A superstrate on top of the patch can help. If permittivity of superstrate is higher than the one of the substrate this can be a solution. Also if you can afford multilayer design Ke Wu had a number of papers on stacked patch Uda Yagi. Having said that I don't think he achieved much more than 11-12dBi with single element design. Fabry-perot antenna can be a possible solution which ties well with your geometry, though the bandwidth may be an issue. But at the end of the day to get a gain you have to have an aperture of appropriate size, that's the physics unfortunately.

If you willing to drop your initial design and look into travelling wave slot antennas, then I would suggest Fermi antenna ( I had a 16dBi antenna and theoretically more can be achieved with very good bandwidth as well).

If it is a patch antenna, increasing the substrate height could lead to gain enhancement. But the bandwidth would become smaller in this way. Also making antenna arrays is a way of increasing the overall gain.

thanx for reply. My design is aperture(slot) coupled patch antenna

Put your structure in an antenna array and you can also add slot on the ground plane

A superstrate on top of the patch can help. If permittivity of superstrate is higher than the one of the substrate this can be a solution. Also if you can afford multilayer design Ke Wu had a number of papers on stacked patch Uda Yagi. Having said that I don't think he achieved much more than 11-12dBi with single element design. Fabry-perot antenna can be a possible solution which ties well with your geometry, though the bandwidth may be an issue. But at the end of the day to get a gain you have to have an aperture of appropriate size, that's the physics unfortunately.

If you willing to drop your initial design and look into travelling wave slot antennas, then I would suggest Fermi antenna ( I had a 16dBi antenna and theoretically more can be achieved with very good bandwidth as well).

Many thanx Mr.Dmitry Zelenchuk

use of an optimization technique (e.g. GA) helps too.

I have done a similar work by using fss superstrate on slot coupled antenna. You can check that one also.

The frequency of my design is 79 GHz

@Ravi Kumar KRS. I would agree that SIW is much better with respect to transmission line loss at 79GHz.

@Basem. Looking at the operating frequency and spec, I guess (correct me if I am wrong) that the antenna is 1D subarray and one to be careful to ensure appropriate beam width in both planes. We can achieve the same gain with different beam width in both planes.

@Ravi Kumar KRS. thank you so much for your help and explanation :)

@Dmitry Zelenchuk I dont understand what u mean 1D subarray ?Also for the beam width u mean be equal for both planes or what?

I simply guessed perhaps incorrectly that your antenna requirement would have different beam width for horizontal and vertical plane. The frequency hints at automotive applications and the antennas are composed from several subarrays or indeed one array with beam squeezed vertically as there is no point to "illuminate the sky and the road" and waste your energy there. 1D is 1-dimensional as in 1x4 array to get the beamwidth.

That is related to the directivty which is a fundamental antenna parameters, we can't speak about gain without mentioning the antenna directivity.

All the above mentioned ways are good in gain improvement of antenna element but I guess one simple answer is making array out of one element, so the gain will rise

and if you choose the circular array the Wobble of Wave (WoW) will stay low

How to improve the mobile phone antenna gain?which technique is the best for improve the gain?

We can use substrate end shaping techniques for enhancing gain.

There are four well known methods of increasing the gain of an antenna. 

i) The magnitude of 'E' field of an antenna is proportional to its length and therefore, the directive gain can be increased by increasing the effective length of the antenna. This is because of the increase the aperture area. 

ii) Gain of an antenna can also be bettered by partially cutting slots in the substrate and by employing multiple layer dielectric substrates.

iii) A reflector can be used behind the antenna so that the EM waves after striking the reflector will form a unidirectional beam thereby increasing antenna gain.

iv) Antennas can be also be arranged in an array to improve the gain.