antenna used in wireless charging

90.8 MHz is in the commercial FM band (that is, in the USA). A common antenna is the 'T' shape. I believe it's simply a single-element yagi. Easy to make. A few feet across. Exact width is wavelength divided by 4 (I think).

Add reflector elements to increase gain.



Now I look again and I realize it's for wireless charging. This suggests you want to pick up magnetic flux (rather than photons). Therefore I don't believe you want an antenna in the conventional sense. I think you want to place the receiving coil as close as possible to the sending coil. There will be need for a lot of experimentation. The goal is to maximize the inductive coupling.

Depends on what you name "high gain" and situation. From a simple dipole you can go on to a yagi. At 90 MHz the size is not too practical, around 1.5 meter half-wave.
Also 90 MHz is within FM radio range and a charging system becomes illegal. Field density hot enough to charge anything.
Try 2.45 GHz or 1...2.2 GHz cell-phone range where field density may be much higher.

i am working on wireless battery charger. i hav done some literature survey and found at 90.8MHz freq level we get required output power

All right, so if you use 90.9 MHz instead of recommended 90.8 MHz, what happen with the charging according to the literature that you read?