A direction finder having an antenna system adapted to receive signals and provided with a right-hand and a left-hand lobe system. By coupling means said lobe systems can be made alternately operative.
http://www.patents.com/Direction-finders/US3965473/en-US/
This device can be used to locate the source of a radio signal. This has many applications, for example you could plant a weak radio transmitter in a model aircraft.
http://www.freeinfosociety.com/electronics/schemview.php?id=1922