How to force to jump eternally object?

How to force to jump eternally object (upwards - downwards, upwards - downwards, upwards - downwards)?

public float jumpSpeed = 8.0f;
public float gravity = 20.0f;
private bool grounded = false;
private Vector3 moveDirection = Vector3.zero;

public void FixedUpdate() 
{
    if (grounded) 
    {
    // We are on the ground so jump
    moveDirection.y = jumpSpeed;
    }

    // Apply gravity
    moveDirection.y -= gravity * Time.deltaTime;

    // Move the controller
    CharacterController controller = (CharacterController)GetComponent("CharacterController");
    CollisionFlags flags = controller.Move(moveDirection * Time.deltaTime);
    grounded = (flags & CollisionFlags.CollidedBelow) != 0;
}

This Script forces a character to jump eternally