Since I know many here are also interested in aviation and radio navigation, I wanted to share an excellent new website:

https://flyingthebeams.com/

The creator,  Doug Davis, has done an exceptionally good job of explaining not only the Low Frequency Radio Range (LFR) but the evolution of airborne navigation to the present day, with many illustrations and photos that I haven't seen elsewhere.   To his credit,  he has also preauthorized the use of material from this site for educational purposes,  as long as it is unaltered and attribution is given.     An enormous amount of effort has been put into researching and developing this site.

To my surprise he was able to find actual audio clips of the LFR in operation, which I didn't know even existed!   There are also on-site photos of the few remaining intact radio range sites that I've only been able to view via Google Maps.   In addition to navigation, the "Early Radio Tech" page provides a great summary of the development of radio with high quality graphical illustrations. 

A terrific resource for anyone interested in the topic.

73, Bob W9RAN