Here's a large antenna farm that does just what you state: http://en.wikipedia.org/wiki/AN/FLR-9
Daniel,
Thanks for the link, those were some antenna's.
Here's another big one.
There are (or were) also much smaller antenna's that will do HF/DF.
You will even see small, hand-held antenna's that will locate a transmitter by pointing the antenna.
As far back as the '60's (that I am aware of and the equipment looked ancient then), there were direction finding antenna's that would work within the standard Aviation Communication Band of VHF (118-136 mHz).
The antenna was a circular affair with many (360) 'arms' or 'spokes' out six feet or so parallel to the ground. When a transmission was received you could look on a circular scope and see a green line that would indicate the direction in degrees from/to the transmitter/receiver.
This way they could 'vector' a lost pilot home and by having him turn a specified number of degrees off the bearing for a few minutes, they could calculate his 'time-to-the-station' in minutes. Two (or more) stations in different locations could pinpoint the transmitter by where the Bearing Lines crossed.
Since the FAA Flight Service Station Specialists needed to log a number of these "DF Steers" to maintain his/her 'Currency' they would encourage pilots to do "Practice DF Steers" so both parties could maintain proficiency in the operation. They would just request that you transmit to them a "count-to-ten" or so and then fly a heading that they would supply. This was what Amelia expected the Itasca could do for her, alas the Coast Guard had no HF/DF capability and she did not know that. Pan Am did and maybe that was where she got the idea.
Aircraft were (after WW2 and still are) equipped with specialized receivers that compute direction from specialized VHF (108-118 MHz) (VOR OmniRange Stations) which transmit a 'mark' signal combined with a circulating beam and the time electronically measured from the 'mark' to the 'beam' is your bearing to or radial from that station. The panel indicator has a rotatable dial to select a radial (time) and a needle to show left/right/center on the selected radial.
A more comparable to the RDF Loop system from Earharts day is still found today in some aircraft. These work in the LF/MF Radio bands (180-1700 {you can listen to AM Radio and steer to the station} kHz) and drive a needle (Automatic Direction Finder) on the panel to show a relative bearing from the aircraft. The Hooven ADF that she unwisely had removed from her Electra (was the absolute latest) worked this way. The earlier style she had installed in its place required manually turning that big loop on top of the fuselage while you listened to the signal strength to locate the direction to/from the station. Had she been proficient with either type, she would have easily located the Itasca.