A Technique to Realize Aperture Coupled Microstrip Patch as a Truly Low Cross-Polar Antenna by Mitigating the Major Issues Over its Skewed Radiation Planes

Aperture-fed microstrip patch is typically known for its low cross-polarized (XP) radiations, however it stands true only over its orthogonal plane. The same, over the diagonal planes are as high as that for any other common feeds. This specific issue has been addressed and successfully resolved by proposing a technique for the first time. This introduces a pair of additional printed loops adjacent to the radiating patch. Detailed investigations leading to the design insight have been presented. A C-band rectangular patch promises as much as 7 dB suppression in the diagonal plane XP level keeping the overall performance across the principal radiation planes unchanged. This is an important step in keeping the XP levels for an aperture-fed antenna truly low thus maintaining more than 22 dB of co- to cross-polar isolation over the entire azimuth.

However, aperture-coupling has its own advantages. Its XP radiation is inherently low over both E-and H-planes [1], [4] and hence, it is considered an ideal candidate for dual-polarized applications [25], [26], [27]. However, this is a paradox since the XP level remains significantly high over the skewed planes that pass through around 45 • azimuth. This is commonly known as a diagonal plane, or D-plane, and is frequently ignored. Such high XP levels sometimes impose limitations on antenna applications [28].
Indeed, these D-plane XP issues are not straightforward to deal with [28]. The sources behind such phenomenon were not known until the recent investigations in [16], [17], [29], [30]. According to [17], the D-plane XP fields do not originate from the so-called orthogonal mode; rather they are caused by antenna near fields. As a result, a balanced optimization of those source fields or their equivalent source currents (J x , J y , the primary resonance being x-polarized) becomes tricky. This paper proposes a compensating current loop for an aperture-fed rectangular patch to obtain a balance between J x and J y and thus to minimize the target XP radiations over D-planes. A comprehensive design along with its physical insight has been presented. An experimental verification at C-band ensures about 6-7 dB reduction in D-plane XP level without compromising the primary radiation and other features of the antenna. This, to the best of our knowledge, is the first attempt without modifying or reshaping the ground plane to make an aperture-fed patch exhibit perfectly low XP over the entire radiation plane. Thus, it solves a longstanding issue and opens enough scopes for practical applications which demand higher cross-polar discrimination (XPD).

II. THE DIAGONAL PLANE ISSUES
This examination using [31] begins with three identical C-band patches fed by three different methods, as shown in Fig. 1. Their H-and D-plane radiation patterns are compared in Fig. 2. The patterns significantly vary only in terms of XP behavior. The H-plane feature is noticeable: the highest XP is produced by a probe-fed patch, which gets lowered by 5 dB in microstrip-fed geometry. It remarkably falls by another 10 dB when fed by aperture-coupling. This H-plane XP source is the weakly generated TM 02 mode [28] due to a kind of perturbation caused by the feeding structure. Its impact is maximum when it is a vertical probe and moderate when it is a planar microstrip line. An aperture, in contrast, couples through an equivalent magnetic dipole and causes the least electrical perturbation. Its H-plane XP thus becomes inherently low. But the scenario is completely different in D-plane. Apart from an asymmetry imposed by the vertical probe, the peak XP levels caused by all three configurations seem to be mutually comparable. Their peaks around 45 • are predominantly high, say by about −6 to −8 dBi. Such D-plane behavior prevents its application as a true low-XP device, even if it offers so impressive features over the H-plane.

III. THE DESIGN APPROACH AND INSIGHT
The definition of XP fields was provided in [32]. Their predominant components occurring across the orthogonal and diagonal radiation planes can be expressed as [16]: where, ι x , ι y are the source current polarizations vectors and θ is the elevation angle. These ι vectors indeed align with the associated surface currents J = ∇ n × H with |H| = |E|/η. Thus, J y appears to be the primary contributor to the XP fields in both planes and J x plays a role over the D-plane only. The best way of controlling it is to maximize J x /J y ratio since J x is indispensable in terms of the main radiation. The proposed method introduces a pair of resonant loops adjacent to the patch as shown in Fig. 3. The idea is to get it electromagnetically coupled with the patch, resonate at the same frequency, and enhance the effective conduction current along the x-axis as well as reduce along the y-axis. Thus, the loop perimeter 2(a + b) becomes strategic to match with λ av (≈ (λ 0 + λ g )/2). It may be noted that the overall deployment area of the proposed looped coupled patch is conveniently accommodated within λ 0 × λ 0 GP. This indeed is found to be the optimum GP dimension required for maximum gain   with minimum XP. A detailed study will be provided in Section IV. The magnitude of J x /J y over the entire GP of a conventional patch is shown in Fig. 4(a). It is equally high across both co-and cross-polar axes and endorses the reality of occurring low H-plane XP values, as obtained in the aperturecoupled version of Fig. 2(a). The physical meaning of the same can be visualized from the simulated current vectors of Figs. 4(b) and (c). However, J x /J y in Fig. 4 (a) is found to be noticeably low over the diagonal axes. Our present approach is to enhance J x /J y ratio, especially around the diagonal axes (BB ). This is possible either by decreasing J y or by increasing J x . Newly introduced EM coupled loops indeed make it happen. Fig. 5 helps in understanding the situation. The coupling phenomenon between the patch edges and the loops redistributes the conduction currents. Their simulated and equivalent schematic portrays are shown through Figs. 5(a) and (b) respectively. It is clearly evidenced from Fig. 5(a) that the resonating nature of the loops creates a situation where J y components turn mutually opposite in phase and cancel out. The J x components in contrast get widely redistributed and add up. This eventually enhances H y across AA ( Fig. 5(c)). Its physical manifestation is examined in Fig. 6. The GP current remains predominantly x-polarized and gets widely distributed compared to that occurring underneath a standalone patch (Fig. 4(c)). Finally, Fig. 6(b) depicts simulated values of J x /J y which compared to Fig. 4(a) appears distinctively improved covering a wider zone of the radiating aperture. Improved J x /J y especially around BB should have a considerable impact in reducing XP fields across the skewed radiation planes.

IV. RESULTS AND VERIFICATIONS
The proximity of the loops has to be maintained as g≈0.01λ av . The overall resonance, as studied in Fig. 7(a), significantly depends on the value of a, the optimum perimeter (2a + 2b) being unaltered. Typically, a < L a and for this specific design a ≈ 0.75L a shows the best possible matching.
The patch gets capacitively coupled with the loops causing a left shift of the resonance and hence an adjustment is required by changing the patch dimension. Figs. 7(b) and (c) examine the radiation properties. The CoP patterns remain close to those caused by an isolated patch except a small reduction in beam-width over H-plane. The expansion of effective aperture caused by the loops is the reason behind this, resulting in an increase in gain by 1.2 dB. More significantly, the targeted achievement over the D-planes is enormous: the XP level reduces from −6 dBi to −13 dBi as evident from Fig. 7(b). E-plane XP values for both geometries are inherently low (below −35 dBi) and hence are not discussed here. It is also important to note that the role of a in controlling radiation patterns is not as sensitive as observed in guiding S 11 in Fig. 7(a). Therefore, a = 8.75 mm ≈ 0.75L a appears to be the best acceptable value. With this a, H-plane XP level is found to rise marginally from −25 dBi to −24 dBi. However, it still remains significantly lower when compared to the values observed in the D-plane. One can visualize the actual impact from the 3D XP patterns as documented in Fig. 8. Both patterns are displayed with identical physical and color scales. The conventional patch ( Fig. 8(a)) reveals a very low XP level over the principal planes (xz and yz) and significantly high values over the D-planes (ϕ = 45 • ). The same gets a drastic reduction ( Fig. 8(b)) when the proposed design is implemented. Our study ensures a maximum of 6-7 dB improvement in the 3D XP scenario. This eventually reveals CoP to XP isolation over 22 dB across all radiation planes.
A relative performance of the proposed configuration as a function of GP size is examined in Fig. 9. Indeed, a conventional patch with a GP below 0.9λ does not appear useful because of lower gains. Hence, the range of the present study is from 0.9λ to 1.3λ, which comfortably accommodates the proposed loop coupled patch. The said GP size reveals consistent improvements both in D-plane XP and peak gain. From the nature of variation (Fig. 9), one may surmise the optimum GP size of the order of λ.
The concept of the loop has been reconfirmed by changing its shape from rectangle to circle as shown in Fig. 10(a). The impedance and radiation performances appear mutually comparable and a representative comparison for D-plane radiation patterns is shown in Fig. 10(b). The rectangular loop appears a bit compact relative to the circle and is hence a better choice for practical applications. It may be noted that in realizing an array each of the loops should serve a pair of adjacent elements and thus no extra space is required to accommodate the loop in between two radiating elements.

V. PROTOTYPES AND MEASUREMENTS
A set of prototypes with and without loops are realized and the photograph for the proposed geometry is shown in Fig. 11(a). Agilent's E8363B precision network analyzer and an anechoic chamber provided with an MI-750 microwave receiver have been used for the measurements. A pyramidal horn with more than 40 dB XP isolation was used as the transmitting radiator.  Fig. 11(b) examines their measured S 11 in comparison with the simulated predictions. Satisfactory impedance matching is evident. The antenna with loops reveals a relatively narrower matching bandwidth. Their radiation patterns across the planes of our interest are shown in Fig. 12 ensuring very close agreement between the measured and simulated data. The measured peak gain of the proposed geometry with loops is found to be 9.1 dBi, which appears improved by about 1 dB compared to that without loop. This gain increment is associated with a minor decrement in beam width. As predicted for H-plane ( Fig. 12(a)), the measured XP levels also show no considerable difference between the configurations with and without loops. Fig. 12(b) indeed experimentally establishes the design target that, D-plane XP values get reduced by about 7 dB over a wide angular range around the boresight. This indeed results in more than 14 dB XPD which is significant to a scanning beam array generating its beams covering the diagonal planes. The consistency of this feature over the entire operating band has been ensured by repeating the measurements as furnished through Fig. 13. Near the lower edge of the band (f = 6.02 GHz), the order of XP reduction is about 4 dB which increases up to 7 dB at the upper edge (f = 6.21 GHz). Table 1 provides a comprehensive overview of the peak gain and XP levels at resonance for the antenna, allowing for a clearer  understanding of the performance differences between the configurations with and without loops. One can estimate the improvement in the XP discrimination over the D-plane as 7 dB.

VI. RADIATION EFFICIENCY
The loop hardly causes any change in the antenna efficiency, confirmed by the measured values obtained using Wheeler cap method [33]. The technique uses measured input resistance of the antenna at resonance with and without Wheeler  (b) measured and simulated S11 of the prototypes. Parameters as in Fig. 5. cap. The cap is actually a metallic shield that covers the antenna under test and prevents radiation from it [34]. This will be evident from our experimental setup as depicted through Fig. 14. The radiation efficiency is calculated as [33]  where, the total resistance (R t ) = radiation resistance (R r ) + loss resistance (R l ). Their measured values and obtained efficiencies with and without loops are documented in Table 2.
The measured efficiencies hardly show any noticeable change caused by the introduction of the loops.

VII. POSSIBILITY IN ARRAY CONFIGURATION
This work examines the suitability of the proposed geometry in an array configuration. An H-plane array in Fig. 15(a) indicates that each loop serves two adjacent elements and therefore, the inter-element spacing becomes 0.63λ (with W/L = 1.6) which is reasonably acceptable for a practical design. It is possible to meet 0.5λ spacing by adjusting the W/L ratio. Its impedance property is compared with the standalone antenna in Fig. 15(b) indicating a marginal enhancement by 40 MHz. The radiation characteristics of the array examined over both D-and H-planes (Fig. 16) promise considerable improvement in terms of polarization purity, as in the case of a single-element study.

VIII. CONCLUSION
This investigation shows how an aperture-fed microstrip patch could be made to qualify as a truly low cross-pol antenna. The new technique without adding extra cost or complexity has been successfully demonstrated for the first time. The typical overall dimension of a standalone antenna appears adequate to accommodate the loops. The antenna structure, therefore, does not demand any extra space or size. A comprehensive comparison is provided in Table 3 indicating adequately improved and useful features. It should find potential applications in base station repeater and telemetry downlink systems.