I do a lot of monitoring of ATC frequencies from home which is maybe about 20 miles from the airport. There seems to be a large variation in the quality of transmissions from plane to plane. I can rarely hear planes once they are on the ground but on occasion I can hear an individual plane ask for pushback quite clearly. Some planes landing are much clearer than others even though they are following the same approach pattern for the same runway. I realize that the reception can vary from day to day but this is using the same scanner in the same location and within a 20 minute period so I'm assuming the variation in the signals received is due to the individual radio equipment in the aircraft. I'm wondering if the radio equipment that comes with each model of plane is standardized or does each customer specify particular equipment. For example does each 777 have the exact same radio equipment. Do the airline's technicians do a lot of tweaking on their own.