Radio Direction Finding

Brooke Clarke 2009 - 2015

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After reading "Secret Weapon" by Kathleen Broome Williams (ISBN: 1-55750-935-2) about shipboard High Frequency Direction Finding (HFDF) and it's impact on sinking German U-boats I wanted to have a look at the patents relating to direction finding.  The model SE950 1918 Navy aircraft LW direction finder started this.

There are different flavors of Radio Direction Finding (RDF).  In navigation systems the transmitter is at a known frequency and transmits pretty much all the time.  But in the case of a distress transmission the frequency is known (was 500 kHz) but the duration is short.  And in the case of a German sub in W.W. II the frequency is sort of known and the transmission time is very short (under one second).



The idea of the SE950 was that ships in W.W.II could locate submarines by using RDF. The target signal might be a transmission or from the Local Oscillator (LO) from a receiver leaking out to the antenna.


One use for RDF is part of a navigation system where the transmitting beacon location is known.  This might be a low frequency, high frequency, VHF or UHF.  The ARN-89 is an example of a LF-HF RDF system and the Light Weight Beacon is an example of an LF beacon that can be backpacked to a known location.

The DU-1

This was made by Bendix.   

1892151 Direction finding system, Hyland Lawrence A, Wired Radio Inc, Dec 27, 1932, 342/445, 343/728 - loops & sense whip, switchable unidirectional or bidirectional 
1981884 System for detecting objects by radio, Hyland Lawrence A, Taylor Albert H, Young Leo C, Nov 27, 1934, 342/27, 367/128, 342/453, 340/991, 342/407 -
                bi-static RADAR, depends on re-radiation rather than reflection since the frequency is much lower than current RADAR systems (i.e. probably HF).

2144310 Radio apparatus and method of manufacture, Hyland Lawrence A, Bendix Radio Corp, Jan 17, 1939, 343/842, 343/866, 29/605, 29/602.1, 343/872, 403/270  
2159379 Radio antenna, Hyland Lawrence A, Bendix Radio Corp, May 23, 1939, 343/705, 343/872, 123/41.56, 244/1.00A, 343/866 - shielded loop

Bendix DU1 Loop

Another use might be when an aircraft is involved in Search And Rescue (Wiki: SAR) where the beacon transmitter is in an unknown location.  Survival Radios such as the PRC-90 transmit on known frequencies so the aircraft direction finder system only needs to operate on a limited number of frequencies.

Modern aircraft ejection seats contain a beacon transmitter, similar to the Vietnam era URT-33.  Also built into aircraft there are locator beacons that are activated by a crash.  Locating a downed aircraft is a problem that's not been solved as of 2015.  The case of MH370 (Wiki) is troubling.  The crash type beacon would be ineffective if a plane crashes into water because it will be under water in a very short amount of time.  Although an improved crash beacon could be made (or probably already exists) that would eject from the aircraft and float to the surface. The ultrasonic Underwater Locator Beacon (Wiki: ULB) only has enough battery life to work for a month.  So in the case of MH370 where the crash location could be in a huge area the battery had gone dead by the time the search location was narrowed.  There needs to be improvement in the ULB.  One area is so that standard Sonobuoys can detect it's signal.  The existing signal was designed for optimum range when a very specialized ULB receiver is used.  But in the case of MH370 where no one knew where to look that's of no value.  But there are a huge number of sonobuoys all over the globe and the related equipment and personnel trained to use them.

There has been talk about live streaming aircraft data so that a plane in trouble would be recognized before it crashed, but that's not practical because if every plane aloft or moving on the ground was streaming data there would need to be a system that could (1) have enough bandwidth and (2) process that large volume of data in real time.  A more practical system would be to put a box between the airplane and Flight Data Recorder (Wiki: Black Box) that would look for signs that a crash was likely.  The problem is if the algorithm is too sensitive there will be a lot of false alarms and if too rigid it may not transmit enough information prior to the crash.  Note this system would probably be put out of action the instant of a crash.

Here's an idea I sent to the FAA:
"A simple solution might be to make an ADS-B transponder with internal rechargeable battery in an enclosure that's strong enough to contain an internal explosion and or fire caused by it's own battery.  This would allow the crew to turn off it's external power, but not turn off the transponder function.  The battery would be charged from the aircraft electrical system and there would be signal from the transponder if it's battery state was low.  That signal can be monitored both by the crew and anyone receiving it's signal.  The battery needs to have enough capacity so that the beacon can operate for more time than the plane it's in.  Rather than just using the internal battery to replace the normal external power when the external power is off the transponder would go into a mode that depends on internal sensors.  Part of this battery mode would be a signal saying it's on battery power.  If a combination of sensors all say the plane is stopped on the ground then the transponder would turn off it's internal battery, but even then would run if external power was applied."

Ground Based

During W.W.II there were HFDF systems called Wullenweber (Wiki), Circularly Disposed Antenna Array or AN/FRD-10 (Wiki).  In these systems which started off using a motor driven high speed scan and later upgraded to solid state beam rotation a very short transmission was all that was needed to get a good bearing to the source.  This eliminated the cover that was provided by burst transmission like the GRA-71 that transmitted a message at 300 Words Per Minute instead of maybe 15 WPM for a manual Morse transmission.

The Army has a number of DF systems to locate enemy ground radio transmitters, for example the PRD-1 HF Direction Finding Radio used in Korea and Vietnam, Radio Receiving Set AN/TRQ-23 which uses the OE-4/GR antenna group.  This system uses a motor drive to rotate the DF antenna and a CRT to provide bearings.

The Drop Zone Assembly Aid System (DZAAS)  is a directional receiver worn on the wrist the points to a package on the ground that's arrived by parachute.

Key, Object & Pet Location Tags are a new class of devices.  Most work in conjunction with a smart phone and can help in locating something.  Most of them do not depend on direction finding, but rather proximity between the tag and smart phone.

I have a couple of Black Boxes that are in machined aluminum boxes with water proof sealing that probably good to very great depths, i.e. they are the most indestructible electronic products I've ever seen.  Not only could a tank drive over them but also they are immune to high levels of shock and vibration.  They transmit a narrow pulse that all my receivers will not receive because a matched detector would require a much wider bandwidth than commercial or military receivers use, so the signal is very stealthy.  I'm sure there's a directional receiver that works with these to help someone find whatever is attached.


They can have systems that are similar to the Wullenweber only at much higher frequencies.  This could be used to locate other ocean going vessels or land based radio transmitters.


Anti Radiation Missiles (Wiki: ARM) are designed to fly into a source of radiation like would be used on a ground to air weapon system. For example the Shrike and HARM.


Many law enforcement agencies use LoJack (Wiki) receivers on their cars.  The cars use 4 roof mounted antennas that work in a pseudo Doppler mode to provide a bearing.
There are also vehicle beacons that allow a tracking vehicle to follow without being in a direct line of sight.  But I expect these have been replaced by tracking devices that combine a GPS receiver with a cell phone.


984108 Apparatus for Determining the Direction of Space Telegraph signals, Oscar C. Roos, Feb 14 1911, [Secret Weapon pg  76] 342/439
System using two whip antennas along the centerline of a ship

1590346 Radio Direction Finding, L.M. Clement (WE), Jun 29 1926,
Bellini-Tosi principle

1606476 Radio Signaling System, J.O. Mauborgne & G. Hill, Nov 9 1926,
Includes loop antenna on with azimuth and elevation adjustments

1717674 Aerial System for Wireless Signaling, C.S. Franklin (RCA), Jun 18 1929
Combines two crossed loop antennas seperated by a wavelength and adds a sense antenna

2083495 Electrical Indicating or Measuring System, H.S. Black, E&J Edison,  (Bell Labs),Jun 8, 1937 [Secret Weapon pg  81, 251]
CRT indication of relative phase or amplitude of two same frequency singals
2423437 Direction Finder, H.T. Budenbom (Bell Labs), filled: Aug 25, 1934, issued: Jul 8 1947, 342/369
Short Wave DF receiver with CRT readout of difference between signals (not bearing)
2208209 Radio Direction Finder, Henri Gaston Busignies (Intl Std Elec Corp), Filed: Dec 17, 1937, Issued: Jly 16 1940,  [Secret Weapon pg  81, 251] 342/428 ; 324/121R; 348/116
Rotating CRT gives beraring to SW transmitter

2314029 SELF-ORIENTING RADIO DIRECTION FINDER, Donald S. Bond, W.L. Carson (RCA), Mar 16 1943,
2360810 SELF-ORIENTING RADIO DIRECTION FINDER, Donald S. Bond, W.L. Carson (RCA), Mar 16 1943,
2234331 Ultra High Frequency Radio Direction Finding, D.S. Bond (RCA), Mar 11 1941, [Secret Weapon pg  82, 251]
Vertical half wave dipoles, drives a L -center-R meter output

2454768 Direction Finding Antenna System, G.S. Burrroughs (Fed Tel & Radio Corp), Nov 30 1948
""H" Adcock type system with sense antenna
2468064 Radiogoniometer, R. Hardy (Intl Std Ele Corp), Apr 26 1949, 342/429 ; 342/431; 342/436; 342/440
Neon lamps for Left or Right, manualy turned antenna array.  "H" Addock with sense ant

2365347 Radio Direction Finding System, W.P. Lear (Lear Avia Inc), Dec 19 1944,
Manually or motor turned loop, sense antenna, for aircraft LF and MF.

My Related Web Pages

Survival Radios
PRD-1 Army HF Direction Finding Reveiver
AN/ARN-89 & other Aircraft Direction Finders


An Evaluation of Short-Based Radio Direction Finding 1978 - for VHF EPRIB beacons with analysis of Doppler and other systems

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page created 5 Oct 2009.