Transparent 2-Element 5G MIMO Antenna for Sub-6 GHz Applications

Round 1

Reviewer 1 Report

A transparent MIMO antenna arranged in an AgHT-8 substrate is proposed for sub 6 GHz 5G applications. 

-The authors are recommended to add the simulated/measured TOTAL efficiency of the proposed design.
-The authors are requested to explain why the ground plane of the antenna is not full and also compare the performance of the design when the ground plane is full.
-Since a large number of the elements are required for 5G application, 4*4 and 8*8 MIMO performance and radiation characteristics should also be discussed.

-For MIMO antennas, it is desired to use a shared ground plane (for practical purposes), which is not considered in this paper and needs to be justified.
-The bandwidth is limited. Any design technique to improve the bandwidth or generate another operation band to make the antenna multi-band.
-What is the purpose to focus 4.65 to 4.97 GHz, while 3.5 and 3.8 GHz are the main candidate bands at sub 6 GHz 5G spectrum?
-How 50-ohm feedline width is calculated to be 2.2 mm?
-Has the SMA connecter is soldered on the antenna surface.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Minor revision

Comments for author File: Comments.pdf

Author Response

Original Manuscript ID: electronics-1536908         

Original Article Title: “Transparent 2-Element 5G MIMO Antenna for Sub-6 GHz Applications”

 

To: Electronics Editor

Re: Response to reviewers

 

Dear Editor,

 

Thank you for allowing a resubmission of our manuscript, with an opportunity to address the reviewers’ comments.

We are uploading (a) our point-by-point response to the comments (below) (response to reviewers), (b) an updated manuscript with yellow highlighting indicating changes, and (c) a clean updated manuscript without highlights (PDF main document).

 

 

Best regards,

Dr. Issa Elfergani et al.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REVIEWER 2

 

Reviewer#2, Concern # 1: In the text, the citation of figures and tables must be before the latter. For example, we find Figures 8 and 9, then we find their description. And the reverse is what we must have. Likewise, check for the other figures.

 

Author response:  Thank you very much for your valuable comment. The manuscript is updated as per the suggestion.

 

Author action: The changes are carried out in the revised manuscript.

 

Reviewer#2, Concern # 2: Figure 9 shows the current distribution of two antenna elements at the frequency 4.81 GHz when the element 1 is powered and the element 2 is adapted to 50 Ohm, and it is for the two cases of positions of the elements: case 1 and case 2. For case 1 the distance between the two patches is d = 5 mm which represents almost the wavelength / 12 and the second case d = 15 mm which is almost the wavelength / 4. Figure 9 shows that element 2 is not affected by current from antenna 1 in either case. So the coupling is negligible. However, by comparing this result of figure 9 with that of figure 8 we notice that in the working frequency band [4.6 - 4.94] GHz, the parameter S12 (normally S21) is between -15 dB and -18 dB which is not low enough to cancel the current in element 2. We must wait for a lower value, for example -25 dB (which represents 3% of transmission). In addition, there is no clear improvement in case 2 over case 1. It is an isolation gain of 2 or 3 dB despite the distance d being increased threefold.

 

Author response: Thank you very much for your valuable comment. The authors would like to emphasize that in case 1, separate ground profile is used which helps in reducing the interelement coupling however the distance between antenna elements is only 5 mm which is not meeting the spatial diversity criteria of maintaining the distance between (λ/2-λ/4). Due to the same reason the S21 > 17.10 dB is achieved.

In case 2, connected ground profile is introduced however the mutual coupling is reduced by placing the elements 15 mm apart (λ/4). The same leads to achieving the S21> 17.38 dB in case 2.  It shows that spatial diversity plays an important role in increasing the isolation between the antenna elements in connected ground profiles.    

Reviewer#2, Concern # 3: Especially in case 1 where the two elements are very close, the effect of antenna 1 on antenna 2 must be present. Then explain your current density results.

 

Author response:  Thank you very much for your valuable comment. In case 1, separate ground profile is used which helps in reducing the interelement coupling however the distance between antenna elements is only 5 mm which is not meeting the spatial diversity criteria of maintaining the distance between (λ/2-λ/4). Due to the same reason the S21 > 17.10 dB is achieved. The current coupled to antenna element 2 is not observed due to the separate ground profile.

Reviewer#2, Concern # 4: Why you used the distance between elements 5 mm and 15 mm.

 

Author response: Thank you very much for your valuable comment. The distance between the elements is chosen after carrying out the parametric variation by moving the antenna elements along X and Y axis while ensuring the minimum isolation levels > 15 dB. The same is achieved for the distances shown in the proposed antennas.

 

Reviewer#2, Concern # 5: Justify why you did not use an isolation technique to better decouple the two elements, such as: Use of metamaterials, neutralization lines, Defected Ground…

 

Author response:  Thank you very much for your valuable comment. The proposed flexible MIMO antenna design avoids alternative complex mechanisms including the introduction of vias and decoupling structures, which is almost impossible to implement in a transparent antenna design due to fabrication complexities.

 

Reviewer#2, Concern # 6: In the title of Figure 10, the 2D radiation pattern is at the frequency 3.81 GHz which is outside the operating frequency band of your antenna. What is the use at this frequency? If this is a typo, please correct it.

 

Author response:  Thank you very much for your valuable comment. The authors apologize for the same. In figure 10. (Figure 11 in the revised manuscript), the 2D radiation pattern is plotted at 4.81 GHz and not 3.81 GHz. It was a typo error and is corrected in the revised manuscript.

 

Reviewer#2, Concern # 7: The gain of the proposed antenna is low (between 1 dBi and 1.8 dBi). Why? What is the point of using this type of antenna with low gain in 5G.

 

Author response:  Thank you very much for your valuable comment. The gain and efficiency values are low in comparison to the conventional Copper-based antenna designs. The gain and efficiency of the transparent antennas are directly dependent on the sheet impedance of the conductive oxide (8 Ω/Sq) that leads to conductivity value of 706.215 S/m which is very less as compared to copper-based antenna (Conductivity = 5.96×10⁷). While the transparency and conductivity are inversely proportional, the conductivity is lower for the higher optical transparency to be achieved. The low value of conductivity is the main reason for the low gain and efficiency of the transparent MIMO antenna. The uses of the transparent antenna are addressed in the below concern.

 

 

Reviewer#2, Concern # 8: What is also the advantage of using a transparent substrate such as plexiglass and transparent conductive oxide 20 (AgHT-8) as a patch and ground. Put applications of this antenna into practice, (such as antennas integrated on photovoltaic cells, or….).

Author response:  Thank you very much for your valuable comment. The proposed optically transparent MIMO antenna finds its applications in smart indoor devices likes wireless repeaters and routers that works at Sub-6 GHz 5G band. It can be easily interfaced on the office and home buildings where glass structures are utilized. That way it maintains the aesthetics owing to its transparency. In addition, such types of transparent antennas can be utilized in the satellites with small form factor and also in cube satellites by repeating the structure to form an array structure thus helping to meet the critical space necessities [A], [B]. Transparent antennas over solar panels helps in revenue generation by leasing out the panels to telecom operators [A-C].

 

[A] White and H. R. Khaleel, ‘‘Flexible optically transparent antennas,’’ in WIT Transactions on State of the Art in Science and Engineering, vol. 82. WIT Press, 2014, p. 59, doi: 10.2495/978-1-84564-986-9/003.

[B] X. Liu, D. R. Jackson, J. Chen, J. Liu, P. W. Fink, G. Y. Lin, and N. Neveu, ‘‘Transparent and nontransparent microstrip antennas on a CubeSat: Novel low-profile antennas for CubeSats improve mission reliability,’’ IEEE Antennas Propag. Mag., vol. 59, no. 2, pp. 59–68, Apr. 2017.

[C] M. J. Roo-Ons, S. V. Shynu, M. J. Ammann, S. J. McCormack, and B. Norton, ‘‘Transparent patch antenna on a-Si thin-film glass solar module,’’ Electron. Lett., vol. 47, no. 2, pp. 85–86, 2011.

 

Authors Action: The above content is added in the salient features just before conclusion and is highlighted in yellow.

Reviewer#2, Concern # 9: The efficiency of the antenna is also low (between 50% and 60%). It must be greater than 70%. Explain why, knowing that your substrate is low loss.

 

Author response:  Thank you very much for your valuable comment. The gain and efficiency values are low in comparison to the conventional Copper-based antenna designs. The gain and efficiency of the transparent antennas are directly dependent on the sheet impedance of the conductive oxide (8 Ω/Sq) that leads to conductivity value of 706.215 S/m which is very less as compared to copper-based antenna (Conductivity =
5.96×10⁷). While the transparency and conductivity are inversely proportional, the conductivity is lower for the higher optical transparency to be achieved. The low value of conductivity is the main reason for the low gain and efficiency of the transparent MIMO antenna

Reviewer#2, Concern # 10: In the curves of figure 11, although all the ECC values are less than 0.1, which is very acceptable. The good ECC values are outside the operating frequency band of your antenna [4.6 - 4.94] GHz (at 5.5 GHz for example) where the coupling of two antennas is very weak. Why? Normally it's the opposite.

Author response:  Thank you very much for your valuable comment. Due to the ineffective excitation of each antenna element of dual-element transparent antenna resulting from the impedance mismatch in out of frequency band, the radiated electric or magnetic field components from each antenna element have minor correlation in between in addition to the lower degree of inter-element near field coupling. This can be correspondingly deduced from Figure 9 where the higher isolation value has been observed in out of frequency band due to the reduced level of near-field coupling among the antenna elements, which results in ECC value to be correspondingly lower.

Reviewer#2, Concern # 11: Review references. There are some missing information: ref. 3, 5, 16. Some references are not recent.

Author response:  Thank you for the valuable suggestion. The references are thoroughly checked for any missing references. The reason for some old references is due to the fact that it is taken as reference for proposing the transparent antennas especially ref [3] and ref [5].

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear authors,

GOod job.

Here are my comments:

*) ABout the transparent materials for the condcutor and subsrate. Please spicify the conductivity for the conductor and the permitivitty and loss tangent for the dielectric. ALso, can you comment on cost of these materials compared to traditional materials? 

*)Eq.5, please explian why you are adding a 0.5. Also, why the equationa only involves S parameter and not efficiency?

Many thanks

Author Response

Original Manuscript ID: electronics-1536908         

Original Article Title: “Transparent 2-Element 5G MIMO Antenna for Sub-6 GHz Applications”

 

To: Electronics Editor

Re: Response to reviewers

 

Dear Editor,

 

Thank you for allowing a resubmission of our manuscript, with an opportunity to address the reviewers’ comments.

We are uploading (a) our point-by-point response to the comments (below) (response to reviewers), (b) an updated manuscript with yellow highlighting indicating changes, and (c) a clean updated manuscript without highlights (PDF main document).

 

 

Best regards,

Dr. Issa Elfergani et al.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

REVIEWER 3

Reviewer#3, Concern # 1: About the transparent materials for the conductor and substrate. Please specify the conductivity for the conductor and the permittivity and loss tangent for the dielectric. Also, can you comment on cost of these materials compared to traditional materials? 

Author response: 

Properties of material used for patch and ground:

AgHT-8 (Sheet Resistance = 8Ω/Sq) according to the datasheet, Thickness = 0.177 mm = 0.000177 m, Conductivity = 706.215 S/m, transparency >75%)

 

Properties of material used as a substarte:

Plexiglas (ɛr = 2.3, tan δ = 0.0003, thickness (T) =1.48mm, transparency of more than 85%.)

 

As the conductive oxide sheet is made up of silver sandwiched between tin oxide, the cost is higher as compared to copper-based antennas. Also, such material needs state of the art fabrication facility since it uses chemical vapor deposition technique.  Plexiglass substrate is easily available and the price of this material is very nominal.

Reviewer#3, Concern # 2: Eq.5, please explain why you are adding a 0.5. Also, why the equation only involves S parameter and not efficiency?

 

Author response: 

(5)

 

The derivation of Equation (5) where multiplication of the factor 0.5 with the efficiency is explained in detail referring to [A] using the concept of polarization. Authors would like to highlight that MEG equation can also be calculated using S parameters but in the present case equation (3) and (4) is used for MEG calculation that uses cross polarization into account.

 

	(3)

Where, in equation (3), Gϕ(q, ϕ) and Pq (q, ϕ) are power gain and the existing power along horizontal and vertical polarization. The XPR is expressed as below in equation (4)

(4)

 

where P_vpa and P_hpa represents the power received by vertically and horizontally polarized antenna, respectively. 

 

[A] Glazunov, A. Alayon, Andreas F. Molisch, and Fredrik Tufvesson. "Mean effective gain of antennas in a wireless channel." IET microwaves, antennas & propagation 3, no. 2 (2009): 214-227.

 

Author action: As equation (3) is used for MEG calculation, equation (5) is removed from the revised manuscript to avoid the ambiguity.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

1. There is no Novelty. Authors should clearly mention the novelty of this work and also the motivation behind this work.
2. Authors have used Plexiglas as a substrate. What is the motivation for using this substrate? What are the factors which make this substrate better than the other rigid/flexible substrates? Please provide a justification why it was decided to use this substrate.
3. Authors should improve the Introduction section and state of the art analysis for the manuscript.
4. Authors should give more details of the design methodology and also provide numerical analysis of the design process. The authors should also elaborate the stepwise design optimization process.
5. It is recommended to enhance the literature review and comparison by considering the below works: https://doi.org/10.3390/app11041635 https://doi.org/10.3390/electronics9010071   “A Flexible and Compact Semicircular Antenna for Multiple Wireless Communication Applications”, DOI:10.13164/re.2018.0671

6. In section 3.1 (Figure 6) current distribution is discussed. Please provide more detail by discussing the contribution of inner and outer circles in attainment of important results or antenna characteristics.
7. Authors should discuss in detail the proposed antenna behavior in terms of radiation pattern. Authors should also provide the detail about the measurement setup used.
8. Authors should improve the quality of figures specially Figure 2, 10, 11, 14 by using different styles and colors of curve lines.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The paper can be accepted in its current form for publication!

Reviewer 4 Report

All the concerns have been well answered and deficiencies been removed. I find the paper suitable for acceptance in present form.