Map of AT&T's "WEAF Chain" radio network, from Radio Broadcast, "How Much It Costs to Broadcast", by Austin C. Lescarboura, September, 1926, page 368:
 
1926 WEAF Chain
HOW  COMMERICIAL  PROGRAMS  REACH  A  PART  OF  THE  NATION
Many well established commercial "courtesy programs" are broadcast, originating from WEAF, in New York, and furnished to the broadcasting stations indicated on this outline map. Officials of WEAF assert that these programs reach 52.2 per cent. of the population of the United States. The chart shows the main telephone "repeater stations" through which the telephone lines pass. The programs are stepped up in volume before delivery to the tubes at each distant broadcasting station