Using IsBlank In Formula

I have a field with 3 possibilities.....

If H22 is less than 30 I want an "X"

If H22 is greater to or equal to 30 I want a "@"

If H22 is a blank field, I want the field containing the formula
to also be blank.

The formula below tests correctly for the "X" and the "@"
but the blank returns a "@".

What am I missing.? Thanks.

=IF(H22<30,"X",IF(H22>=30,"@",IF(H22,IsBlank," ")))