Pascual Marques 
Radio Nav [PDF ebook] 

This manual describes the fundamental principles of the radio signal, NDB navaid and ADF avionics for radio navigation. The manual contains an extensive range of exam-type questions and practical exercises. The exercises have been designed for the student pilot to carry out mental arithmetic calculations and to promote a sense of situational awareness so that you know where the aircraft is heading and its position relative to the ground station at all times.