Margaret Penrose 
The Campfire Girls of Roselawn [EPUB ebook] 

‘Showing how Jessie Norwood and her chums became interested in radiophoning, how they gave a concert for a worthy local charity, and how they received a sudden and unexpected call for help out of the air. A girl who was wanted as a witness in a celebrated law case had disappeared, and how the radio girls went to the rescue is told in an absorbing manner.’
Originally published as 
The Radio Girls of Roselawn, and then republished as 
The Campfire Girls of Roselawn.