Hi kijangnim,
From memory, the first G/S antenna is near the nose of the aircraft and is used initially. When the gear is lowered, then a second antenna on one of the gear doors is used for the rest of the ILS.
Maybe it's because the 747 pilots are on the second floor higher up than 777. This old link has some more info: see AirRabbit's
archive/index.php/t-273878.html