# A Wideband Transition from Microstrip Line to Microstrip Spoof Surface Plasmon Polariton Line for Microwave/Millimeter-Wave Applications

Authors: Somia Sharma; Rajesh Kumar Singh; Ananjan Basu; Shiban K. Koul
Journal: International Journal of RF and Microwave Computer-Aided Engineering
 (2023)

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## Abstract

In this article, a wideband transition from a microstrip transmission line to a microstrip spoof surface plasmon polariton (SSPP) transmission line is reported. The proposed transition is realized to achieve better performance starting from low frequencies up to 40 GHz. The proposed shape of the unit cell is taken in such a manner that the momentum, impedance, and polarization of the microstrip line can be matched with that of the SSPP transmission line. The wide operating band of the transition can be achieved by perturbing only the shape of the unit cell. Extra care is taken in choosing the shape of the unit cell so that the fabrication of the circuit becomes easier, especially at millimeter-wave frequencies. The proposed transition has a good transition-occupation ratio and transmission efficiency. The achieved transition-occupation ratio and transition efficiency for all three designed cases are excellent, and it is 34.18%, 44%, and 39% forp=1 mm, 2 mm, and 3 mm, respectively. It is a potential candidate for realizing millimeter-wave antennas and devices due to its attractive properties, such as compact layout, low fabrication cost, ease of fabrication even at millimeter-wave frequencies, and good operational characteristics.


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## Body

## 1. Introduction

Transition is a very important part of the transformation for converting a conventional technology into a currently developing technology. Spoof surface plasmon is an active research area that has numerous advantages over existing technologies. Although spoof surface plasmon has all the properties of surface plasmon (normally used in optics) at the microwave to terahertz frequencies, it is difficult to feed into and extract RF signals directly from the components or circuits realized using plasmonic technology. An efficient transition, which can convert an existing mode (e.g., quasi-TEM mode of a microstrip line) into an SSPP mode, is needed so that the signals can be extracted from or fed to the circuits efficiently. These conversions from spatial waves to SSPP were obtained using prism and gratings [1]. Nowadays, many antennas, filters, and power dividers are being developed using SSPP. Both grounded and ungrounded SSPPs are used to develop microwave components and circuits [2]. Transitions from a conventional transmission line to an ungrounded SSPP have been reported in [3–8]. Although, the microstrip to ungrounded SSPP or CPW to ungrounded SSPP transitions have been realized, grounded SSPP-based transitions find application in realizing passive microwave circuits such as filters where the ground is needed for the realization of short or open-circuited stubs.Poor grounding degrades the transmission of a signal and leads to unwanted radiation. Also, an amplifier possessing high power gain requires a good grounding to suppress feedback (output to the input) signal to prevent oscillation. The advantage of using grounded SSPP is that it can be easily integrated with the microstrip and CPW. As the SSPP technology is in the growing stage [9–14], we need a ground to solve the above-mentioned issues. It is beneficial to realize a transition with the ground on the back side so that microwave active and passive circuits can be easily integrated.An ultrathin-corrugated metallic strip with mirror symmetry on both sides of the dielectric substrate has been used to integrate the amplifier chip with SSPP [15]. In [16–18], transitions from conventional microstrip lines to grounded SSPPs have been proposed. In [16], a miniaturized SSPP low-pass filter has been designed with a compact transition where the cut-off frequency of the filter was 14 GHz. A gradient corrugated strip with “under layer ground” was used to match the impedance of the microstrip with the grounded SSPP, the realized transition had high conversion efficiency, but the proposed transition was not compact in size, and the transition operated well up to 6.5 GHz [17]. A broadband transition from QTEM mode to SSPP mode has been reported in [18], where it was shown that conversion of the wave vectors could be achieved by gradually varying the groove height. Although that transition has achieved high-conversion efficiency, the operating frequency of the transition is limited by the groove height. According to available reports, it is observed that it is difficult to realize a wideband transition while maintaining good performance. To realize a transition for higher frequencies, the groove height becomes very less, and it is difficult to realize such transitions using conventional photolithography or machining processes.In this article, we propose a compact and wideband transition from a conventional microstrip line to an SSPP transmission line. All three matchings, i.e., momentum, polarization, and impedance, are achieved simultaneously that are not reported earlier in the scientific literature. The shape of the corrugation is varied to match the momentum, polarization, and impedance matching while keeping the groove height fixed. The proposed transition is robust; it can be used at microwave and millimeter-wave frequencies without encountering any fabrication issues. The transition shows excellent transmission efficiency over a wide frequency range and a good transition-occupation ratio. This transition finds application in microwave/millimeter-wave integrated circuits, phase shifters [19], RF transceiver circuits, and 5G systems [20].


## 2. SSPP Transition and Transmission Line

Figure1(a) shows the geometry of an SSPP unit cell. The width of the groove is “a” on the upper side and “b” on the lower side. The periodicity of the unit cell is “p”, and the height of the groove is “h”. Figures 1(b)–1(d) show the dispersion relation of a single unit cell by considering different values of “a” and “b”. The unit cells used to design transitions for p=1 mm, p=2 mm, and p=3 mm are shown in the inset of graphs of Figure 1. Figures 1(b), 1(c), and 1(d) show the effect of variation of “a” and “b” on the asymptotic cut-off frequency at p=1 mm and h=0.4 mm, p=2 mm and h=0.8 mm, and p=3 mm and h=1.7 mm, respectively. Figure 1 shows the sensitivity or gradual increase in the wave momentum with the change in the shape of a unit cell and helps in realizing a smooth transition.Figure 1
(a) Unit cell, (b–d) dispersion diagrams of unit cell with shape variation (“a” and “b” variation) for (b)p=1 mm and h=0.4 mm, (c) p=2 mm and h=0.8 mm, and (d) p=3 mm and h=1.7 mm.
(a)(b)(c)(d)Figures2(a)–2(c) show the extracted characteristic impedance plots. Characteristic impedances are plotted using the cubic spline interpolation method [21]. Figures 2(a)–2(c) show the effect of variation of “a” and “b” on the characteristic impedance against the frequency for p=1 mm and h=0.4 mm, p=2 mm and h=0.8 mm, and p=3 mm and h=1.7 mm, respectively. The transition structure is not only realized by getting impedance and momentum matching but also by transforming quasi-TEM mode to SSPP mode. Figures 3(a) and 3(b) show the normal (|Ex|) and azimuthal Ez2+Ey2 component of the electric field at different positions on a plane perpendicular to the transition at 10 GHz for p=2 mm.Figure 2
Extracted characteristic impedances of SSPP-TL for (a)p=1 mm and h=0.4 mm, (b) p=2 mm and h=0.8 mm, and (c) p=3 mm and h=1.7 mm.
(a)(b)(c)Figure 3
Electric field distribution of SSPP-TL forp=2 mm (a) normal component and (b) azimuthal component.
(a)(b)In Figure3, plane 1 shows that the normal component for the microstrip line which is zero, and most of the electric field is concentrated in the azimuthal component. Further, as we move far from the plane perpendicular to the transition, the azimuthal component decreases gradually, and the normal component increases gradually. One can clearly observe that the electric field concentration is higher for normal component than the azimuthal component in plane 4. The electric field study shows that the field is concentrated on the surface of the metal dielectric interface. This indicates that the transition efficiently converts a QTEM mode into the SSPP mode.Figure4 shows the SSPP-TL with the proposed transition. It is concluded from the above study as shown in Figures 1–3 that the momentum, impedance, and polarization matching can be achieved using this transition. It is designed on a F4B dielectric substrate whose dielectric constant is 2.65, loss tangent is 0.001, and height of substrate is 1.5 mm for p=3 mm and 0.5 mm for p=2 mm. For p=1 mm, the SSPP-TL with the proposed transition is designed on Rogers 4350B with a dielectric constant of 3.66, loss tangent of 0.0037, and height of 0.254 mm. The SSPP-TL has a microstrip line section of length L1 and width Ws.Figure 4
SSPP-TL with proposed transition.The length of the transition and transmission line isL2 and L3, respectively. The transition is designed by varying the shape of the unit cell. The parameters “a” and “b” of the unit cell are varied to realize a smooth transition. For p=1 mm, “a” is varied from 0 mm to 0.9 mm while “b” is varied from 1 mm to 0.1 mm. The length of the transition, L2, is 5 mm for p=1 mm. For p=2 mm, “a” is varied from 0 mm to 1.8 mm while “b” is from 2 mm to 0.2 mm, and the length of the transition, L2, is 12 mm for p=2 mm. For p=3 mm, the variation in “a” and “b” is from 0 mm to 2.8 mm and 3 mm to 0.2 mm, respectively, and the length of the transition, L2, is 18 mm.The transition is compact and robust, and one can easily tune the low-pass characteristic of the line up to 40 GHz and by varying the geometrical parameter of the unit cell. When the SSPP line is designed with the conventional transition withp=1 mm and h=0.4 mm, the height of the unit cells is varied from 0.04 mm to 0.4 mm which is not possible to fabricate using conventional photolithography or machining processes. The objective of the proposed work was to realize a compact and wideband transition, and the calculated dimensions of the transition should have been larger than 0.1 mm so that it can be realized using conventional photolithography or machining processes.TheS-parameters of the SSPP-TL with the proposed transition are shown in Figures 5(a)–5(c). These transmission lines work as low-pass filters whose cut-off frequency can be varied by varying the height and shape of the unit cell. Figures 5(a)–5(c) show the S-parameters of SSPP-TL by varying the height (h) for p=3 mm, p=2 mm, and p=1 mm, respectively. The graphs depict that the low-pass characteristic can be varied from lower to higher frequencies with good transition-occupation ratio and transmission efficiency by varying the geometrical parameters.Figure 5
S-parameters of SSPP-TL at different groove height for (a) p=3 mm, (b) p=2 mm, and (c) p=1 mm.
(a)(b)(c)


## 3. Simulated and Measured Result

Three transmission lines with different cut-off frequencies are designed in a full-wave CST Microwave Studio and developed. Figures6(a)–6(c) show the fabricated SSPP-TLs with the proposed transition for p=3 mm, 2 mm, and 1 mm, respectively. Figures 7(a)–7(c) show the simulated and measured S-parameters of the SSPP-TL for p=3 mm, p=2 mm, and p=1 mm, respectively. Measured and simulated results in Figures 7(a) and 7(b) show that the reflection coefficient is better than -10 dB, and the transmission coefficient is better than -3 dB in the frequency range from 1-12 GHz and 1-22 GHz, for p=3 mm and p=2 mm, respectively. In Figure 7(c), the simulated and measured reflection coefficient is better than -10 dB, but the transmission coefficient is lower than -3 dB at the higher frequencies. The insertion loss of the coaxial connector is increased at the higher frequencies.Figure 6
Photographs of fabricated SSPP-TLs with proposed transition for (a)p=3 mm, (b) p=2 mm, and (c) p=1 mm.
(a)(b)(c)Figure 7
Simulated and measuredS-parameters of SSPP-TL for (a) p=3 mm and h=1.7 mm, (b) p=2 mm and h=0.8 mm, and (c) p=1 mm and h=0.4 mm.
(a)(b)(c)A small discrepancy in simulated and measured results is due to fabrication imperfection and coaxial connectors that are not considered in the simulation. Table1 shows the comparison of the proposed transition with the published reports. The transition-occupation ratio is defined as the ratio of the length of transition to the wavelength which is calculated based on the center frequency of the operating range. The transition-occupation ratio defines the compactness of the transition, the smaller the value, the more compact the transition. Compared to the previously reported literature, the proposed transition is compact and has good fractional bandwidth. In this article, we have not only discussed a compact and wideband transition but also provided the design guidelines for realizing transitions to be used at low as well as high frequencies. This design of transition can be translated to other low-cost substrates such as FR4 glass epoxy substrate, for commercial uses. Due to the limited resources, the validation of the transition operating up to 40 GHz is shown here. Due to the unavailability of high-frequency coaxial connectors, we were not able to measure the performance of the proposed transition beyond 40 GHz. The performance of the proposed transition can be improved beyond 40 GHz using advanced fabrication processes and feed connectors. In previously reported papers as listed in Table 1, it would be difficult to achieve or translate the transition to the millimeter-wave frequencies because of the fabrication complexity. In the proposed idea, the transition is made in such a way that it can be easily translated to the millimeter-wave frequencies.Table 1
Comparison with previously reported transitions.
Type of SSPPBandwidth (GHz)F.B.W. (%)Transition lengthTransition occupation ratio (%)[3]Groundless SSPP22.5-4260.5%0.6933λ39.13%[4]Groundless SSPP4.5-11.587.5%0.933λ48%[5]Groundless SSPP5-846.2%0.589λ58.9%[6]Groundless SSPP3.2-5.827.3%2.403λ146.85%[7]Groundless SSPP6-1266.67%1.8λ180%[8]Groundless SSPP2.2-1063.9%0.4473λ57.95%[15]Dual metallic strip3.1-17.5139.8%1.236λ103%[16]Grounded SSPP0-13200%0.1456λ14.56%[17]Grounded SSPP0.1-8197.5%0.567λ56%[18]Grounded SSPP1-7150%0.533λ53.33%This work (p =1 mm)Grounded SSPP1-40190.24%0.3417λ34.18%This work (p =2 mm)Grounded SSPP1-22182.6%0.46λ44%This work (p =3 mm)Grounded SSPP1-12184.61%0.39λ39%


## 4. Conclusion

In this article, a compact and wideband transition from microstrip line to microstrip SSPP-TL is presented. It has been demonstrated that the low-pass characteristic of the SSPP-TL with the proposed novel transition can be easily varied up to 40 GHz by changing the periodicity and the height of the unit cell. This shows the robustness of the transition. The proposed transition is different from the conventional transition as the shape of the unit cell is varied instead of height. The advantage of varying only height is the ease of fabrication at higher frequencies. At higher frequencies, the gradient height of the unit cell for transition makes the fabrication complex. The obtained transition-occupation ratio and transition efficiency for all three designed cases are excellent. The proposed transition is a potential candidate to be used in circuits designed at microwave and millimeter-wave frequencies.



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