Planar Sensing Platform with Hexagonal Complimentary Split Ring

Resonators Excited by Spoof Surface Plasmon Polaritons and

Director Elements

Rishitej Chaparala

, Y Vijay Kumar

, Jyoti S

, Ravisankar Ponnada

and Brahmaraju M

SR University, Hasanparthy, Warangal, Pin: 506371, Telangana, India

Department of ECE, Pragati Engineering College, Surampalem, ADB Road, Andhra Pradesh, India

Keywords: Planar Sensor, Printed Yagi-Uda Antenna, Quality Factor, Split-Ring Resonator (SRR), Complimentary Split

Ring Resonators (CSRRs), Spoof Surface Plasmon Polaritons (SSPPs), Radio Frequency Identification

(RFID).

Abstract: This work presents excitation of hexagonal complimentary split ring resonators (CSRRs), which function as

sensor using SSPP and directors. This configuration removes the requirement for transmission lines (TLs)

and minimizes the impact of external circuitry on sensing accuracy. A compact design was achieved by

developing the complete sensing platform, which includes the CSRRs and director elements, for the C band

and integrating them onto a single substrate with dimensions of 151 x 60 x 0.8 mm

. Investigation results

showed a resonant frequency to permittivity variations for Material Under Test (MUT). The measurements

indicated that the sensor’s |S

| (dB) parameters are influenced by the dielectric properties of the samples.

1 INTRODUCTION

Microwave resonant-based sensors have recently

attracted widespread interest for a variety of

applications, including biomedicine, chemistry (P.

Vélez, J. 2019, M. C. Cebedio, M. Moradpour 2021),

water quality control, and aviation (O. Malyuskin, I.

Frau, S. R. Wylie, I. Frau, O. Korostynska 2018).

These sensors demonstrate strong capabilities in

detecting solid, gas, and liquid substances present in

their environment (B. D. Wiltshire et al, A. Tricoli, N.

Nasiri, B. D. Wiltshire, M. Alijani). Their ability to

provide real-time, noncontact detection, combined

with low-cost manufacturing, has facilitated their

widespread use in industrial applications (M. H.

Zarifi, H. Sadabadi, J. Kilpijärvi). Furthermore,

microwave sensors can be designed in compact, low-

profile configurations with reconfigurable features,

making them compatible with CMOS technology and

expanding their application range (A. Ebrahimi, W.

Liu) A low-cost, low-profile microwave resonator

sensor is the planar sensor, typically fabricated using

PCB etching technology (A. Ebrahimi et al., O.

Korostynska,). Split-ring resonator (SRR) sensors, in

particular, have attracted significant attention for their

compact design, flexibility, and potential to offer

improved sensitivity and resolution (Z. Abbasi, M. H.

Zarifi, M. Abdolrazzaghi, M. H. Zarifi). Various

excitation methods have been investigated for SRRs,

such as transmission lines (TL) and incident plane

waves (D. Baena,2005). TL excitation is commonly

used in SRR-based sensors due to its compatibility

with planar platforms, providing a low-profile design

and straightforward implementation. Nonetheless, in

the TL-excitation technique, modifications in

external circuitry, including the coupling gap, can

influence crucial resonant parameters such as

frequency, amplitude, and the loaded Q-factor (D. M.

Pozar, 2012). Problems occur when a large material

obscures the coupling gap between the SRR and TL

(K. Luckasavitch) resulting in modified resonant

responses and loaded quality factors (QL) that can

interfere with the sensing process (M. H. Zarifi).

While methods like positive feedback loops have

been implemented to mitigate sensor losses and

enhance resolution (M. H. Zarifi, J. B. Pendry), The

reliance of sensing performance on variations in

external coupling circuitry has not been entirely

eliminated. To lessen this effect, resonators can be

excited using incident plane waves emitted from an

Chaparala, R., Kumar, Y. V., S, J., Ponnada, R. and M, B.

Planar Sensing Platform with Hexagonal Complimentary Split Ring Resonators Excited by Spoof Surface Plasmon Polaritons and Director Elements.

DOI: 10.5220/0013587600004664

Paper published under CC license (CC BY-NC-ND 4.0)

In Proceedings of the 3rd International Conference on Futuristic Technology (INCOFT 2025) - Volume 2, pages 87-92

ISBN: 978-989-758-763-4

antenna instead (W. Wang). This approach is similar

to the excitation of frequency selective structures (H.

Torun, 2019) and effectively separates the resonators

from the excitation probes (antennas), thus reducing

the impact of external circuitry variations on the

resonant characteristics.

Few studies have explored planar sensing

platforms based on antenna-driven SRR excitation (S.

Zahertar, C. A. Balanis), Recent studies have focused

on similar structures that use printed monopoles and

loop antennas for excitation. However, these designs

place the resonators in the near-field zone of the

antennas, resulting in heightened near-field coupling

between the SRR and the excitation probes, which

can exacerbate the loading effects of external

circuitry on the resonators (W. Wang et al).

Furthermore, this planar arrangement is analogous to

detection methods that employ surface acoustic

waves and eddy currents (W. Wang et al.,).

In this study, a planar microwave sensing platform

was introduced that employs the excitation of

hexagonal CSRRs situated in the Fresnel zone of

Yagi-Uda antennas, with all components

consolidated onto a single substrate. This approach

offers a different excitation mechanism for CSRRs,

eliminating the need for transmission lines (TLs) to

drive resonator elements in large platforms or arrays

of adjacent resonators. By employing SSPP for

excitation, the method reduces near-field effects from

excitation probes (such as TLs) These factors

influence the resonator’s performance and field

distribution, rendering it beneficial for sensing

applications. The Yagi-Uda antenna topology was

selected for its simplicity and endfire radiation

characteristics (M. N. Abdallah), although other types

of endfire radiating antennas could also be utilized.

To address polarization mismatch, identical Yagi-

Uda antennas were simulated and oriented toward the

direction of maximum radiation. The SRR and

director elements were strategically placed in the

Fresnel zone to minimize near-field coupling with

probing structures and transmission lines (TLs) (N.

Katsarakis). To enhance intensity, the gaps between

the SRR and director elements were aligned with the

polarization of the antenna (Hosseini, Arezoo, et al,

2023). Hexagonal CSRRs were used instead of

traditional circular SRRs and simulated using CST

Microwave Studio software for optimal performance.

2 DESIGN AND PRINCIPLE OF

OPERATION

2.1 Designing Spoof Surface Plasmon

Polariton (SSPP)

The SSPP feeder element was designed to excite the

CSRR, replacing the conventional microstrip feeding

technique. The proposed design’s top and bottom

views are illustrated in Fig. 1(a) and 1(b). It is

composed of a single dielectric material, Rogers

TMM3. Port 1 of the SSPP structure serves as the

input, connecting to the director elements. This sensor

operates across a frequency range of 18 GHz. SSPP

feeding provides several benefits over microstrip

feeding, such as enhanced field confinement,

wideband operation, customizable dispersion

characteristics, and compact integration. The SSPP

structure, acting as the feeding element, features

corrugated grooves with heights ranging from 0.5 mm

to 3.5 mm on both sides as shown in fig. 1(b). These

grooves are fed by a Yagi-Uda antenna, which

includes director elements spaced 1.45 mm apart on

both sides.

2.2 Excitation and Resonance

Characteristics of CSRRs

The design of the sensor features two wide-band

YagiUda antennas that act as radiators, in conjunction

with planar CSRRs that serve as sensing elements.

These components are excited via SSPPs. The

structure is simulated using a single dielectric

material, Rogers TMM3, and employs SSPP as the

feeding element, featuring corrugated grooves with

heights varying from 0.5 mm to 3.5 mm on both sides.

Fig. 2 shows the hexagonal CSRR positioned at the

center of the structure, with four director elements

located beneath the resonator. Two planar, concentric

CSRRs, composed of metal and having a thickness of

0.035 mm, are placed on the upper plane, accurately

positioned at the center to create the sensing region.

Furthermore, director elements are positioned on the

lower plane, directly under the CSRR, to redirect

extra electromagnetic energy for the optimal

excitation of the CSRR. A planar configuration is

established using an SSPP feed, with radiating

elements arranged on both sides. Five director

elements are incorporated to produce an end-fire

radiation pattern. Additionally, the spacing of the

director elements is optimized to improve the field

intensity in the sensing region. As shown in Fig. 3,

the optimized width of the CSRR gap (g) is 1 mm,

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along with the following dimensions: W

= 1 mm, W

= 0.5 mm, W

= W4 = 0.5 mm, L

= 33.8 mm, L

5.75 mm, and L

= 5.89 mm. The spacing (S

)

between the director elements is 1.45 mm. The outer

) and inner radius (r

) of the hexagonal CSRR

measure 2.4 mm and 1.4 mm, respectively. The

CSRR serves as the sensing elements and is expected

to absorb the radiated electromagnetic power at the

resonant frequency, creating a notch in the

transmission coefficient.

3 SIMULATION RESULTS AND

DISCUSSION

(a)

(b)

Figure 1: (a) Upper view displaying the CSRR and (b).

Lower view of the director elements powered by SSPP

feeding.

Figure 2: CSRR and below director elements.

Figure 3: Measurement of director elements serving as

radiating components

Figure 4: Front and back view showing radiation from

director elements fed by SSPPs, with field confinement in

hexagonal CSRRs observed in the top view.

Planar Sensing Platform with Hexagonal Complimentary Split Ring Resonators Excited by Spoof Surface Plasmon Polaritons and Director

Elements

The Yagi-Uda array, comprising half-wavelength

dipoles and an SSPP feed, was designed to operate

within a frequency range of roughly 5 to 8 GHz,

enabling a compact physical footprint for the sensing

platform. Fig. 4 shows the field confinement on the

CSRR sensors at far field region when excited by the

SSPP and the directors. At far field they are

visualized with the radiating behavior characterized

by planar wave fronts indicative of traveling plane

waves. To lessen the near-field effects of the radiating

and resonating components, the two director elements

were adequately spaced apart. The design and

simulation of the structure were conducted using CST

Microwave studio. Simulation of the sensor was

obtained at the frequency of 5.79 GHz, as depicted in

Fig. 5. Initially, the design involving two Yagi

antennas was simulated to analyze their impact on the

transmission coefficient (S

), with results presented

in Fig. 5. The integrating CSRR featuring directors

positioned at the centre created a stop band notch near

its fundamental resonant frequency of 5.7 GHz,

achieving amplitude of -38.14 dB. This highlights the

crucial role played by the directors in exciting the

CSRR by redirecting additional EM power towards

them. As a result, the CSRR resonated, minimizing

the transmission of power between the antennas and

producing a unique band-stop resonance properties.

The proposed hexagonal CSRR designed for the

sensor application were simulated using the CST

Microwave studio transient solver. As shown in Fig.

6 the material under test (MUT), a solid dielectric

element, with a relative permittivity ranging from 1 to

5, is positioned on the surface of the two concentric

CSRRs. When testing dielectric materials, CSRR can

resonate at particular frequencies based on the

material's dielectric characteristics, which may prove

beneficial for sensing applications, potentially

including RFID. Due to their unique electromagnetic

properties, CSRR can be used for effective control of

resonance, enabling more compact and efficient

RFID system designs, especially at microwave and

millimetre wave frequencies.

The CSRR sensor is modelled from metal that has

a thickness of 0.035 mm. The size of the CSRR sensor

is as follows: l × b × h= 5.75×6 ×0.035 mm.

The

measurement is performed with MUT 1&2 by

successively changing the dielectric constant values

from 1 to 5. Fig. 7 illustrates the simulated responses

for dielectric constant values ranging from 1 to 5. It

has been noted that as the dielectric constant rises, the

curve moves towards lower frequencies. In

RFIDbased sensing systems, CSRRs might be used to

detect changes in environmental conditions (like

humidity or pressure) by monitoring shifts in resonant

frequency as a result of variations in the dielectric

material properties. CSRRs with dielectric material

testing may not be a typical RFID device, but could

be employed in advanced RFID systems, particularly

for specialized sensing or antenna miniaturization

tasks. The improvements of this work over existing

designs are summarized in Table 1. By incorporating

SSPP feeding, the operating frequency is reduced

compared to conventional design.

Figure 5: Simulated S

response of the proposed CSRR.

Figure 6: Placement of solid dielectric samples 5.75 × 6 mm

on the surface of the two hexagonal CSRR.

INCOFT 2025 - International Conference on Futuristic Technology

Figure 7: Measured transmission coefficient S

for solid

dielectric samples with different relative permittivity values

from 1 to 5.

Table 1: Comparison with existing literature works.

Ref

Freq

uency

Feeding Sensor

(GHz)

(dB)

[27]

1-3

GHz

microstrip SRRs 1.8

-25

[31]

5-25

GHz

microstrip MIMO 16

-35

[26]

5-8

GHz

microstrip

Loop

sensor

6.6

-25

[34]

14-16

GHz

microstrip

SRRs

15.4

-34

This

work

5-8

GHz

SSPP CSRRs 5.79

-38

4 CONCLUSIONS

A planar configuration combining Complimentary

Split Ring Resonators (CSRRs) with Yagi-Uda

antennas fed by SSPP was developed and studied for

material sensing applications, targeting solid

materials. By incorporating the CSRRs and director

elements onto a single substrate, the need for

transmission lines (TLs) was eliminated. The

introduction of CSRRs shared by the director

elements created a band-stop notch in the

transmission coefficient. Various materials were

placed over the sensitive region of CSRRs and the

| response in decibels was analyzed to distinguish

between them. The results showed a correlation

between changes in resonant frequency and the

attributes of the solid samples evaluated particularly

their dielectric constants and sample sizes. This

excitation using director element for CSRR on a

planar platform using incident plane waves not only

extends the potential for microwave sensors in planar

configurations without the need for wiring but also

represents a significant advancement in reducing the

reliance on external coupling circuitry, such as

transmission lines. This sensor is highly appropriate

for the application owing to its excellent sensitivity,

cost-effectiveness, and ability to provide realtime

sensing.

ACKNOWLEDGEMENTS

The author extends sincere thanks to Jyothi,

Ravisankar, and Brahmaraju for their financial support

for the conference and for their valuable contributions

in discussing the results and providing feedback.

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