Substrates with higher thickness and high dielectric constant results in smaller bandwidths and lower efficiencies due to the possibility of surface-wave excitation. Also, higher dielectric constant materials with longer length of feed line result in increased losses and chances of spurious radiation. The substrate thickness should be made as thin as possible to eliminate multiple surface waves. It is preferable to use low dielectric constant material and a substrate of optimum thickness for containing the spurious feed radiation to a minimum.

The substrates that can be used for the design of microstrip antennas, its dielectric constants are usually in the range of 2.2 to 12. The ones that are most desirable for good antenna performance are thick substrates whose dielectric constant is in the lower end of the range because they provide better efficiency, larger bandwidth.

You can have a look to this paper: D. M. Pozar, “Microstrip Antennas,” Proc. IEEE, Vol. 80, No. 1, pp. 79–81, January 1992.

Small antenna size and high dielectric constant usually go with narrower bandwidth or lower efficiency or both. Typically you need higher fields and/or current densities to radiate from a smaller antenna, so this requires higher Q, which has lower bandwidth, and the material losses also increase because of the higher stored energy.

Use of magnetic materials rather than dielectric can improve this (among other things your magnetic substrate will be thinner than a dielectric one). Look up articles on electrically small antennas. Your antenna is probably not small enough to be in that category but it will help you to see the problem you are at the edge of.

Thank you so much everyone.

In general, higher dielectric constants lead to lower bandwidth. The bandwidth is inversely related to the quality factor. The loss in the substrate is an other factor that reduces the efficiency of the antenna. The thickness of the substrate is more effective in the case of microstrip antennas as compared to printed monopole antennas (where there is no ground plane behind the monopole).

Thank you so much. I will look into printed monopole antennas

may be you should enhancement the bandwidth by slot method .you can see the enhancement method that has been used in this papers

https://www.researchgate.net/publication/323761856_Circular_slotted_monopole_printed_antenna_with_grounded_stub_WLAN_band-notch_for_UWB_applications

or this paper

https://www.researchgate.net/publication/319953887_Mutual_Coupling_Reduction_of_a_21_MIMO_Antenna_System_Using_Parasitic_Element_Structure_for_WLAN_Applications_1

S, all tiogether from the ground plane of metamatera

Agree with the comments from able fellows above. You may look into printed monopoles for UWB or even planar monopoles could do the job. But as far as the properties of dielectrics are concerned, very good knowledge has been shared above.