You may look here
http://news.nationalgeographic.com/news/2014/07/140727-amelia-earhart-history-flight-airplanes-adventure-explorer/ to read about how Earhart used radio on her Honolulu Oakland flight. She also carried a simple to operate, a single needle pointed to the station automatically, radio compass by Bill Lear
http://mlsandy.home.tsixroads.com/Corinth_MLSANDY/rt105.html reads in part
So he, Bill Lear, took the next logical step.
He brought his direction finder to the Bureau of Air Commerce, which
wanted to improve private flying as well as commercial aviation and which
had been working on a radio compass for fifteen years without much
success. If Lear could get the bureau to endorse his radio compass, he
figured he would have a better shot at reaching the roughly 7,500 private
pilots in the country as well as the major airlines. He convinced
Director Eugene Vidal to commission the bureau to test his Learoscope to
see if it would be of any value to the private pilot. Vidal hired his
friend Amelia Earhart, for $1 a year, to fly Lear's direction finder
(slightly rebuilt according to bureau specfications) in her bright red
Lockeed Vega primarily to determine its possibilities as an air
navigation aid for private owners of aircraft." To draw Earhart as a test
pilot was a promotional coup, for the aviatrix- the first woman to cross
the Atlantic, one year after Charles E. Lindbergh- was a darling of the
media. Vidal asked her to make notes and observations to assist [the
bureau] to perfect the instrument."
Neff