Compact CPW-fed Antenna with Controllable WLAN Band-rejection for Microwave Imaging

This work presents the design and experimental validation of a compact frequency-reconfigurable coplanar waveguide (CPW)-fed ultra-wideband (UWB) antenna for application in microwave imaging systems with a capability to switch between UWB modes, with and without WLAN band-notched features. The suggested topology and design process are used to achieve an enhancement in the bandwidth using T-shaped slots between the feedline and the ground plane of the antenna, whereas the WLAN band is rejected using an open loop resonator (OLR) placed on the antenna backside. Switching between UWB with and without WLAN band-notched modes is performed using a single PIN diode. The simulation results corroborate well with the experimental data and clearly show an interesting frequency reconfigurable behavior for use in microwave imaging applications. The antenna performance simulation and analysis model is presented for breast tumor detection.


I. INTRODUCTION
icrowave imaging has emerged as a promising tool in the field of medical diagnostics, particularly in the context of breast cancer detection.This modality exploits disparities in dielectric properties between healthy and malignant tissues.In comparison to established imaging techniques such as magnetic resonance imaging and X-ray mammography, microwave imaging offers advantages in terms of cost-effectiveness and the utilization of nonionizing radiation [1].For effective implementation of microwave imaging, ultrawideband (UWB) antennas are indispensable due to their capability to generate short pulses in the time domain, facilitating high data rates [2].Furthermore, the flexibility of UWB antennas allows for the combination of low frequencies, enabling enhanced penetration depth, and high frequencies, contributing to superior image resolution [3].A comparative study of antennas for microwave imaging systems, as presented in [4], shows that the gain varies from 2.43 dBi to 8.4 dBi depending on the antenna characteristics.Increasing the antenna size leads to increased gain; however, there is a trade-off between achieving acceptable gain and managing the antenna size.Numerous UWB antennas have been proposed for microwave imaging applications, encompassing diverse designs such as square microstrip monopole antennas [5], CPW taper arc slot antennas [6], flexible dual-polarized antennas [7], hibiscus petal pattern-shaped antennas [8], slotted patch antennas [2,9,10], and vivaldi antennas [11,12].On the other hand, to avoid potential interference between UWB and other communication standards, deploying UWB antennas featuring band-notch characteristics has been suggested for microwave imaging.In [13], a UWB antenna incorporating a semi-elliptical defected ground structure, and featuring an X-band notch using a C-structured reflector has been reported.Furthermore, in [14], a UWB polygon antenna with WiMAX and WLAN bands notched has been proposed; the band-rejections were obtained using ring slot and EBG structures.Moreover, [15] described a monopole antenna featuring double U-shaped slots on the radiating element to reject WLAN and WiMAX bands.Additionally, in [16], a Multiple Input Multiple Output (MIMO) UWB slot antenna with WLAN band-notched using U-shaped slot has also been presented.The concept of frequency reconfigurable antennas (FRA) offers the advantage of integrating various operating modes, such as narrowband, multi-band, UWB, and bandnotch, within a single antenna.The transition between these operating modes can be achieved through mechanisms like Micro-Electro-Mechanical Systems (MEMS) [17], varactor diodes [18], or PIN diodes [19,20].Whereas FRAs find widespread use in emerging communication standards [20,21], their application in microwave imaging systems has been relatively limited.In the literature, a CPW antenna with a tunability feature using varactors [22] has been M reported, as well as a tapered slot antenna that switches between UWB and narrowband modes [23].This paper introduces a compact coplanar waveguide (CPW)-fed antenna with a capability to switch between UWB modes, with and without WLAN band-notched features, specifically designed for microwave imaging applications.The antenna design and simulation are conducted using CST Microwave Studio, comprising a square radiating patch fed by a 50-ohm CPW line, T-shaped slots, and an open-loop resonator (OLR).The novel design strategies involve using a single OLR on the antenna backside to create a band-notch and employing a single PIN diode to achieve a straightforward frequency-reconfigurable antenna with a simple switching mechanism.Additionally, the antenna is proposed for application in microwave imaging systems, operating in both interfered and uninterfered environments.

A. DESIGN OF UWB ANTENNA
The geometric layout of the initial UWB antenna is shown in Fig. 1(a).The antenna is printed on an FR4 substrate of 1.6 mm in thickness, a relative permittivity of 4.  I.In order to ensure that the proposed antenna performance could meet the desired bands, parametric studies are carried out.The effect of varying the length of the radiating element "Lp", and the ground plane length "Lg" are investigated as shown in Fig. 2. It is clearly demonstrated that the "Lp" value influences significantly on the first resonant frequency.Increasing the length of "Lp" results in a downward shift of the lower edge frequency, while decreasing the length has the opposite effect.Additionally, the "Lg" affects the impedance matching of the antenna.
Interesting results were obtained with Lg = 11.8 mm, which allows a better coupling between the ground plane and the radiating element.The fabricated UWB antenna is depicted in Fig. 1(b).Measurements are performed using the KEYSIGHT PNA N5224A network analyzer.Fig. 3 shows the measured and simulated magnitude of the reflection coefficient.
Based on [5], the dimension of T-shaped slots can be approximated by the following equation: where λg is the guided wavelength of the newly created resonance at 9.2 GHz.The measurement setup for the radiation pattern of the proposed UWB antenna in the anechoic chamber is shown in Fig. 4, the simulated and measured radiation patterns at both 4 GHz and 8 GHz are presented in Fig. 5.One can observe that the proposed antenna exhibits radiation patterns similar to those of a monopole antenna.It has an omnidirectional radiation pattern in the H-plane and a bidirectional radiation pattern in the E-plane.The input transmitted pulse (Tx) and the received one (Rx) for two identical antennas (face to face and side by side configurations) separated by a distance d of 150 mm are shown in Fig. 6.It can be seen that the shape of the pulse is conserved for both configurations.

B. FILTERING MECHANISM
To create a band-notch characteristic, an open loop resonator is inserted on the back face of the proposed UWB antenna (Fig. 7), close to the feedline and the radiating element, to enhance the coupling between the OLR and the antenna.The central frequency and filtering level of the band-notched depend on the OLR dimensions and its position.The considered dimensions of the OLR (placed at the position "O" with x=5.65 mm, y=11.8 mm), in order to obtain WLAN band-notch with a central frequency at 5.66 GHz, are: R=2.6 mm, W=0.6 mm, Gap=0.4 mm.Based on [24], the relationship between the central frequency of the band-notched and the OLR dimension is expressed by the following equation:  The equivalent circuit of the proposed UWB antenna with WLAN band-notched is shown in Fig. 8.It is approximated by using multiple cascaded LC resonator circuits [25].The filtering effect generated by the OLR is represented by a parallel RnLnCn network [26].The considered values of the lumped elements are listed in Table II.The electromagnetic simulation, equivalent circuit simulation and measurement of S11 are shown in Fig. 9.  Fig. 10 shows the surface current distribution at a frequency inside the rejected WLAN band (5.66 GHz), and at a frequency outside the WLAN band (8 GHz), respectively.The current is concentrated on the OLR at 5.66 GHz and is less dense at 8 GHz, which demonstrates the implication of the OLR on the WLAN band rejection.It captures frequencies proportional to its electrical length and lets the other frequencies pass to be radiated by the square patch.

C. RECONFIGURATION MECHANISM
For the first phase of test and measurement, the switch states (ON/OFF) are modeled by the presence and absence of cooper (ideal switch).The optimal position to integrate the switch is in the middle of the OLR (Fig. 11(a)).The surface current distribution at 5.66 GHz for both switch states is shown in Fig. 12, it demonstrates that when the switch is disabled, the current density is very low on the OLR, so the filter effect is disabled in this case and the UWB band is achieved without any frequency band rejection.When the switch is activated, an UWB antenna with WLAN band-notched is obtained (filter is enabled).The simulated and measured S11 of the two prototypes are presented in Fig. 13.By comparing the realized gain as a function of frequency for the two modes illustrated in Fig. 14, it is observed that the gain drops significantly to the negative value at the notched WLAN band, regarding other operating frequencies the gain remains almost identical.For the final implementation with a real switch, a PIN diode SKYWORKS SMP1321-079LF up to 10 GHz, two inductors of 100 nH, which block the RF signal and allow the DC signal to pass through, and printed DC bias lines of 0.4 mm width are used (Fig. 11(b)).The PIN diode is simulated with the s2p file available on the manufacturer's website [27].From the SMP1321 Series datasheet, at the ON state, the PIN diode is equivalent to a resistance Rs=2 Ω in series with parasitic inductance Ls=0.7 nH and, at the OFF state it is equivalent to a capacitor Ct=0.25 pF shunted by a parallel resistance Rp of high value similar to an open circuit in series with Ls.In the case of a real switch, some changes are made to the dimensions of the OLR (R=2.5 mm, W=0.8 mm) along with its position (x=3.75,y=12) in order to reduce the inductive and capacitive effects of the diode and remain centered on the notched WLAN band.The photograph of the fabricated reconfigurable antenna using a PIN diode is shown in Fig. 15, whereas Fig. 16 shows the simulated and measured S11, the reconfiguration ability of the antenna is clearly seen, the slight variations in the measured results are primarily due to the impact of soldering the SMA connector to the feed line, and DC wires to DC bias lines.

III. BREAST TUMOR DETECTION
A hemispherical breast phantom is designed, with dimensions specified by a skin layer of 2 mm thickness, a fat layer of 48 mm thickness, and a spherical tumor with a radius of 5 mm lodged in the fat layer.The breast phantom is built using materials from the CST bio tissue library, except the tumor was created with a relative permittivity of 50, and 4 S/m of conductivity [7].The dielectric properties of skin and fat are shown in Fig. 17  The antenna is set in a monostatic microwave imaging setup placed at 10 mm from the breast phantom, as presented in Fig. 18.Finally, a comparison in terms of bandwidth, peak gain, antenna size, and reconfigurability between the proposed antenna and some existing antennas for microwave imaging applications is illustrated in Table III.

IV. CONCLUSION
A compact and reconfigurable monopole antenna able to switch between UWB and WLAN band-notched modes is presented.The switching between the two modes is possible using a PIN diode inserted on an OLR.The simulated and measured results are in good agreement.The proposed antenna is suited to use for microwave imaging applications in the presence and/or absence of interferences.
3, and a loss tangent of 0.025.The proposed antenna consists of a square radiating patch fed by a 50 Ohm CPW line, a pair of T-shaped slots is introduced into the ground plane to create a new resonance at 9.2 GHz and allow the bandwidth to be increased from [2.94-8.75GHz] to [2.94-11 GHz], it signifies an improvement of 16.23 % in bandwidth and covers the entire UWB band.T-shaped slots influence the surface current distribution along both X and Y axes, whereas the vertical (or the horizontal) rectangular slot affects only one axis.The optimized antenna dimensions are listed in Table

FIGURE 2 .
FIGURE 2. Effect of varying the lengths.(a) Length of the patch and, (b) length of the ground plane.

FIGURE 3 .
FIGURE 3. Magnitude of S11 for the designed UWB antenna.

FIGURE 6 .
FIGURE 6. Transmitted and received pulses.(a) Face to face and, (b) Side by side.
and LOLR are speed of the light, effective permittivity, and total length of the OLR, respectively.

FIGURE 7 .
FIGURE 7. Geometry and position of the integrated OLR.

FIGURE 8 .
FIGURE 8. Equivalent circuit of the proposed UWB antenna with WLAN band-notched.

FIGURE 9 .
FIGURE 9. Magnitude of S11 for the proposed UWB antenna with WLAN band-notch characteristic.

FIGURE 10 .
FIGURE 10.Simulated surface current distributions of the proposed antenna at: (a) 5.66 GHz and, (b) 8 GHz.

FIGURE 12 .
FIGURE 12. Simulated surface current distributions of the proposed antenna at 5.66 GHz. a Switch ON and, b Switch OFF.

FIGURE 13 .
FIGURE 13.Magnitude of S11 for reconfigurable UWB antenna using ideal switch.

FIGURE 14 .
FIGURE 14. Realized gain of the reconfigurable antenna for both UWB and Band-notched modes.

FIGURE 15 .
FIGURE 15.Photograph of the fabricated reconfigurable antenna.(a) Top view, (b) Bottom view, and (c) During measurement.

FIGURE 16 .
FIGURE 16.Magnitude of S11 for reconfigurable UWB antenna using a PIN diode. .

FIGURE 17 .
FIGURE 17. Dielectric properties of skin and fat layer.(a) Relative permittivity, and (b) Conductivity.

FIGURE 18 .
FIGURE 18. Simulation setup of the antenna and breast phantom.

Fig. 19
Fig.19depicts the reflected signal in both the presence and absence of the tumor.It is noticeable that there is a contrast between the reflected signals, and that a signal indicating the existence of the tumor can be generated by subtracting the reflected signals (with tumor and without tumor).For the two operating modes of the antenna (UWB and band-notched UWB) it can be seen that the tumor detection is possible, which is a good solution for an interfered or an un-interfered environment.

TABLE I .
Physical dimensions of the UWB antenna Parameter Value (mm) Parameter Value (mm)

TABLE II .
Optimized lumped elements values of the equivalent circuit

TABLE III .
Comparison between the proposed antenna and other previously published antennas