Editing Radio navigation (section)

=== ADF and NDB===
{{Main|Non-directional beacon}}
A great advance in the RDF technique was introduced in the form of phase comparisons of a signal as measured on two or more small antennas, or a single highly directional [[solenoid]]. These receivers were smaller, more accurate, and simpler to operate. Combined with the introduction of the [[transistor]] and [[integrated circuit]], RDF systems were so reduced in size and complexity that they once again became quite common during the 1960s, and were known by the new name, [[automatic direction finder]], or ADF.{{fact|date=July 2022}}

This also led to a revival in the operation of simple radio beacons for use with these RDF systems, now referred to as ''non-directional beacons'' (NDB). As the LF/MF signals used by NDBs can follow the curvature of earth, NDB has a much greater range than [[VHF Omnidirectional Range|VOR]] which travels only in ''line of sight''. NDB can be categorized as ''long range'' or ''short range'' depending on their power. The frequency band allotted to non-directional beacons is 190–1750&nbsp;kHz, but the same system can be used with any common AM-band commercial station.{{fact|date=July 2022}}