The 1920s was an era when retail sales were dominated by large downtown department stores, which offered a wide variety of consumer goods. Some stores even had auditoriums, for providing entertainment, so it was natural for them to expand into radio broadcasting for publicity purposes. The Leo J. Meyberg Co.'s store in Los Angeles, California was known as Hamburger's, and it began radio broadcasting activities under an experimental licence, 6XAK. (The article's estimated range for this station, 100 miles by day and 200 miles by night, appears to be very generous for a 5-watt transmitter, which more likely had a normal range of perhaps 10 miles.) Effective December 1, 1921, the Commerce Department would establish regulations for a formal broadcast service, and the Meyberg Company was one of the first to get a licence under the new regulations, for station KYJ, which lasted from December 9, 1921 until it was deleted on May 1, 1923
Radio News, December, 1921, page 485:

Radio  in  Department  Stores

ON the roof of a downtown department store in Los Angeles, California, is located one of the most up-to-date and modern radio stations found in the west. The station is a 5-watt installation, the current going into the antenna when the station is in operation is 1.6 amperes, with 500 volts applied on the plates of the 5-watt tube. The station has a sending day range of 100 miles and a night range of twice that distance. Its official call letters are 6XAK.
    With the installation of this equipment on the roof of Hamburger's department store, commerce and science joined hands and met on a common ground. The station will be used for experimental purposes, and in its use the firm aims to aid in the popularizing of radio work in the southwest, and at the same time to carry on considerable worth-while publicity for the firm as well.
    As yet only telephone equipment is in use, but telegraphy will be added later. 6XAK Equipment
    Every afternoon, from four to five the store gives a free radio concert to all stations in Southern California who desire to tune to the Hamburger wave-length (310 meters) and listen in. These concerts are becoming more and more popular, and it is estimated that several thousand stations daily enjoy them by wireless.
    Classes in radio operation, code work, theory of wireless, and so forth are being planned by the store, and since the first of November, 300 boys have been taking at least one lesson a week in the Hamburger station. A class room with a capacity of 40 boys is being fitted up, and it is planned to hold from eight to ten classes a week.
    The day classes open, all boys who have registered will attend a Hamburger's "at home" on the roof, a special program for this day being worked out. Moving pictures of the event will be taken, these pictures to be later released by one of the moving picture weekly services. Colonel Hersey, head of the Los Angeles Weather Bureau, and during the war in charge of the Balloon school at Fort Omaha, will lecture to the boys on the need of wireless at war times. The boys will then be marched through the wireless rooms, given time to inspect the installation, and then assigned to class days.
    So rapidly did the fame of the Hamburger wireless spread, that when the Scotti Grand Opera Co. was in Los Angeles, in October, four famous opera singers sought out the management of the store, and arranged to sing into the wireless transmitter. Arrangements for this event had been made some time in advance and considerable publicity given it, with the result that thousands were listening in on instruments all over Southern California. Several ships at sea staged afternoon concerts and entertained their passengers with real grand opera coming from the store roof. The singers who participated in the concert that afternoon doubtless sang to the largest audience any singer ever had the privilege of appearing before.
    Results of the world series, play by play, were broadcasted each day the players were waging their strife at the Polo Grounds, and many a station located in the mountains and deserts of California had news of the plays long before Baseball Fans on Manhattan Island.
    The store operates the station as one of its services, no charge being made for the lessons to boys, or for any of the other features being handled out of the station.
    Close co-operation has been established between the radio station on Hamburger's roof and the various physics classes of Southern California High Schools, and a number of professors who are teaching in the High Schools are coming to the store in order that they may view the operations of a radiophone station. At this time the store classes will deal with the fundamentals of radio, only, later, however, it is planned to put in a large commercial installation and then instruction will be given in advanced theory, and boys will be given work that will enable them to take the examination for a commercial operator's license.