Open AccessArticle

A Cross-Shaped Slotted Patch Sensor Antenna for Ice and Frost Detection

Rula Alrawashdeh

Electrical Engineering Department, Mutah University, Al-Karak 61710, Jordan

Technologies 2025, 13(1), 5; https://doi.org/10.3390/technologies13010005

Submission received: 8 November 2024 / Revised: 7 December 2024 / Accepted: 18 December 2024 / Published: 25 December 2024

(This article belongs to the Section Information and Communication Technologies)

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Abstract

Beyond data transmission, antennas have recently been utilized as sensors, offering the advantage of reducing hardware requirements and power consumption compared to systems where sensors are separate from antennas. Patch antennas, in particular, are widely used across various applications, including sensing, due to their attractive features like compact size and conformability. In addition, they can be easily designed in different ways to sense variations in certain variables. Adding a slot to the patch antenna introduces several advantages, including multiband, wideband operation, and improved impedance bandwidth. Slots also provide a concentrated region of electromagnetic fields, which increases the antenna’s sensitivity for sensing and detection purposes. In this paper, a rectangular patch antenna with a cross slot is designed and proposed for water, ice, and frost detection. Detection is achieved by measuring variations in the resonant frequency in response to water, ice accumulation, and frost. The results indicate that the proposed antenna can detect both water and ice accretion with a frequency shift of up to 1.538, 0.358, and 0.056 GHz, respectively, which reflects good sensitivity levels of the antenna. The effect of the slot on strengthening the near electric field and antenna sensitivity is discussed in this paper. The antenna is fabricated and measured and the indicators of each detection scale have been extracted. The proposed antenna has a simple structure and a small size of (40 × 40 × 1.53 m³). In addition, it can be precisely used to sense different environmental parameters such as frost and ice. Thus, it can serve as a strong candidate for detecting natural disasters like frost damage. Furthermore, the findings in this paper offer valuable insights into how the presence and structure of slots influence the sensitivity response of patch antennas, supporting ongoing research in this field.

Keywords:

cross slot; ice; frost; patch antennas; resonant frequency; sensor antennas; water

1. Introduction

In recent years, the integration of sensing capabilities into antenna systems has garnered significant attention, leading to the development of sensing antennas [1,2,3]. Sensing antennas have already been proposed for cancer [4,5,6], frost, and wildfire detection [7,8], which supports a wide range of applications, including healthcare and early warning of natural disasters. In early warning systems, different disasters, such as frost damage, can be detected at an early stage by providing timely alerts of specific parameters indicating the presence of a disaster from the sensed area to an external receiver [9,10]. The antenna plays a major role in building a robust wireless connection for sending the sensed data promptly and continuously. However, it can also be used to work as a sensor at the same time. This dual functionality reduces the need for separate components such as sensors, simplifying system design and integration. Moreover, many antennas require less power to operate compared to some types of active sensors, making them more suitable for remote or resource-constrained environments [11,12]. An example of a wireless early warning system based on antennas for sensing and data transmission is shown in Figure 1. Data is sent from the antenna after detection to an external receiver, where it is processed. The sensor antenna in such systems is required to satisfy a number of requirements as outlined below:

-: It should have a design that responds to specific variable changes, with alterations in parameters like the reflection coefficient or resonant frequency that correspond to shifts in weather or environmental conditions.
-: It must integrate smoothly with the application system.
-: Rigid structures are preferred to prevent performance variations due to deformation.
-: Good far-field characteristics are essential, as the antenna needs to transmit data in addition to sensing; these include parameters such as radiation efficiency and gain.

In comparison to conventional thermometer sensors, the sensor antenna not only transmits data but also performs sensing, which reduces both hardware and power requirements. Unlike traditional sensors that rely on external RF systems for communication, the proposed sensor antenna simultaneously senses and transmits data, eliminating the extra energy typically needed for separate data storage and transmission later [8,13]. A set of antennas has already been proposed in the literature for sensing applications. In [14], a T-shaped slot patch antenna was proposed at 2.378 GHz for wireless ice and frost detection. The detection process was based on varying the amplitude and frequency of the transmission coefficient between the antenna sensor and a wide band receiver with total shifts in the resonant frequency of 32 and 36 MHz in the presence of frost and ice, respectively. A flexible loop antenna was proposed in [15] at 2.4 GHz for ice detection. That antenna was flexible, which can be easily deformed. Moreover, it was not sensitive enough to detect small variations of the resonant frequency. An antenna based on split ring resonators was proposed in [16] to determine the real parts of the complex permittivity and permeability of magnetodielectric composites at 2.74 and 2.34 GHz. A compact double-ring microstrip resonator structure was fabricated in [17]. It detected water, frost, and ice based on the measured resonant frequency, amplitude, and quality factor variation in the scattering parameter of the sensor in the frequency range between 3 and 5 GHz. Measurements were conducted over a temperature range between −10 and 19 °C. However, the far-field performance was not evaluated and validated in most of these designs. This is required to assess the antenna’s capability of sending the sensed data as alarm signals about the natural disaster status. A frequency of around 2.45 GHz was mainly exploited for the patch antennas proposed for frost detection. However, higher frequencies in the sub-6 band (<6 GHz), including the 5.6 GHz, offer a number of advantages such as:

-: The 5.6 GHz band offers wider bandwidth compared to lower frequency bands, which allows for faster data transmission. This increased capacity can reduce delays in communication, supporting low-latency performance, especially for applications that require rapid data exchange or real-time control systems [18].
-: Higher-frequency bands, such as 5.6 GHz, often experience less interference than lower bands (e.g., 2.4 GHz), which are more congested due to the presence of many devices operating in those frequencies. Reduced interference results in more stable connections, minimizing retransmissions and communication delays, thus contributing to lower latency [19].
-: Sufficient penetration for propagation during fog and snow [20,21].
-: This frequency enables smaller sizes in comparison with other lower frequencies, such as around 2.4 GHz, as size is inversely proportional to frequency [22]. This is preferred for compact, modern devices.

Hence, the frequency of 5.6 GHz is selected for the investigations in this paper.

Beyond the operating frequency, the antenna structure is a crucial parameter in designing an effective sensor antenna. Although a T-slot was added to the antenna structure in [14], it did not influence the antenna’s sensitivity. Instead, sensitivity in that design was adjusted by including a rectangular slab element separated from the patch by a gap, creating a coupling area with the material under test. This configuration, with the slab and patch edges, resembled a parallel plate capacitor, where overall capacitance impacted the resonant frequency and sensitivity. However, the combined impact of the antenna structure and slot on sensor performance was not thoroughly analyzed. Additionally, incorporating the slab in that design led to a gain reduction of 2.2 dBi.

In this paper, a cross-slotted rectangular patch antenna is designed to sense and detect water, ice, and frost. The contributions of this work can be summarized as

-: A single-slotted patch element structure is used to work as the sensor. This has the advantage of increasing the antenna gain and optimizing the antenna radiation shape and direction. The cross slot inside the single patch element has the advantage of increasing the gain by 5 dBi for the investigated structure. This is different from previous designs that used extra slab elements external to the slotted patch, which worked to reduce the antenna gain.
-: The slot effect on the performance of the antenna is studied not only for the functionalities of sensing but also for that of far-field data transmission.
-: This study presents, for the first time in the literature, link budget calculations for sensor antennas used in environmental monitoring. These calculations are crucial for assessing the antenna’s ability to transmit sensed data effectively. While the far-field radiation of the substrate-integrated waveguide sensor antenna was analyzed in [8], the study did not include link budget calculations. Additionally, the impact of the antenna structure on its radiation characteristics was neither evaluated nor addressed in that work.
-: Investigations at 5.6 GHz help steer research in this field toward higher frequencies and provide clear insights into the benefits and limitations of using these frequencies compared to lower ones.

This paper is arranged as follows: In Section 2, the design procedure and methods are first provided. In Section 3, the performance of the antenna as a sensor and data transmitter is evaluated, validated, and analyzed. Link budget calculations are also provided in this section. The paper is finally concluded in the final section.

2. Design Procedure and Methods

In this work, a rectangular patch sensor antenna with a cross slot is designed. The design process involves two main steps:

1. A rectangular patch antenna has first been designed (Equations (1)–(5) [23]). Resonances at around 3.3, 4.63, and 5.8 are obtained. An FR-4 substrate (ɛ_r = 4.3, tanδ = 0.025 [24]) of 1.53 mm thickness is used. The antenna is fed with a 50 Ω microstrip line. The structure and overall dimensions are illustrated in Figure 2.

W = \frac{1}{2 f_{r} \sqrt{ε_{0} μ_{0}}} \sqrt{2 / (ε_{r} + 1)}

(1)

ε_{r e f f} = \frac{ε_{r} + 1}{2} + \frac{ε_{r} - 1}{2 \sqrt{1 + 12 d / W}}

(2)

f_{r} = \frac{1}{2 L_{e f f} \sqrt{ε_{r e f f}} \sqrt{ε_{0} μ_{0}}}

= \frac{1}{2 (L + 2 Δ L) \sqrt{ε_{r e f f}} \sqrt{ε_{0} μ_{0}}}

(3)

Δ L = 0.412 d (ε_{r e f f} + 0.3) (\frac{W}{d} + 0.264) / [(ε_{r e f f} - 0.258) (\frac{W}{d} + 0.8)]

(4)

L = L_{e f f} - 2 Δ L

(5)

where W (mm) is the rectangular patch width, L (mm) is the rectangular patch length,

ε_{r}

is the dielectric constant of the substrate,

f_{r}

(Hz) is the resonant frequency,

ε_{0}

(F/mm) and

μ_{0}

(H/mm) are the permittivity and permeability of free space, respectively, and

d

(mm) is the substrate thickness. Due to the fringing field effect, where the electric field lines bend at the patch edges and create an additional source of radiation, the antenna becomes electrically longer than its physical dimensions. The elongation in the patch length is represented as ∆L (mm), as defined in Equation (4). This elongation depends on several factors, including the substrate thickness (d), width (W), and length (L), as well as the effective relative permittivity (

ε_{r e f f}

). The effective permittivity arises from the fringing field effect, as the electric field lines partially extend into the surrounding air rather than being fully confined within the substrate. Since the fringing occurs at both edges of the patch (one at each side along its length), the total additional effective length becomes (2∆L). The physical length of the patch (L) and the additional length due to fringing fields together form the effective length (

L_{e f f}

(mm)), as indicated in Equation (5). The effective length accounts for the actual electrical length, which is slightly longer than the physical length, and determines the resonant frequency of the patch antenna, as illustrated in Equation (3).

2. A cross slot has also been etched from the rectangular patch in step 1. Its dimensions are also indicated in the same figure. The idea behind integrating a slot into the patch structure is to provide a concentrated region of electromagnetic fields, which increases the antenna’s sensitivity. The cross-slot structure provides a relatively large contact region integrating two rectangular slots in two orthogonal dimensions. The dimensions and position of the rectangular slot, as depicted in Figure 2, are selected to shift the resonance frequency downward to approximately 5.6 GHz, aligning with the targeted frequency band of investigation. The slot forces the surface current to follow a longer path around it. This longer current path makes the patch behave as though it is electrically larger than its physical size. Since the resonant frequency is inversely proportional to the effective length, increasing the effective length lowers the resonant frequency by 266 MHz [25,26].

The cross slot can be viewed as two rectangular slots arranged In a parallel configuration in which the capacitance (

C_{e q}

(F)) can be calculated as [27]

C_{e q} = C_{1} + C_{2}

(6)

C_{1} = C_{2} = C

(7)

C = ε \frac{L_{s} W_{s}}{2 d} \cos^{- 2} (\frac{π x_{f}}{L_{s}})

(8)

where

C_{1}

(F) and

C_{2}

(F) are the equivalent capacitance of the first and second slots, respectively, which are equal for the same slots’ dimensions,

ε

(F/mm) is the effective permittivity,

L_{s}

(mm) is the slot length,

W_{s}

(mm) is the substrate width, d (mm) is the substrate thickness, and

x_{f}

is the feed point location in (mm).

The capacitance introduced by the slot increases with its width and length. However, this capacitance works to shift the resonant frequency down [28,29], and hence, the overall slot’s dimensions should be carefully selected to obtain a good level of sensitivity within the intended frequency range.

f_{r} = \frac{1}{(2 π \sqrt{L_{e q} C_{e q}})}

(9)

where L_eq (H) is the total equivalent effective inductance of the antenna.

As a starting point, simulations were conducted. The −10 dB reflection coefficient is shown in Figure 3. The CST (Computer Simulation Technology) 2023 software is used for simulations. Hexahedral meshes and a Time domain solver are used [30]. It is important to note that all frequency bands are narrow, as expected for a typical rectangular patch. However, this does not impact the antenna’s performance for the proposed application, as only the resonant frequency shift will be monitored.

3. Performance and Analysis

3.1. Working Principle and Sensing Mechanism

The sensor antennas of investigations are aimed at sensing water and ice accretion and detecting frost. By monitoring the antenna’s response, particularly the resonant frequency, changes associated with the accumulation of water and ice or the occurrence of frost presence can be detected. The antenna’s resonant frequency is affected by environmental changes, such as moisture or frost on its surface. The presence of water, ice, and frost changes the dielectric properties around the antenna, leading to a detectable shift in the resonant frequency. Frost will be resembled by a dielectric material of relative permittivity ranging between 1.5 to 2.5 and conductivity of 1 × 10⁻⁵ S/m [14]. Water and ice have a permittivity of 78 and 3.2 and an electric conductivity of 1.59 and 1 × 10⁻⁵ S/m, respectively [14,30]. Water has a larger permittivity than ice, and hence, a larger shift in the resonant frequency Is expected.

{f_{r}}^{'} = \frac{f_{0}}{\sqrt{{ε_{r e f f}}^{'}}}

(10)

where

{f_{r}}^{'}

(Hz) is the resonant frequency with the material under test,

f_{0}

(Hz) the original resonant frequency without the material under test, and

{ε_{r e f f}}^{'}

is the effective permittivity with the material under test.

3.2. Performance as a Sensor

In conditions conducive to frost formation, water can exist in various forms, including frost itself, ice, liquid water, and water vapor. Frost occurs when the temperature of the surface is below freezing (0 °C or 32 °F), while liquid water can exist as dew when temperatures are just above freezing and the air is saturated with moisture [31]. The three conditions investigated in this work, and are simulated as follows:

A full layer of water and ice covers the antenna’s entire top. The thickness is 1 mm for each layer.
Partial layers of water and ice covering the slot only. This is to study the effectiveness of the slot structure in controlling the detection, which could influence realization issues later.
A layer of material with the following dielectric properties (ɛ_r = 1.5, 2, 2.5 and σ = 1 × 10⁻⁵ S/m) which resembles frost.

The water and ice layers on the antenna top can be seen in Figure 4.

The results of the antenna −10 dB reflection coefficient with a full layer of water and ice are shown in Figure 5. It can be seen from the figure that the resonant frequency of the antenna shifts down from 5.607 GHz to 91 MHz when a 1 mm layer of ice is placed on its top. However, a shift of 1120 MHz is obtained when a layer of water is placed instead. This is because of the larger relative permittivity for water in comparison with ice. The resonant frequency also shifts down from 4.886 GHz to 77 MHz in the second frequency band for the case of ice top loading. This is also larger for the case of loading with water in which a frequency shift of 1106 MHz is obtained at this band. A slight shift in the resonant frequency has also been obtained for the case of ice at the lowest frequency band around 2.5 GHz, although it was not matched, while deep matching at 2.1 GHz is obtained when water loaded the antenna.

The simulation results for the case of the slot coverage only are shown in Figure 6. It can be seen from the figure that the resonant frequency also changes and shifts down when either water or ice is placed on the slot only. However, the downshift was smaller than that for the full layers. This is expected as the change to the effective permittivity will affect a smaller part of the radiator. The frequency difference between full (on the entire patch) and partial (on the slot only) loading was smaller for the case of ice (49, 35, and 7 MHz, for the higher, middle, and lower bands, respectively) than that for water (98, 259, and 140 MHz for the previously mentioned bands, respectively). Again, this was because of the smaller permittivity of ice in comparison with that of water.

The results for the case of frost accumulation for the three layers (ɛ_r = 1.5 (F1), 2 (F2), 2.5 (F3), and σ = 1 × 10⁻⁵ S/m) are shown in Figure 7 and Figure 8, respectively.

The results in Figure 7 indicate that the resonant frequency shifts the 5.607 GHz frequency down by 21, 49, and 70 MHz for the three layers as the relative permittivity of the layers increases by 0.5 from one layer to another. It also shifts down the 4.886 GHz by 21, 42, and 56 MHz. The downshift was negligible at the lowest frequency band at around 2.59 GHz. However, matching is still not obtained. The results in the figure above also reveal a minor frequency shift at a lower frequency band, around 2.5 GHz.

For the resonant frequency of the partial frost layers indicated in Figure 8, it shifts down the 5.6 GHz also but by smaller values of frequency (7, 21, and 28 MHz for layers F1, F2, and F3, respectively) in comparison with that for the full layer. It also shifts down the 4.886 GHz by 14, 21, and 35 MHz for layers F1, F2, and F3, respectively.

The effect of adding the slot to the antenna can be explained with the aid of Figure 9. The near electric field becomes stronger around and inside the cross slot. A stronger near-electric field allows the antenna to better sense variations in the immediate vicinity. In the near field, the electromagnetic waves are not as fully radiated as they would be in the far field, and they can more effectively couple with other nearby objects or antennas [32,33,34]. This strong coupling is beneficial in sensor applications where accurate detection of nearby objects is critical. The increase in the near electric field is attributed to the capacitance introduced by the slot, as described with reference to Equations (6)–(8). This enhances the electric field intensity around the slot, which acts as a capacitor.

The effect of the ice thickness on the antenna performance is investigated. Three thicknesses of 2, 3, and 4 mm are simulated. The −10 dB reflection coefficient results are shown in Figure 10.

Thicker layers cause the resonant frequency to shift down. However, an average shift of 14 and 7 MHz at the upper and middle-frequency bands have been obtained for each 1 mm increase in thickness. A small shift has also been obtained for the case of frost accumulation. Thicker layers are not investigated for the case of water, as water is unlikely to accumulate on the antenna’s smooth top surface.

3.3. Validation via Measurements

In this section, the antenna is fabricated, and its reflection coefficient is measured. The antenna is measured as follows:

Without loading its top with either water or ice.
With a layer of water covering the entire top of the antenna.
With a layer of ice covering the entire top of the antenna.
With a layer of water covering the slot of the antenna only.
With a layer of ice covering only the slot of the antenna.

The fabricated antenna is shown in Figure 11.

The antenna with water and ice on its top is shown in Figure 12.

The reflection coefficient S₁₁ was measured using a Vector Network Analyzer (VNA). The SOLT (Short-Open-Load-Through) calibration method was employed to shift the reference plane to the connector interface, ensuring accurate characterization of the antenna. Additionally, care was taken to ensure that the connector did not contact any part of the material under test, avoiding unintended interactions in the measurements. All measurements were conducted in a laboratory at the University of Liverpool, UK, during the summer, under room-temperature conditions.

The measured results for the antenna without either water or ice and with a layer of water covering its overall top and the slot only are shown in Figure 13.

The results indicate that a good matching between simulations and measurements is always obtained. Almost the same resonance is obtained for the case of no loading for all three frequency bands. For the case of loading with a full layer of water, a difference of up to 0.5 GHz between simulation and measurements is obtained. A smaller difference between simulations and measurements not exceeding 0.125 GHz is obtained when water covers the slot only.

The measured results of the reflection coefficient (S₁₁) for the antenna with a layer of ice covering its overall top and the slot only are shown in Figure 14. A good matching between simulation and measurement results not exceeding 0.3 GHz is also obtained.

The resonance shift obtained after loading the antenna with water and ice from the reference frequency obtained without loading for all cases of simulations explained above is summarized in Table 1.

Based on the results above, cases of detection for the different tested materials can be extracted. However, it is worth noting that resonance was not obtained through measurements when the slot was only used to sense water.

To provide a more comprehensive and realistic scenario, measurements are also conducted for the antenna over time starting with ice on its top until it melted and converted to water. This case includes variations in the effective permittivity during conversion from ice to water. Figure 15 shows the results of the measured reflection coefficient in dB for four samples taken every 1.5 min. It is worth indicating that measurements are taken at room temperature during summer and this explains the short overall time taken for the ice to melt. However, this does not affect the reliability of the experiment in which variations in the ice state and its reflection on the effective relative permittivity value are more important than the time needed for the status to change. The resultant resonant frequency and shifts are summarized in Table 2.

The results in the figure and table indicate that the frequency begins to shift down gradually as the status of the ice changes from solid to liquid. Each case obtained a distinct resonant frequency. Generally, the results obtained in this case support the detection criteria and results obtained in Table 2. For example, a deviation of only 0.004 GHz between the measured cases at t = 6 min and that for water covering the entire patch surface was obtained, which ensures that a frequency of 4.34 GHz is reliable enough to detect or sense water in this case. However, to guarantee an accurate detection of the material state, decisions based on the frequency values registered in the two frequency bands at the same time are considered for decisions, as summarized in Table 3. This provides more accurate decisions as “and” conditions should be satisfied for each material status to be detected.

Based on the results provided in Table 1 and Table 2, it can be clearly seen that using the entire patch surface as a sensor supports the capabilities of more reliable detection. Exploiting the slot only for the detection process minimizes the antenna sensitivity for some cases, as noted for the 4.844 GHz frequency that was obtained two times for two different materials (ice on the slot only and frost). Thus, the results based on sensing with the entire patch surface are only presented in Table 3. While some frequency bands, such as the (5.537–5.586 GHz) band, which is allocated for frost detection, lie within the frequency band (5.516–5.784 GHz) allocated for ice detection, the other frequency band (4.83–4.865 GHz) allocated for frost has no values or range in common with (4.528–4.809 GHz) allocated for ice. This ensures an accurate distinction between the different materials under test. A sufficient gap between the resonant frequencies across the detection ranges for the different materials under test is maintained, which validates the effective sensitivity of the proposed antenna.

3.4. Performance for Data Transmission

3.4.1. Radiation Characteristics

In this section, the antenna performance in the far-field region is evaluated. The far-field parameters involve radiation pattern, gain, and radiation efficiency.

The 3D simulated far-field radiation patterns at 5.6 GHz of the proposed antenna without loading and with water/ice are shown in Figure 16. The main radiation is obtained at the direction of the patch in the azimuth plane at an angle of 10 degrees. The same shape and orientation are almost kept for the cases of the three layers resembling frost, whether they cover the entire patch or slot only. They are also maintained for the case of ice loading. However, the maximum radiation angle is tilted by up to 50 degrees when water covers the entire patch.

A summary of the realized gain and radiation efficiency for the cases of investigations at 5.6 GHz is provided in Table 4.

The largest radiation efficiency and gain are obtained when it is not loaded with any material. The smallest values are obtained for the case of a full layer of water due to its highest conductivity and corresponding loss. Partial layers of both water and ice obtained larger radiation efficiency and gain than their corresponding full layers, which is due to their smaller effective conductivity than that for full layers. The small values of conductivity for ice made its radiation efficiency and gain values close to that case of no loading.

The radiation pattern of the frost cases (partial) is the same for the three layers and identical to that case of no loading. It is also almost the same for the case of the three frost layers covering the entire patch. This is because their dielectric properties are close to each other.

In addition to enhancing the antenna’s sensitivity, the cross slot improves the antenna gain by 5 dBi in the proposed design. The cross slot helps to effectively reshape the radiation pattern, reducing side lobes and focusing more energy into the main beam direction. The slot works to concentrate the field, achieving a maximum value in a specific direction at the front of the antenna. This contrasts with the antenna without the cross slot, where radiation is scattered in various directions, reducing its directivity in a single direction within the desired plane, as shown in Figure 17. This effective concentration of the radiation caused by the cross slot enhances the antenna’s directivity by around 3.3 dBi, as indicated in the figure, leading to an increase in the antenna gain as follows [22,35]:

G = η \times D

(11)

where

G

is the antenna gain,

η

is the radiation efficiency, and

D

is the directivity, all of which are unit-less for this case. However, gain and directivity are typically expressed in dBi, while radiation efficiency is usually expressed in percentage or dB.

In addition, the slot enhances radiation efficiency by increasing power radiation and reducing energy confinement within the structure, resulting in an approximately 2 dB increase in the radiation efficiency of the proposed antenna.

The 3D radiation plot of the antenna demonstrates that it supports single polarization only. Therefore, it is best suited for operation with a receiving antenna of matching polarization. However, as a patch antenna, its structure can be modified to obtain dual, reconfigurable, or circular polarization [36], which will be a part of our extended work in the future.

3.4.2. Link Budget Calculations

One of the main parameters that should be evaluated for the antenna is the distance of communication. This distance can be estimated based on a number of parameters, including the gain, radiation efficiency, input power, and receiver sensitivity. These parameters are summarized in Table 5, with values considered based on a similar communication case at almost the same frequency [37,38].

The distance can be calculated with reference to Equation (12).

P_{r x} = P_{t x} + G_{t x} + G_{r x} - 10 n l o g \frac{d}{d_{0}} - 10 l o g {(\frac{4 π d_{0}}{λ_{0}})}^{2} - L_{C} - L M

(12)

where

P_{t x}

is the input transmitted power,

P_{r x}

is the receiver sensitivity,

G_{t x}

is the transmitting (our proposed) antenna gain,

G_{r x}

is the receiving antenna gain, which can attain a value of 13 dBi at 5.6 GHz [39], LM is the link margin with a value of 10 dB, which is considered large enough to handle any unexpected losses in such systems [37,40], n is the path loss exponent which is assumed 3 for an outdoor propagation [41], and

L_{C}

is the cable loss.

Based on the above-mentioned parameters, the distances over which the antenna can work to send the sensed data are 80 and 208.6 m for the cases of water and frost, respectively.

4. Conclusions

In this paper, a cross-shape slotted patch antenna is designed and proposed for ice, water, and frost detection in the sub-6 GHz band. With a designated sensing region, the antenna can detect water, ice, and frost through a small frequency shift of just 7 MHz when the slot is loaded with a frost-mimicking material. It can also measure variations effectively when the material being tested is fully loaded with the detected material. The proposed antenna is capable of providing a clear decision to detect water, ice and frost based on a corresponding received frequency range for the case of full loading with the material under test. In addition, the antenna has obtained a good radiation performance for transmitting the sensed data over a communication range of up to 208.6 m. The slot effect on the antenna performance as a sensor and as a data transmitter is investigated, indicating that the slot works to strengthen the sensing capabilities of the antenna by strengthening the near electric field coupling. It also improves the transmission capabilities of the antenna by increasing the radiation directivity and gain by around 5 dBi.

The investigations in this paper have shed light on the effectiveness of the slot structure in controlling the antenna sensitivity and gain. However, further slot structures can be investigated. Moreover, the relationship between the equivalent circuit model parameters and sensitivity may also be extracted. The effect of the slot on other antenna types may also be studied. Complementary split-ring-based slots are recommended for future investigations due to their unique ability to control near-field electromagnetic behavior, which may influence sensitivity levels, as suggested by the results obtained in this paper.

Funding

This research was funded by the North Atlantic Treaty Organization (NATO)/Science for Peace and Security (SPS): funder reference SPS G5932, RESCUE; project reference WT 433209.

Informed Consent Statement

Not applicable

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Acknowledgments

We would like to thank the funder (NATO/SPS), in addition to Mutah University and the University of Liverpool, for the support of this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. A block diagram of a wireless detection system based on antennas.

Figure 2. The proposed antenna design: (a) typical patch without a slot, (b) with a cross-shaped slot; L_G = 40, W_G = 40, L = 21, W = 30.5, L_m = 15, W_m = 3, L_S = 15, W_S = 4, S₁ = 3, S₂ = 4, S₃ = 7; dimensions in mm.

Figure 3. The −10 dB reflection coefficient of the proposed cross-slotted patch antenna in comparison with the corresponding typical rectangular patch antenna.

Figure 4. The antenna with a (a) full layer of water, ice, or frost and (b) partial layer of water, ice, or frost on the slot only; loading layers are presented in blue.

Figure 5. The reflection coefficient (S₁₁) of the simulated antenna with a 1 mm layer of water and ice on its top.

Figure 6. The reflection coefficient (S₁₁) of full and partial layers of water and ice on the antenna top.

Figure 7. The reflection coefficient (S₁₁) of the three frost layers (ɛ_r = 1.5 (F1), 2 (F2), 2.5 (F3), and σ = 1 × 10⁻⁵ S/m) covering the entire patch surface.

Figure 8. The reflection coefficient (S₁₁) of the three frost layers (ɛ_r = 1.5 (F1), 2 (F2), 2.5 (F3), and σ = 1 × 10⁻⁵ S/m) covering the slot only.

Figure 9. The near electric field of the antenna: (a) without the slot and (b) with the slot. The top and bottom arrows indicate the maximum and minimum visualized magnitudes of the near electric field, respectively.

Figure 10. The reflection coefficient (S₁₁) of different ice layers of 2, 3, 4, and 5 mm thicknesses.

Figure 11. The fabricated cross-shaped slot antenna.

Figure 12. The fabricated cross-shaped slot antenna loaded with (a) water and (b) ice.

Figure 13. The measured results compared to simulated ones of the reflection coefficient (S₁₁) for the antenna without loading and with water covering the overall antenna top (full) and the slot only (part).

Figure 14. The measured results compared to simulated ones of the reflection coefficient (S₁₁) for the antenna with a layer of ice covering the overall antenna top and the slot only.

Figure 15. The measured results of the reflection coefficient (S₁₁) for the antenna for four samples starting with an ice layer taken every 1.5 min.

Figure 16. The radiation pattern at 5.6 GHz for the antenna: (a) without loading and (b) loaded with water and ice.

Figure 17. The 3D directivity pattern for the antenna: (a) without the slot and (b) with the slot. The top and bottom arrows indicate the maximum and minimum visualized magnitudes of the directivity, respectively.

Table 1. A summary of the resonant frequencies and their shift in GHz for the cases of no loading, loading with water, loading with ice, and loading with frost.

Case	Band #1		Band #2
	Resonant Frequency	Frequency Shift	Resonant Frequency	Frequency Shift
No water/no ice
Simulations Measurements	5.607	-----	4.886	-----
Simulations Measurements	5.61		4.834
Water/on the entire patch
Simulations Measurements Water/on the slot only	4.487	1.120	3.78	1.106
Simulations Measurements Water/on the slot only	4.34	1.267	3.348	1.538
Simulations Measurements	4.585	1.022	4.039	0.847
Simulations Measurements	4.482	1.125	No resonance	------
Ice/on the entire patch
Simulations Measurements	5.516	0.091	4.809	0.077
Simulations Measurements	5.784	0.177	4.528	0.358
Ice/on the slot only
Simulations Measurements	5.565	0.077	4.844	0.042
Simulations Measurements	5.44	0.167	4.849	0.037
Frost/on the entire patch
Simulations (F1)	5.586	0.021	4.865	0.021
Simulations (F2)	5.558	0.049	4.844	0.042
Simulations (F3)	5.537	0.07	4.83	0.056
Frost/on the slot only
Simulations (F1)	5.6	0.007	4.872	0.014
Simulations (F2)	5.586	0.021	4.865	0.021
Simulations (F3)	5.579	0.028	4.851	0.035

Table 2. A summary of the resonant frequencies and their shifts in GHz for four cases at different times, starting from ice at the entire patch top.

Case	Band #1		Band #2
	Resonant Frequency	Frequency Shift	Resonant Frequency	Frequency Shift
t1 = 1.5 min	5.792	0.185	4.496	0.39
t2 = 3 min	5.744	0.137	4.456	0.43
t3 = 4.5 min	5.544	0.063	4.376	0.51
t4 = 6 min	5.488	0.119	4.344	0.542

Table 3. A summary of the detected cases based on observed resonant frequencies for the case of entire top loading.

Received Frequency Range 1	Received Frequency Range 2	The Detected Materials Based on the Received Frequency Range
4.34–4.487	3.348–3.78	Water
5.516–5.784	4.528–4.809	Ice
5.537–5.586	4.83–4.865	Frost
5.488–5.792	4.344–4.496	In between water and ice

Table 4. A summary of the radiation efficiency and gain of the proposed antenna at 5.6 GHz.

Case			Radiation Efficiency (%)	Gain (dBi)
No water/no ice			40	4.2
Water	On the entire patch		4.2	−8.34
Water	On the slot only		7.43	−6.15
Ice	On the entire patch		36	3.52
Ice	On the slot only		39.5	4
Frost	On the entire patch	F1	38.15	3.95
		F2	38	3.884
		F3	37.4	3.764
	On the slot only	F1	38.3	3.99
		F2	38.8	4.03
		F3	39.2	4.052

Table 5. Parameters for the evaluated communication link.

Parameter	Symbol	Unit	Value
Frequency	f	GHz	5.6
Input transmitted power	$P_{t x}$	dBm	17
Receiver sensitivity	$P_{r x}$	dBm	−83
Transmitting antenna gain	$G_{t x}$	dBi	−8.34 (worst case—water) 4 (best case—frost)
Receiving antenna gain	$G_{r x}$	dBi	13
Link margin	LM	dB	10
Reference distance	$d_{0}$	m	1
Free space wavelength	$λ_{0}$	m	0.054
Path loss exponent	n	----	3
Cable loss	$L_{C}$	dB	3

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Alrawashdeh, R. A Cross-Shaped Slotted Patch Sensor Antenna for Ice and Frost Detection. Technologies 2025, 13, 5. https://doi.org/10.3390/technologies13010005

AMA Style

Alrawashdeh R. A Cross-Shaped Slotted Patch Sensor Antenna for Ice and Frost Detection. Technologies. 2025; 13(1):5. https://doi.org/10.3390/technologies13010005

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Alrawashdeh, Rula. 2025. "A Cross-Shaped Slotted Patch Sensor Antenna for Ice and Frost Detection" Technologies 13, no. 1: 5. https://doi.org/10.3390/technologies13010005

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Alrawashdeh, R. (2025). A Cross-Shaped Slotted Patch Sensor Antenna for Ice and Frost Detection. Technologies, 13(1), 5. https://doi.org/10.3390/technologies13010005

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A Cross-Shaped Slotted Patch Sensor Antenna for Ice and Frost Detection

Abstract

1. Introduction

2. Design Procedure and Methods

3. Performance and Analysis

3.1. Working Principle and Sensing Mechanism

3.2. Performance as a Sensor

3.3. Validation via Measurements

3.4. Performance for Data Transmission

3.4.1. Radiation Characteristics

3.4.2. Link Budget Calculations

4. Conclusions

Funding

Informed Consent Statement

Data Availability Statement

Acknowledgments

Conflicts of Interest

References

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