A Flexible and Compact UWB MIMO Antenna with Dual-Band-Notched Double U-Shaped Slot on Mylar^® Polyester Film

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript introduces a MIMO antenna with dual band notched double U-shaped slot. The proposed antenna is fabricated and measured. I think the paper is well organized, but there are the following minor issues that need to be addressed before publication:

1. The benefits of using Mylar® polyester film can be explained in the manuscript.
2. What is the radiation efficiency of the antenna?
3. The reflection coefficient at the notch location does not approach 0 dB, which means that energy is radiated out. Can you give the radiation patterns at the notch frequencies (3.5 and 5.5 GHz)?

Author Response

1. The benefits of using Mylar® polyester film can be explained in the manuscript.

Ans: We revised and added the benefits of using Mylar polyester film in the statement, “This flexibility and robustness are achieved by the utilization of Mylar® polyester film, which provides advantages such as strength, durability, lightweight characteristics, flexibility, and exceptional resistance to moisture, chemicals, and extreme temperatures.” with red text in page 3, paragraph 4, line 3.

 

2. What is the radiation efficiency of the antenna?

Ans:  We revised and added the antenna efficiency in statement, “Moreover, the simulation results of antenna efficiency, depicted in Figure 15, clearly demonstrate the antenna's ability to sustain high radiation performance over its operational bandwidth. The antenna exhibits an efficiency surpassing 80% within the specified ultra-wideband (UWB) frequency range, confirming that the designed structure allows effective impedance matching and ensures efficient radiation with minimal power losses from absorption. Conversely, at the purposely notched bands—aligned with the WiMAX band (3.3–3.7 GHz) and the WLAN band (5.1–5.8 GHz)—the antenna efficiency significantly declines to below 50%. This reduction directly results from the effective application of double U-shaped slot structures, which attenuate radiation in certain frequency ranges through destructive interference and obvious resonances. As a result, negligible electromagnetic energy is emitted at the notched frequencies, thus reducing the possibility of interference with current WiMAX and WLAN systems while maintaining ideal efficiency in other segments of the UWB spectrum.” with red text in page 17, paragraph 1, line 1.

 

3. The reflection coefficient at the notch location does not approach 0 dB, which means that energy is radiated out. Can you give the radiation patterns at the notch frequencies (3.5 and 5.5 GHz)?

Ans:  The reflection coefficient at the notch location cannot approach 0 dB due to the persistence of some radiated energy. The radiated energy at the notch frequencies of 3.5 and 5.5 GHz exhibits a gain of -5 dB at boresight (0 degree), which is below 0 dB, as illustrated in Figure 12. Consequently, the presented antenna can diminish the radiated energy by at least -5 dB at the notch frequency.  

We added the radiation patterns at the notch frequencies (3.5 and 5.5 GHz) in Figure 13 and explained the statement, “Figure 13a illustrates that the radiation patterns at 3.71 GHz exhibit an almost omnidirectional configuration in the X–Z plane and a bidirectional characteristic in the Y–Z plane. Co-polarization predominates over cross-polarization, signifying steady radiation with high polarization clarity. It is important to notice that the gain levels at the boresight direction (0°) are below -5 dB for both planes. The considerable decrease in radiation intensity immediately indicates the suppression effect of the designed notch band at this frequency, hence reducing radiated energy and providing successful interference mitigation. At 5.78 GHz, as depicted in Fig. 13b, the antenna demonstrates complicated multi-lobed radiation patterns in both the X–Z and Y–Z planes because of the activation of higher-order modes. Similarly to the 3.71 GHz situation, co-polarization continues to dominate, although cross-polarization becomes marginally more prominent. The boresight direction demonstrates a gain level below –5 dB, confirming that the antenna efficiently attenuates radiated power at the notched frequency. This confirms that the notching technique effectively diminishes unwanted radiation in the primary direction, thereby ensuring efficient rejection of potential interference sources, including WLAN networks at around 5.8 GHz.

The ongoing finding of diminished gain (< –5 dB) at the boresight for both notched frequencies signifies that the antenna's radiated energy is significantly dampened at these bands. This behavior validates the efficacy of the notched design in attenuating extraneous signals, while the maintained omnidirectional or multi-lobed radiation at alternative angles guarantees dependable coverage within the operating UWB spectrum. Moreover, these radiation attributes correlate well with the return loss (S₁₁) measurements, exhibiting pronounced notches at 3.71 GHz and 5.78 GHz, as well as with the port-to-port isolation findings, confirming that mutual coupling does not impair antenna performance at these attenuated frequencies. Consequently, the suggested dual-notch antenna attains frequency-selective rejection and stable radiation properties, rendering it appropriate for contemporary UWB communication systems.” with red text in page 15, paragraph 1, line 1.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors present a planar antenna following a coplanar excitation with two slits that work as a notch filtering. The prototype was manually manufactured, and it seemed to provide good results. Some observations:

- Fig. 5 seems to be low resolution.
- Fig. 11 low resolution as well, particularly those exported from CST 3d power pattern.
- In fig. 12 authors should mention that the gain is in the broadside direction.
- With regard to eq. 1, it should be mentioned that the use of S parameters for the ECC is a simplification from the farfield result.
- measured gain and scattering parameters should be better appreciated together with the simulations, to see the correlation.
- From fig. 14 one can see that the antenna was manually fabricated, without the use of a milling machine (LPKF). It can be seen that the slits were irregular as well as the SMA soldering. I can guess that deviations from the ideal scenario of simulation might appear, therefore both results in the same plot should be present, with the justification for eventual discrepancies.
- still the manual fabrication, how come are you assuring that the 0.5mm gap was kept in the prototypes? It seems a very demand requirement to be manually adjusted.
- Plots in general seem to be too large - for instance figs. 3, 4 6 to 13, 16

Author Response

1. Fig. 5 seems to be low resolution.

Ans:  Figure 5 has been enhanced to a higher quality resolution as indicated by the reviewer.

 

2. Fig. 11 low resolution as well, particularly those exported from CST 3d power pattern.

Ans:  Figure 11 has been revised and changed to Figure 13. It was enhanced to a higher quality resolution as indicated by the reviewer.

 

3. In fig. 12 authors should mention that the gain is in the broadside direction.

Ans: Figure 12 has been revised and changed to Figure 14. We revised by adding the statement, “Simulated gain of the proposed antenna with and without the notch in the broadside direction as altering r = 0, 10, 20 mm.” below Figure 14 with the red text on page 16.

 

4. With regard to Eq. 1, it should be mentioned that the use of S parameters for the ECC is a simplification from the Farfield result.

Ans:  We revised and added the statement, “Furthermore, the application of the S parameter for the ECC is a simplification from the far-field result” with the red text in page 17, paragraph 2, line 3.

 

5. Measured gain and scattering parameters should be better appreciated together with the simulations, to see the correlation.

Ans:  We revised and explained the different results of the simulation and measured S-parameters in Figure 18 on page 18, paragraph 2, line 8, and gain in Figure 20 at page 20, paragraph 2, line 7 with the red text.

 

6. From fig. 14 one can see that the antenna was manually fabricated, without the use of a milling machine (LPKF). It can be seen that the slits were irregular as well as the SMA soldering. I can guess that deviations from the ideal scenario of simulation might appear, therefore, both results in the same plot should be present, with the justification for eventual discrepancies.

 

Ans:

For S-parameter:

We revised and explained the different results of the simulation and measured gain in the statement, “It is also evident from Figure 18 that slight shifts occur between the simulated and measured resonance frequencies of the dual notch bands. These variations are primarily ascribed to manufacturing tolerances and the manual assembly of the prototype. Nonetheless, the overall concordance between the two results affirms the consistency and reliability of the suggested antenna.” with red text in page 18, paragraph 2, line 8.

 

For gain:

We revised and explained the different results of the simulation and measured gain in the statement, “It is also observed in Figure 20 that slight discrepancies exist between the simulated and measured results in terms of notch frequencies and gain levels. These deviations are primarily attributed to fabrication tolerances and the manual assembly of the antenna prototype. Nevertheless, the overall agreement between simulation and measurement validates the proposed design and confirms the reliability of the antenna performance.” with red text in page 20, paragraph 2, line 7.

 

7. Still the manual fabrication, how come are you assuring that the 0.5mm gap was kept in the prototypes? It seems a very demand requirement to be manually adjusted.

 

Ans:  The manual fabrication indicates that the gap ranges from 0.5 mm to 1 mm, leading to discrepancies in the resonance frequency between the simulation findings and the observations, as explained and illustrated in Figure 18. Nonetheless, the measurement outcomes exhibit a resonant frequency trend that aligns with the simulation results, maintaining functionality within the operational frequency range and preserving the indicated notch frequency.

 

8. Plots in general seem to be too large - for instance figs. 3, 4 6 to 13, 16

Ans:  Figures 3, 4, 6 through 13, which were shifted to 16, and 16 shifted to 19, have been improved and reduced in size according to the reviewer's recommendation.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors present an interesting study on a UWB MIMO antenna implemented on a flexible Mylar dielectric substrate.

The manuscript is well written and has a good logical flow.

In order to further improve its quality, I suggest the following revisions:

• Literature review enhancement:

The review of the literature should be expanded to include more references to flexible or conformal antennas.

It would also be valuable to emphasize that conformal antennas are a topic of growing interest in the literature. Enriching the Introduction with references that extend the concept of conformal antennas to other technologies, such as reflectarrays, transmitarrays, and metasurfaces, would help underline their broad applicability. For example:

1) C. Fan, W. Zhao, K. Chen, J. Zhao, T. Jiang, Y. Feng, Optically Transparent Conformal Reflectarray with Multi-Angle Microwave Efficient Scattering Enhancement. Adv. Optical Mater. 2024, 12, 2400151. https://doi.org/10.1002/adom.202400151

2) M. Beccaria, P. Pirinoli and F. Yang, "Preliminary results on Conformal Transmitarray Antennas," 2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Boston, MA, USA, 2018, pp. 265-266, doi: 10.1109/APUSNCURSINRSM.2018.8608872.

3) I. Yoo and D. R. Smith, "Design of Conformal Array of Rectangular Waveguide-Fed Metasurfaces," in IEEE Transactions on Antennas and Propagation, vol. 70, no. 7, pp. 6060-6065, July 2022, doi: 10.1109/TAP.2022.3140822.

Any papers recommended in the report are for reference only. They are not mandatory. You may cite and reference other papers related to this topic.

• Validation of flexibility:

Since the claim is that the antenna is flexible and can conform to non-planar surfaces, it would be beneficial to include at least a preliminary analysis of the electromagnetic performance in a bent configuration.

Even simulated results for a curved geometry, compared with the planar configuration already presented, would strengthen the claim.

Key parameters to compare could include radiation patterns, gain, and impedance bandwidth.

 

Author Response

1. Literature review enhancement: The review of the literature should be expanded to include more references to flexible or conformal antennas. It would also be valuable to emphasize that conformal antennas are a topic of growing interest in the literature. Enriching the Introduction with references that extend the concept of conformal antennas to other technologies, such as reflectarrays, transmitarrays, and metasurfaces, would help underline their broad applicability. For example:

1) C. Fan, W. Zhao, K. Chen, J. Zhao, T. Jiang, Y. Feng, Optically Transparent Conformal Reflectarray with Multi-Angle Microwave Efficient Scattering Enhancement. Adv. Optical Mater. 2024, 12, 2400151. https://doi.org/10.1002/adom.202400151

2) M. Beccaria, P. Pirinoli and F. Yang, "Preliminary results on Conformal Transmitarray Antennas," 2018 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, Boston, MA, USA, 2018, pp. 265-266, doi: 10.1109/APUSNCURSINRSM.2018.8608872.

3) I. Yoo and D. R. Smith, "Design of Conformal Array of Rectangular Waveguide-Fed Metasurfaces," in IEEE Transactions on Antennas and Propagation, vol. 70, no. 7, pp. 6060-6065, July 2022, doi: 10.1109/TAP.2022.3140822.

Any papers recommended in the report are for reference only. They are not mandatory. You may cite and reference other papers related to this topic.

Ans:  We revised the introduction and added the statement, “In addition to advancements in UWB MIMO development, recent research has shifted towards conformal antenna technologies that incorporate radiating elements into curved or non-planar structures, preserving aerodynamic and structural integrity. The concept has been expanded to include conformal reflectarrays, transmitarrays, and metasurfaces, providing lightweight, low-profile, and highly directed radiation systems. In [26], it is proposed that the optically transparent conformal reflectarrays utilizing indium-tin-oxide (ITO) rings and patches exhibit extensive phase coverage, minimal return loss, and improved multi-angle scattering in the X-band, rendering them appealing for stealth and target echo manipulation. Moreover, conformal transmitarrays demonstrate that effective phase control and beam steering can be accomplished on curved surfaces [27], while conformal metasurface arrays utilizing waveguide feeding and semi-analytical dipolar modeling offer systematic frameworks for radiation synthesis with diminished feed complexity [28]. These improvements collectively broaden the potential applications of antennas in communications, radar, stealth technology, and electromagnetic compatibility, hence inspiring the creation of next-generation compact and multifunctional UWB MIMO antennas.” with the red text in page 3, paragraph 3, line 1. Also, the references [26] – [28] were added on page 24.

 

2. Validation of flexibility:

Since the claim is that the antenna is flexible and can conform to non-planar surfaces, it would be beneficial to include at least a preliminary analysis of the electromagnetic performance in a bent configuration.

Even simulated results for a curved geometry, compared with the planar configuration already presented, would strengthen the claim.

Key parameters to compare could include radiation patterns, gain, and impedance bandwidth.

Ans:  We added the simulation results of impedance bandwidth and gain for curved geometry and compared with the planar configuration as varying parameter r = 0, 10, 20 mm.

 

For impedance bandwidth with curve:

The simulation of impedance bandwidth for curved geometry r = 0, 10, and 20 mm is added and explains the statement, “Figure 10a depicts the conformal UWB MIMO antenna configuration with the bending parameter r, representing the extent of curvature when affixed to cylindrical surfaces. To assess the resilience of the suggested design under varying bending situations, the S-parameter responses for r = 0, 10, and 20 mm are illustrated in Figure 10b. The impact on the Reflection Coefficients (S₁₁ and S₂₂) is depicted in Figure 10b. The reflection coefficients for both ports consistently remain below –10 dB throughout the majority of the UWB working band, hence affirming effective wideband impedance matching. When the antenna is flat (? = 0 mm), pronounced notch bands manifest around 3.71 GHz and 5.78 GHz, where S₁₁ and S₂₂ increase sharply above –5 dB, signifying a deliberate impedance mismatch that attenuates energy at these frequencies. As the antenna curvature increases to r = 10 mm and r = 20 mm, the overall impedance matching experiences modest perturbations, accompanied by tiny frequency shifts and fluctuations in notch depth. Nonetheless, the notches remain distinctly visible, indicating that the dual-notch mechanism is resilient to conformal bending. The transmission coefficient (S₂₁) demonstrates inter-port isolation consistently below –15 dB throughout the majority of the UWB band for all curvature scenarios, signifying minimal mutual coupling appropriate for MIMO functionality. At the notch frequencies of 3.71 GHz and 5.78 GHz, S₂₁ exhibits a further decline (becoming more negative), indicating that the antenna not only mitigates radiation but also improves port isolation at the notched bands. This is advantageous in dual-port operation, as it diminishes the likelihood of interference coupling between parts at the rejected channels.

Therefore, it can be noticed that, although slight frequency shifts and variations in impedance matching occur with increasing curvature due to bending, the dual-notch characteristics at 3.71 GHz and 5.78 GHz, as well as the port-to-port isolation, remain preserved, confirming the antenna’s stable performance under mechanical deformation.” with red text in page 10 paragraph 2.

 

 For gain with curve:

      The simulation of gain for curved geometry r = 0, 10, and 20 mm is added and explained the statement, “Additionally, the parametric modification of the blended antenna length with parameters r = 0, 10, and 20 mm indicated that an increase in the parameter r somewhat diminished the notch depth (from −7.3 dBi at r=0 mm to around −5 dBi at r=20 mm) while concurrently enhancing the out-of-band gain. The gain enhancement was particularly significant in the mid- and high-frequency regions, with improvements of 0.9–2.2 dB recorded at 10–14 GHz for r=20 mm relative to the no-notch condition. Nonetheless, given that the specified operating band of the proposed antenna is 3.1–10.6 GHz, the configuration with r=0 mm is considered the most appropriate for practical application, as it provides the greatest rejection at 3.5 and 5.5 GHz while ensuring consistent gain performance throughout the intended UWB band.” with red text in page 16 paragraph 2, line 6.

 

According to the results, the radiation pattern of curved geometry will not significantly differ from that of a flat plane, as the gain in curved geometry does not significantly deviate from that of a flat surface throughout the UWB frequency range. Consequently, it is unnecessary to consider and include the radiation pattern figure with curved geometry in the investigation.  

Author Response File: Author Response.pdf