Most of the research has shown the use of metamaterial superstrate for improving the directionality of planar antennas, and for that purpose Yes the gap is required, you may have air or material like foam that has dielectric constant nearly equal to that of air. In case of you use air in between you may use dielectric screws to hold metamaterial superstrate above your antenna, in case of foam no screws are necessary. Having said that few researchers including me has worked upon how to use the metamaterials for gain improvement without using any air gap, and introduced them in the same layer in which the antenna is modeled. If you are interested in my research you may find my paper over the same topic in my publications.

Thanks

Thank you Nitin. I would love to read your paper.

I will look into your publications in your profile. Thank you

You can have an antenna embedded in a metamaterial, or have the antenna on the surface of the metamaterial, or separated from the metamaterial. All these configurations, and others, are possible. Your metamaterial can be the antenna, with the feed being inside metamaterial. An example of this is a meta-material slab with infinite phase velocity which can be fed at any point to get uniform illumination over its surface.

Thank you Malcolm. Is there any specific paper I can follow which you can recommend. Where antenna can be on surface of metamaterial.

I read Metamaterials edited by Engheta and Ziolkowski

Page 80 describes a zero index antenna.

see Article Propagation in and scattering from a matched metamaterial ha...

Hi, metamaterial structures such as CSRR or SRR can be inserted in the patch or feed of microstrip antennas. So, considering gap is not necessary in many applications.

Hello,

Follow this link:-

https://www.comsol.co.in/model/antenna-decoupling-using-an-electromagnetic-band-gap-meta-material-12045

Chapter Metamaterials in Application to Improve Antenna Parameters

few papers:-

A comparative study on metamaterial for antenna in space application(Pankaj Rameshchandra Katiyar EMRD, Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia; Wan Nor Liza Binti Wan Mahadi)

Thanks,