A modified slotted microstrip patch sensor antenna has been developed for sensitivity enhancement to measure the moisture content of leaves. The design incorporates a rectangular slot positioned above the feed of a rectangular patch antenna. The antenna achieves resonant frequency at a lower range than the operating frequency of a traditional patch antenna without slot while maintaining a miniaturized form factor. To evaluate the sensitivity of the proposed modified slotted patch antenna, the variation in the resonant frequency and the reflection coefficient (S11) is observed with the sample under test (SUT), i.e., leaf is placed above the patch. A comparative analysis with regard to sensitivity and shift in frequency is performed between the designed antenna structures. The designed antenna is fabricated on a 0.8 mm F4B substrate in order to resonate within the 2.40–2.80 GHz range under unloaded conditions. The antenna’s sensitivity is tested by performing parametric analysis with respect to the permittivity of the leaf ranging from 20 to 30. The experimental results reveal that the sensitivity of the proposed patch antenna is enhanced by a factor of 1.67 as compared to traditional patch antenna for the four levels of SUT, demonstrating an enhanced sensitivity in detecting the moisture content of leaves.

The application of microwave technology in various fields, such as agriculture, sports, and medicine, has led to the development of antennas specifically designed for wireless sensor applications. Antenna sensor technology is gaining significant interest because of its cost-effectiveness, passive operation, small size, and ability to sense in multiple ways.1 One of the major advantages of antenna sensors is their flexibility to be used as wireless sensing devices or as standalone sensors, requiring only a minimal number of components.2 An example of an electromagnetic (EM) resonant sensor that enables distributed sensing is the microwave patch antenna sensor.3,4

The microstrip patch antenna design utilizes a broadband electromagnetic signal input to the radiating patch element, which is coupled through a microstrip transmission line. The patch selectively radiates the components of the input signal that correspond to its resonant frequency while reflecting the remaining spectral components.5 Consequently, the reflected signal’s frequency spectrum exhibits pronounced attenuation at the antenna’s resonant frequencies, indicative of the radiated energy at those frequencies. Hence, accurately measuring the permittivity of a material has become a crucial parameter in microwave antenna design due to its simplicity and non-destructive applications.6 

Planar resonator designs, such as split ring resonators, have become increasingly prevalent among various techniques used for determining permittivity. Such strategies are preferred due to their cost-effectiveness, low size, simplistic shape, and relatively simple fabrication.7–9 In such experiments, the sample under test (SUT) is taken as part of the resonator and the permittivity is determined by analyzing the shift in resonant frequency. Such methods involve addressing the sample under test (SUT) as a component of the resonator, and the permittivity is calculated by studying the variation in resonant frequency. Microstrip patch antennas, resembling resonators, have been researched as a sensor for measuring the permittivity of solid or liquid substances. In the literature, a method is suggested for determining the resonant frequency of a patch antenna clad with a dielectric substrate. The technique relies on the efficient dielectric constant of complete design that is proposed to be determined by use of the variational approach.10 

Consequently, in the field of precision farming and plantation, measuring the moisture content (MC) is vital for evaluating harvest times,11 assessment of numerous quality parameters of animal food,12 granular materials in containers,13 wheat,14 grain,15 rice,16 corn kernel,17 and various crops.18–21 MC monitoring extends beyond agriculture and is also crucial for tracking body hydration, skin moisture levels,22–24 microplastic presence in liquids,25,26 and the sensitivity of mineral resources27,28 and fabric29,30 to dampness. Accurate and timely measurement of leaf moisture is essential for optimizing water usage, ensuring plant health, and improving crop yield. While utilizing a coaxial probe is a method that can potentially damage the sample, employing antennas is a preferred non-contact approach for MC monitoring. It is important to note that temperature fluctuations can introduce inaccuracies in MC measurement due to cross-interference.31 As a result, it is advisable to avoid frequency bands where the dielectric constant of water experiences abrupt changes with temperature.

The proposed study presents a modified slotted patch antenna in comparison with conventional patch antenna designed for moisture content sensing in leaves. The antenna is equipped with few rectangular slots that are positioned above the feed line to miniaturize the proposed antenna and increase the sensitivity in the frequency range of 2.20–2.80 GHz. The proposed antenna achieves a better sensitivity and relatively lower MRE at the desired frequency than the previously reported microstrip patch antenna sensors and the conventional patch antenna. The proposed antenna is fabricated on a 0.8 mm-thick F4B substrate. Full-wave simulations using ANSYS EM Suite 2022 are performed, and the sensors’ performance is verified by experimental processes. Moreover, a thorough sensitivity analysis is presented for the validation of the proposed slotted patch antenna sensor.

Studies have shown that the resonant frequency of a microstrip patch antenna, designed for integrated sensing functionalities, is influenced by both the dielectric constant of the substrate material and the dimensions of its radiating element. These radiation properties of the microstrip patch antenna naturally vary with frequency.32 The design of such an antenna encompasses three crucial elements: a radiating surface, a microstrip feed line, and a dielectric substrate. The traditional rectangular microstrip patch antenna structure is depicted in Fig. 1(a).

FIG. 1.

Microstrip patch sensor antenna structures: (a) standard rectangular patch antenna; (b) modified slotted patch antenna.

FIG. 1.

Microstrip patch sensor antenna structures: (a) standard rectangular patch antenna; (b) modified slotted patch antenna.

Close modal

The standard microstrip patch antenna (CPA) incorporates a patch having a rectangular shape that is connected via a 50 Ω microstrip feed line. It is manufactured on an F4B substrate that has a dielectric constant (εr) of 2.65 and a thickness of 0.8 mm, as illustrated in Fig. 1(a). The dimensions of the design are as h1 = 19.6 mm and w1 = 15.5 mm, respectively, as depicted in Fig. 2(a). The width (w2) and length (L1) of the feed line are determined as w2 = 3.2 mm and L1 = 8 mm, respectively. The length (L) of the substrate and the width (W) are chosen as 28.1 and 39.08 mm, respectively.

FIG. 2.

Reflection coefficient values (S11) of the designed antennas: (a) standard microstrip patch antenna; (b) modified slotted microstrip patch antenna.

FIG. 2.

Reflection coefficient values (S11) of the designed antennas: (a) standard microstrip patch antenna; (b) modified slotted microstrip patch antenna.

Close modal
The dimensions, width (W), length (L), change in length (ΔL), and relative permittivity (Ɛr) of the rectangular microstrip patch are calculated using the following equations:33,
W=c2fr2εr+1,
(1)
εreff=εr+12+εr+121+12hW0.5,
(2)
ΔL=0.412hεreff+310Wh+2.6410εreff2.5810Wh+810,
(3)
L=c12frεreff2ΔL,
(4)
where c is the speed of light in free space, h is height of the substrate, and fr is the desired resonant frequency. In order to achieve size reduction at lower frequencies compared to the design reported in Ref. 34, slots have been incorporated into the antenna structure.

Therefore, a thin rectangular slot is etched radiating above the microstrip feed line, as shown in Fig. 1(b). The slotted modified patch antenna (MPA) features a rectangular patch with dimensions of “h1 = 28.3 mm” in length and “w1 = 25.4 mm” in width, which are connected with the microstrip line. In this configuration, the microstrip line is designed using length L1 = 12 mm and width w2 = 3.2 mm. The designed slotted patch antenna resonates at 2.80 GHz under unloaded conditions.

The designed antenna sensor is simulated using the electromagnetic (EM) simulation software ANSYS Electronic Desktop HFSS. The computed S11 parameter of the standard inset-fed rectangular microstrip patch antenna sensor revealed that the antenna resonates at 5.807 GHz, exhibiting a bandwidth of 400 MHz, as depicted in Fig. 2(a). Furthermore, the fabricated antenna sensor demonstrated a resonant frequency at 5.82 GHz, corroborating the simulation results with a slight deviation attributable to fabrication tolerances and substrate material actual properties.

In addition, the proposed slotted rectangular microstrip patch antenna sensor produced a resonant frequency at 2.85 GHz based on simulation results and a bandwidth of 300 MHz. On the other hand, the fabricated design depicted a similar resonant frequency significantly lower at 2.82 GHz, as indicated in Fig. 2(b).

The sensing ability of the modified slotted microstrip patch antenna displayed in Fig. 1(b) is compared with the traditional patch antenna depicted in Fig. 1(a) by measuring the variation in the resonant frequency f (GHz) vs S11(dB). The capacitor area resultant from the slot loaded area possesses the highest electric field intensity. The sensor observes fluctuations in resonant frequency, which demonstrates the leaf’s dielectric property. Through experimental data, the link between resonant properties and leaf moisture content can be established, which is mentioned in Sec. V. Therefore, when plants undergo water stress, the sensor may detect a significant variation in leaf moisture content. To achieve precise sensing data, the leaf that is measured must be positioned close to the location with the most intense electric field. The relationship between frequency and capacitance is given by
f=12πLsCs,
(5)
where f is frequency and Ls and Cs are the inductance and capacitance of the designed sensor. In accordance with Eq. (5), the more concentrated the moisture content of the leaf, the larger the permittivity and capacitance. Consequently, the shift in resonant frequency can be exploited to detect leaf moisture. A leaf model, also called the sample under test (SUT), with a thickness of 0.2 mm and loss tangent of 0.2 is considered in this experiment. The SUT is positioned over the patch, and its relative permittivity (εr) is diversified in the range of 20–30 with an increment of 1.

Figure 3 depicts the S11 characteristics of (a) the traditional patch antenna and (b) the proposed modified slotted patch antenna. For the traditional patch antenna, it is observed that the resonant frequency is achieved at 5.12 GHz as the test sample dimension having εr1 = 30. Moreover, the frequency response of 5.70 GHz is achieved with a value of εr1 = 20.

FIG. 3.

S11 (dB) vs frequency (GHz) for varying relative permittivity of the SUT: (a) traditional patch antenna; (b) proposed slotted patch antenna (MPA).

FIG. 3.

S11 (dB) vs frequency (GHz) for varying relative permittivity of the SUT: (a) traditional patch antenna; (b) proposed slotted patch antenna (MPA).

Close modal

However, for the proposed slot-loaded antenna, with the SUT having a permittivity value of εr1 = 30, the antenna demonstrates its lowest frequency response at 2.24 GHz. Conversely, a higher frequency response of 2.48 GHz is achieved when the test sample’s permittivity is set to εr1 = 20.

In order to verify the sensitivity enhancement of the proposed slotted patch antenna compared with that of the standard patch antenna, the frequency change (Δf), relative frequency change in percent (PRFS), enhancement in PRFS (PRFSE), sensitivity (S), and sensitivity enhancement (SE) are calculated and plotted with respect to S11 responses using the following equations:35,
Δf=fufl(GHz)
(6)
PRFS=Δffu×100(%),
(7)
PRFSE=PRFSproposedPRFSconventional,
(8)
S=ΔfΔε,
(9)
SE=SproposedSconventional,
(10)
where fu and fl represent the resonant frequency under unloaded and loaded (with leaf sample) conditions. The relative position of the sample under test (SUT) just above the patch’s surface plays a crucial role in determining the total capacitance and effective relative permittivity of the microstrip patch sensor. As a result, the resonant frequency of S11 displays as a nonlinear function with regard to the effective relative permittivity.36,37 Figure 4(a) indicates that the sensitivity, S, varied as the relative permittivity changed, with higher values for lower relative permittivities. In this case, when the permittivity of the SUT is Ɛr = 20, the resonant frequency shift, Δfr, for the traditional patch antenna is 0.10 GHz, compared with 0.32 GHz for the proposed slotted patch antenna. The percentage resonant frequency shift (PRFS) for the traditional patch antenna (CPA) is 1.72%, whereas for the proposed slotted patch antenna (MPA), it reached 5.52%, as shown in Fig. 4(b). Therefore, the percentage resonant frequency shift enhancement (PRFSE) of the proposed patch antenna is 3.2 accordingly.
FIG. 4.

Comparative analysis plots for sensitivity performance of the traditional microstrip patch antenna and the proposed slotted patch antenna: (a) sensitivity (S); (b) % relative frequency shift (PRFS); (c) variable resonant frequency (Δfr).

FIG. 4.

Comparative analysis plots for sensitivity performance of the traditional microstrip patch antenna and the proposed slotted patch antenna: (a) sensitivity (S); (b) % relative frequency shift (PRFS); (c) variable resonant frequency (Δfr).

Close modal

Moreover, it is evident from Fig. 4(c) that as the permittivity of the SUT is increased to Ɛr = 30, the shift in resonant frequency, Δfr, for the typical patch antenna (CPA) reached 0.68 GHz. In comparison, it reached 0.56 GHz for the proposed slotted patch antenna (MPA). The PRFS for the traditional patch antenna is 11.72%. Meanwhile, for the proposed slotted patch antenna, the percentage is 9.66%. Subsequently, the PRFSE for the designed slotted patch antenna is 0.84. In addition, the S (sensitivity) of the slotted patch antenna is 0.06 GHz. Consequently, the sensitivity enhancement (SE) is found to be 1.61 GHz. Hence, it can be concluded that the sensitivity of the proposed slotted patch antenna at the resonant frequency of interest is 1.61 times higher compared with that of the traditional patch antenna at a relative permittivity in the range of 20–30.

The fabricated models of the traditional and proposed slotted patch antennas, as detailed in Sec. IV, are constructed on an F4B substrate, as illustrated in Fig. 5.

FIG. 5.

Fabricated patch antennas: (a) traditional antenna; (b) proposed slotted microstrip patch antenna.

FIG. 5.

Fabricated patch antennas: (a) traditional antenna; (b) proposed slotted microstrip patch antenna.

Close modal

The S11 parameters of the designed fabricated traditional and slotted patch antennas shown in Figs. 5(a) and 5(b) have been analyzed by means of an Agilent N5230A network analyzer as indicated in Fig. 6(a). The leaf is placed above the designed antenna sensor, and the shift in resonant frequency is measured.

FIG. 6.

(a) Test bed with network analyzer; (b) weighting process of the leaf after every added (ml) of water.

FIG. 6.

(a) Test bed with network analyzer; (b) weighting process of the leaf after every added (ml) of water.

Close modal

In order to experimentally verify the sensitivity analysis, the modified slotted patch antenna (MPA) is tested under moisture stress. The first step involves measuring the mass of leaf under dry condition. In addition, the mass of leaf after every added (ml) of water is measured. The weighing process is depicted in Fig. 6(b). Consequently, in the moisture content analysis setup, four (4) distinct levels are chosen with water content that constitutes to mass ranging approximately from 4.62 to 5.52 g. The type of leaf is a freshly plucked leaf from the famous Ginkgo tree. It should be noted that the sensitivity levels could be different for dead and alive leaves as well as the type of leaf as every plant has different characteristics to handle water stress.

Initially, with an empty sample holder and the dry leaf weighing 4.62 g, there is negligible change in the resonance frequency, indicating the baseline measurement scenario. As the leaf’s mass increases due to water absorption, observable shifts in the resonant frequency occur, attributable to the varying dielectric properties of the leaf as it transitions from dry to increasingly moist. For the simplicity of discussion, four distant moisture levels, i.e., 0%, 8.33%, 14.60%, and 22.48% of moisture content, are selected. Consequently, the relative moisture content, mc, in leaf samples in percentage can be calculated using the following equation:
mc=mwater(mwater+mdryleaf)×100%,
(11)
where mwater and mdryleaf are the weight of water and the weight of the dry leaf sample.

Table I provides the results of an experiment where the moisture content in leaves is calculated according to the added volume of water (ml). The idea is that as the moisture content in the leaf increases (indicated by increasing Mwet and added water), there is a corresponding decrease in the resonant frequency of the antenna. This is due to the higher dielectric constant of the moist leaf compared to its dry state, which affects the antenna’s frequency response.

TABLE I.

Mass of wet leaf and moisture content (%) by adding water.

MwetMdryMoisture content (%)Added water (ml)
4.62 4.62 0.00 
5.04 4.62 8.33 25 
5.41 4.62 14.60 45 
5.96 4.62 22.48 65 
MwetMdryMoisture content (%)Added water (ml)
4.62 4.62 0.00 
5.04 4.62 8.33 25 
5.41 4.62 14.60 45 
5.96 4.62 22.48 65 

In order to find out the effect of moisture content on the resonant frequency of the proposed slotted patch antenna sensor, the change in the resonant frequency of the slotted patch antenna sensor due the different amounts of moisture content in leaf samples is measured after every added 25 ml of water during the experiment. Initially, with an empty sample as a dry leaf weighing 4.62 g, there is negligible change in the resonance frequency, indicating the baseline measurement scenario.

The resonant frequency of the designed slotted patch antenna sensor decreases as the moisture content of the sample increases as shown in Fig. 7. With an empty sample (dry leaf), the resonance frequency is 2.80 GHz. At level 1 (6.85% moisture content), it decreases to 2.78 GHz. At level 2 (14.60% moisture content), it further decreases to 2.76 GHz. Finally, at level 3 (22.48% moisture content), the resonance frequency is 2.74 GHz. The shift to lower frequencies occurs due to the higher dielectric constant of water as compared to air. As a result, when the moisture content is increased, the dielectric constant of the SUT also increases, resulting in a decrease in the resonant frequency of the sensor antenna. The sensitivity of the experimental data is found out by Eq. (9), whereas the MRE (mean relative error) is found out by the following equation:
MeanRelativeError MRE=MCActualMCPredictedMCActual,
(12)
where MCActual is the moisture content being measured via experimental analysis depicted in Table I. MCPredicted is the predicted moisture content being calculated.
FIG. 7.

Resonant frequency variation by virtue of different volumes of moisture content.

FIG. 7.

Resonant frequency variation by virtue of different volumes of moisture content.

Close modal
In order to find out the MCPredicted value, the calibration curve technique is utilized for frequency with respect to moisture content and the predicted moisture content (PMC) is found out using the calibration equation given by
MC=0.0345f+3.323.
(13)
In order to validate the test results, the PMC is calculated at 2.48 GHz, which is the highest resonant frequency achieved in the experiment as discussed in Sec. IV. The PMC is found to be 3.43%. The mean relative error (MRE) at the selected highest resonant frequency response of 2.48 GHz found out using Eq. (13) is 0.338, which is an indication of a very low error between the predicted and measured values. Moreover, the measured sensitivity (S) at the desired resonant frequency is 2%, which is found out using Eq. (9), and it proves that the sensor is able to sense very precisely for any available moisture content.38 

Table II provides a comparative analysis of three sensor designs used for moisture content detection in different materials. The proposed design has a larger physical size than the previous designs, which allows for a broader frequency range suitable for leaf moisture sensing.

TABLE II.

Comparative analysis with previous designs.

ParametersProposed designPrevious design 132 Previous design 239 
Size 34.8 × 31.8 mm2 30 × 30 mm2 17.96 × 8.92 mm2 
Operating frequency 2.20–2.82 GHz 5.2 and 6.8 GHz 4.36 and 10.69 MHz 
Sensing material Leaf Rice Paper 
Substrate F4B FR4 RT-duroid 5880 
Sensor cost Cheapest Relatively high Expensive 
R2 value 0.954 0.411 and 0.379 0.9105 and 0.9383 
Moisture content (%) 0.86%–22.48% 10.71%–21.87% 10%–28% 
MRE 0.338 0.55 0.85 
ParametersProposed designPrevious design 132 Previous design 239 
Size 34.8 × 31.8 mm2 30 × 30 mm2 17.96 × 8.92 mm2 
Operating frequency 2.20–2.82 GHz 5.2 and 6.8 GHz 4.36 and 10.69 MHz 
Sensing material Leaf Rice Paper 
Substrate F4B FR4 RT-duroid 5880 
Sensor cost Cheapest Relatively high Expensive 
R2 value 0.954 0.411 and 0.379 0.9105 and 0.9383 
Moisture content (%) 0.86%–22.48% 10.71%–21.87% 10%–28% 
MRE 0.338 0.55 0.85 

The use of the F4B substrate makes the proposed sensor the most cost-effective option. In addition, the proposed design demonstrates a high (R2) value via regression analysis, indicating a strong correlation between the sensor’s readings and the actual moisture content. This design also shows the lowest Mean Relative Error (MRE), suggesting a higher accuracy in measurements compared with the previous designs. The operating frequency range of the proposed design is more suitable for the intended sensing material (leaf), whereas the previous designs are optimized for rice and paper.39 Overall, the proposed design offers a balance of size, cost-efficiency, and accuracy, making it a competitive option for moisture sensing in leaves.

Hence, it can be proved with this method that the resonant frequency of the microstrip patch antenna decreases as the moisture content of the leaf increases because of variation in relative permittivity as proved by previous research.40 It is due to the fact that the water molecules in the leaf absorb some of the electromagnetic radiation, which reduces the resonant frequency of the antenna. This method of measuring leaf moisture content is non-destructive and can be used in a variety of applications, such as agriculture, environmental monitoring, and plant physiology research.

The proposed modified slotted microstrip patch sensor antenna has demonstrated an enhanced sensitivity in measuring the moisture content of leaves compared to the traditional patch antenna. The mathematical data indicate that the sensitivity levels of the proposed slotted patch antenna are 1.67 times greater than those of the standard patch antenna across various benchmarked dielectric samples, which have dielectric constants between 20 and 30. However, for the mathematical sensitivity, it is two times higher than that of the traditional patch antenna. The proposed antenna is fabricated on a 0.8 mm-thick F4B substrate, achieving resonance frequencies from 2.40 to 2.80 GHz in unloaded scenarios. The increased sensitivity is attributed to the incorporation of slots above the feed of the patch antenna proving enhanced coupling. The enhanced sensitivity of this modified slotted patch antenna sensor makes it a promising candidate for various applications, including proximity sensing for automation purposes, the measurement of permittivity in planar solid and microfluidic liquid materials, wireless monitoring of biological specimens or dangerous substances, and non-destructive assessment of moisture in soil, foodstuffs, or liquids.

The research received financial support from the National Natural Science Foundation of China (NSFC) under Grant No. 61571084 and was also partly funded by the Fundamental Research Funds for the Central Universities (FRFCU) under Grant No. ZYGX2019Z022. The authors acknowledge the institutional and financial backing that facilitated this study.

The authors have no conflicts to disclose.

Muhammad Talha Khan: Conceptualization (equal); Data curation (equal); Writing – original draft (equal). Xian Qi Lin: Conceptualization (equal); Funding acquisition (equal); Methodology (equal); Supervision (equal); Writing – review & editing (equal). Zhe Chen: Conceptualization (supporting); Methodology (supporting); Resources (supporting). Abid Muhammad Khan: Methodology (equal); Resources (equal).

The data that support the findings of this study are available within the article.

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