] dealt with theory only without any attempt of minimizing it. The studies in [19], [20], [21] demonstrated some degree of improvements for standalone probe-fed patches by introducing defects on the ground plane and as such none of these were found suitable to accommodate corporate feeds required for the array configurations.
The present investigation addresses a different engineering concern that relates to the aperture-fed version of a microstrip patch. It is commonly known for its lowest XP feature [22], [23] but that is true only over ϕ = 90 • plane. Consistent high XP level remains alarming over the D-plane (around ϕ = 45 • ). This work, to the best of our knowledge, addresses the above concern based on a theoretical understanding and proposes a simple commercially viable solution for the first time. The analysis reveals that, besides the orthogonal modes, near magnetic field components such as H X and/or H Y also play vital roles in contributing to the XP radiations over the ϕ planes. This knowledge has been explored in optimizing the favorable near field components which are caused by the surface conduction currents. Removal of conduction currents around the ground plane (GP) corners has been identified and executed in the present study. But the actual solution is not straight forward at all. It needs further consideration of introducing compensatory current paths which have been thoroughly studied in steps.
The final outcome appears a fully planar GP with strategic shaping. This is commercially viable and promises overall XP reduction by 14 dB over the skewed planes and about 7 dB across the so called H-plane. Most significantly, the engineered ground hardly causes any change in the primary radiation or gain values. A thorough comparison with respect to the earlier endeavor has been presented.
The proposed idea has further been examined for an array of elements. A representative sample of 2×2 aperture coupled array with corporate feed bearing identical ground plane engineering has been demonstrated with a promise for 12dB reduction in XP level over the skewed or D-plane. That in turn results in overall 3D co-to-cross isolation around 34dB.
This method is straightforward, planar, and does not incorporate any additional structures. The final product is easy to fabricate and cost effective. Moreover, it allows adequate flexibility in the design based on the primary element shape and size. Utilizing the concept in array may find a possible scope for further exploration because D-plane rectification is indeed a challenge in view of a key need of adaptive arrays that requires wide three-dimensional (3D) signal coverage throughout the entire azimuth plane [24], [25], [26].

II. THEORY AND ANALYSIS
The proposed design has been guided with the help of theoretical interpretation about the XP controlling factors as a function of GP currents and its associated magnetic fields in the substrate. Total radiation fields due to a microstrip patch is contributed by two sources: (i) equivalent magnetic current source across the radiating aperture, and (ii) the induced current on the finite ground [27], [28]. This leads to the instantaneous near-field as [28]: where, E G (θ , ϕ) and E P (θ , ϕ) are near electric fields contributed by ground and patch currents respectively. Since our aim is to investigate the contribution of the GP only, (1) can be written exclusively: where A G (θ , ϕ) represents the magnetic vector potential derived from GP current sources (J S ) and can be expressed as: It splits into two parts in the Cartesian coordinate as As one can represent J S (x, y) = J(x)î + J(y)ĵ, (4) takes the form as: Ludwig's 3 rd definition [29] provides cross-polarized radiation employing only ground plane contribution as Which with the help of (2) -(4) helps in estimating XP over ϕ = 45 • (D-plane) and 90 • (H-plane): Again, Js is related to its source, the near magnetic field H which manifests the signature of GP conduction current as: n being unit vector normal to GP surface.  The theory indicates higher J X (& H Y ) in (7) (& (8)) should results in lower XP over the D-pane. Lower J Y (& H X ) values should also help in reducing XP in both D-and H-planes. Interestingly, a strategic increment of J X can successfully dominate the influence of J Y in the structure. This has been shown explicitly in Table 1.

III. CONCEPT BUILDING AND DESIGN A. DESIGN INSIGHT: GP CONFIGURATION -I
We, therefore, address case (i) in Table 1 and primarily focused on enhancing the source field H Y . Simulated portrays in Fig. 1 reveal the identical patterns of H and J S . Both H Y and J X are noticeably low around the GP corners of a conventional aperture-fed patch antenna. In contrary, H X (also J Y ) appears high towards the corner region, which according to conditions (ii) and (iii) of Table 1 preferably need to be low. Therefore, one may recommend removal of GP corner metal beneath the substrate to get rid of low H Y (unfavorable) and high H X (unfavorable). Then one Table 2.

TABLE 2. Antenna parameters / dimensions (mm).
may surmise a modified or favorable geometry as shown in Fig. 2. It improvises the GP corners and transforms it to a rhombic-like shape. Henceforth, we examine the modified surface currents for a representative unit that operates in S-band. Fig. 3 portrays the surface currents components and provides some useful information of GP configuration-I.
(a) Compared to Fig. 1(b), 'Low J X ' area has been significantly reduced in Fig. 3 (a). Thus, it supports condition (i) of Table 1 favorably.   (b) Adequate 'Low J Y ' (observed in Fig. 1 (b)) is maintained in Fig. 3(b).
A more indicative result is shown in Fig. 4 which compares the change in J X /J Y ratio and ensures improved J X /J Y  in the proposed configuration. Their impact on the radiation characteristics has been examined in Fig. 5. The peak gain is found to be reduced by about 1.5 dB compared to the reference antenna and this justifies reduction in the effective radiating aperture caused by GP shaping. The H-plane pattern gets noticeably wider due to the same reason. But the most important achievement is its XP suppression over Dplanes ( Fig. 5(a)). It promises to be around 10dB although an equal order of degradation happens in H-plane ( Fig. 5(b)). A tendency of a relative hike is also apparent in the Eplane XP values (Fig. 5(c)). Such XP behavior may be attributed to the improvised J X and J Y distributions on GP  Table 2. configuration -I. That complies with our primary goal, i.e., to obtain the lowest possible XP over the D-plane. The new balance in J X and J Y however, does not support the most ideal situation (case (iii) in Table 1: maximum J X with minimum J Y revealing no degradation in the primary radiation) in producing the lowest XP radiation over E-or H-plane. Those values in aperture-fed patch are inherently low (∼ −35dBi) and hence even after some degrees of increment, the XP remains adequately low, say about −28dBi in H and −40dBi in E-planes. Hence, the improvisation as in GP configuration-II does not cause any harm from  engineering point of view. Rather it improves the overall XP scenario from the 3D perspective.
But, this trend of increment in E-and H-pane XP is not a healthy indication of the new design. So, in here, a further scope arises to improvise the electrical parameters of   Parameters as in Fig. 12 and Table 2.

B. GP CONFIGURATION-II
The aim of this whole investigation is to enhance x-polarized current but at the same instance the configuration-I should remain unperturbed. A pair of thin metal strips of variable length S L as shown in Fig. 6 has been introduced. We prefer to call it as 'GP Configuration-II'. Its impact on overall J X as a function of S L dimension has been examined in Fig. 7. GP-II with S L = 0 degenerates to GP-I ( Fig. 8(a)). As S L is increased to S L ≈ 0.3λ, the GP area containing weak J X increases (region R.1, R.2 as shown in Fig. 8(b)). At the same time, J X concentrates on the narrow strips. But with S L ≈ 0.8λ = GL (Fig. 8 (c)), J X regains its distribution over relatively larger area. Its impact has been studied in Fig. 9. The full-length S L (≈ 0.8λ= GL) appears relatively improved compared to other versions bearing S L < GL. GP-II is larger in size compared to GP-I and that is manifested through an increment in peak gain by about 1.5 dB and narrowing down the H-plane pattern.
One may notice in Fig. 8 that both D-and H-plane XP levels drastically increase when the newly introduced S L ≈ 0.3λ. As per Table 1, condition (ii), increase in J Y might be a reason behind this. This has been cross verified in Fig. 10. Parameter J Y has been compared across two arbitrary lines LL 1 & PP 1 (on Fig. 6) and they are plotted in Fig. 9. The case S L ≈ 0.3λ indicates a sharp rise in J Y over LL 1 and a nominal rise over PP 1 .
A further increase in S L as shown in Fig. 10(a) is also possible but that does not help in achieving our goal; both H-and D-plane XP values become worse as shown in Figs. 10 (b, c). Thus, GP-II with S L = GL appears to be the best choice and its XP performance compared to that in GP-I and a conventional reference patch has been shown in Fig. 11. The simulated portrays indeed (Fig. 11) help in visualizing the degree of 3D relative suppression by GP-II. As a result, we limited the rest of our investigations to GP configuration-II exclusively.

IV. DESIGN FEASIBILITIES IN ARRAYS
The studies so far were focused to a standalone patch on GP config-II ensuring its utility in reducing the overall XP level Fig. 12 and Table 2. encompassing both diagonal and orthogonal planes. Whether the same GP-II is equally applicable to array configuration is examined in this section. A 2×2 array of identical patches (Fig. 12) indicates almost no change in S 11 characteristics with respect to its conventional version as shown in Fig. 13. But as predicted, the XP radiations get significantly reduced in D-plane as shown in Fig. 14. Over the H-and E-plane, the XP values show no noticeable relative changes. Thus, about 10 dB suppression in D-plane eventually results in uniform cross-polar discrimination over entire 3D patterns by nearly 30dB. This scenario can be visualized from the  simulated portray of the same as depicted in Fig. 15. In a color diagram, 'red' indicates the strongest and 'blue' indicates is weakest radiation and thus the figure is selfexplanatory. The strip loaded rhombic GP is also successfully suppressing the XP radiations across the entire azimuth plane (ϕ).

V. IMPACT OF ANTENNA PARAMETERS: POSSIBLE DESIGN VARIATION
An engineering aspect of the proposed design using a range of commercially available PTFE substrates has been critically studied. Substrate permittivity ε r2 for the feed line has been VOLUME 4, 2023 467 Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply. varied, keeping that for the microstrip element unchanged. As indicated by the plots in Fig. 16 (a), the peak XP levels (for both H-and D-planes) decrease with an increase in ε r2 values up to a certain minima and then rise up. It is important to note that the lowest possible XP values are closely associated with the maximum order of XP suppression as highlighted by a shaded area. The optimum results are revealed with ε r2 ranging from 9.5 to 12.2 for the present study. But its impact on the bandwidth and gain as examined in Fig. 16(b) does not appear so significant. It, therefore, seems that the relative dielectric constant of the bottom substrate has to be restricted within the range of 10 to 12 to guarantee the lowest possible XP with highest possible reduction.

VI. PROTOTYPES AND EXPERIMENTAL RESULTS
A set of S-band prototypes, each being split in two units, are shown in Fig. 17. The final design using the proposed configuration-II has been compared with a conventional counterpart. The feed line unit for aperture coupled patch is shown as inset. They were measured using E8363B network analyzer and an automated anechoic chamber. Fig. 18 compares the measured S 11 values with simulated predictions for both conventional and config.-II types. Excellent mutual agreement is revealed.
The proposed antenna experiences a little left shift in resonance which perhaps is caused by some inductive loading through redistributed conduction current.
Their radiation characteristics along with respective simulated predictions are examined in Fig. 19. The measurement setup allows capturing reliable data up to 150 • azimuth plane. The co-pol patterns closely follow the simulated predictions and indicate no major change in gain and beamwidth compared to the conventional geometry. Reduction in D-plane XP level by about 11dB is ensured by the measured data, but the dips around ±55 • present in the simulated XP pattern is found to be absent. Some degree of misalignment especially for D-plane measurement may be attributed to this. Measured H-plane XP values, as predicted by the simulated data, remain comparable with the conventional case, and they appear below −30dB. It ensures peak to peak co-to-cross-polar isolation about 34 dB in H-plane. E-plane XP level is identical with the H-plane values and thus, our proposed configuration II experimentally promises over 26dB co-to-cross-polar isolation in 3D scenario. A comprehensive comparison with the earlier investigations is presented in Table 3 which has been discussed later in conclusion section.

VII. CONCLUSION
The design is straightforward, low cost, commercially viable, and easy to implement. It, for the first time, shows how a strategic perturbation causes minimal removal of effective ground plane that results in no loss in gain. This indeed is one of the major advantages compared to earlier DGS based D-plane XP reduction techniques [19], [20], [21].
The proposed design does not require any additional hardware or critical machining. It, therefore, should support both standalone patch and planar arrays effectively in terms of improved polarization purity over the entire radiation planes.
A comprehensive comparison with a set of earlier representative designs has been documented in Table 3. Very few of them addressed D-plane properties. The major advantage is its ability in accommodating corporate feed networks which was noticeably missing in the earlier investigations. Our proposed approach appears relatively improved and commercially viable compared to all of those.