In the days before GPS systems, air routes were marked by transmitters at known positions on the ground. Planes could navigate from one transmitter to another using radio direction finding equipment. This cache gives you a clear view (to the southwest) of one of the buildings that held such a transmitter. Nothing like a little bit of history, eh? See below.
Unfortunately the old transmitter building has been removed.
A non-directional (radio) beacon (NDB) is a radio transmitter at a known location, used as an aviation or marine navigational aid. As the name implies, the signal transmitted does not include inherent directional information, in contrast to other navigational aids such as low frequency radio range, VHF omnidirectional range (VOR) and TACAN. NDB signals follow the curvature of the Earth, so they can be received at much greater distances at lower altitudes, a major advantage over VOR. However, NDB signals are also affected more by atmospheric conditions, mountainous terrain, coastal refraction and electrical storms, particularly at long range.