Commission proceedings. While Motorola was developing the cellular phone itself, from 1968 to 1983, Bell Labs worked on the system called AMPS, while others designed cell phones for that and other cellular systems. Martin Cooper, a former general manager for the systems division at Motorola, led a team that produced the DynaTAC 8000x, the first commercially available cellular phone small enough to be easily carried, and made the first phone call from it. Martin Cooper was the first person to make an analog cellular mobile phone call on a prototype in 1973. The Motorola DynaTAC 8000x was very large compared to phones today. This first cell phone was very expensive when it was released in the US in 1984. The DynaTAC's retail price, $3,995 (about $ in ), ensured that it would not become a mass-market item (the minimum wage in the United States was $3.35 per hour in 1984, which meant that it required more than 1192 hours of work, or more than 1.6 months - just working for the phone, without taxes); by 1998, when Mitchell retired, cellphones and associated services made up two thirds of Motorola's $30billion in revenue. On October 13, 1983, David D. Meilahn placed the first commercial wireless call on a DynaTAC from his 1983 Mercedes-Benz 380SL to Bob Barnett, former president of Ameritech Mobile Communications, who then placed a call on a DynaTAC from inside a Chrysler convertible to the grandson of Alexander Graham Bell, who was in Germany for the event. The call, made at Soldier Field in Chicago, is considered to be a major turning point in communications. Later, Richard H. Frenkiel, the head of system development at Bell Laboratories, said about the DynaTAC: "It was a real triumph; a great breakthrough." Publications U.S. Patent 3,906,166, September 16, 1975 for a Radio Telephone System for the first cell phone was granted by Martin Cooper, Richard W. Dronsurth, Albert J. Leitich, Charles N. Lynk, James J. Mikulski, John F. Mitchell, Roy A. Richardson, and John H. Sangster. Two names were botched in the original filing; Albert Leitich's surname was erroneously omitted, and Dr. Mikulski's first name was omitted. The original document was refiled by Motorola's legal staff, but has not yet been identified. The seeds of the idea for a portable cell phone can be traced to Dr. James J. Mikulski, which were rejected by Mitchell for lack of sufficient business justifications.