Abstract: A theory is presented for frequency responses of backfire helical antenna. Based on assumption of single travelling wave of current, the approximate analytical solution of far field is obtained by summing the contributions of the current distribution. The analytical results are consistent with H. Nakano s numerical results. A new type of reflecting antenna, whose feed is a backfire helix and whose reflector is a conductive disk, is investigated by using this theory. The results are available to practical applications.