It is unimaginable now, but in the early days of broadcasting, it was possible to operate a legal broadcast from a moving vehicle. Not a relay to a stationary transmitter, but to originate programming, perhaps even while steering.In 1919 I find the earliest reference to a license to broadcast from a car. Alfred H. Grebe broadcasted from both cars and boats with the call letters WGMU. Grebe manufactured radios. the purpose of the traveling radio show was of course... to sell them. they were nice radios, usually a chassis of Bakelite and/or nice hardwood like walnut. Grebe was from Richmond Hill, NY, born in 1895. He also founded WAHG, WBOQ, and other less formal stations right out of his factory in Queens.

His mobile station used a 6-wire flat top antenna but it was hardwired to the frame and body of the car! It operated at 150 meters. He did observe the the spark plugs of the other motor vehicles caused interference even then. In advertisements he called it the grebe Auto Radiophone. Grebe said in a Radio Amateur News article:

"The auto-radio-phone is entirely practical, and the near future should bring extensive developments along these lines..."

In the late 1920s Jay W. Peters was broadcasting in Inglewood, CA as 1470 KGGM. Then in 1927 he loaded his transmitter with a collapsible antenna and moved to Albuquerque, New Mexico. I've also seen images with a rig attached to a bus! He too was demonstrating radio equipment that he was trying to sell. Peters traveled the Southwest doing demos. In 1928, he sold the license to the New Mexico Broadcasting Company. In 1928 he moved to a terrestrial stationary radio license. He went to Reno and tried to start another station near Blanch Field Airport in an Elks lodge. He applied for a license, and got the calls KOH. it was the first commercial station in Reno.