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Radio Direction Finding Forum - Discussions regarding direction finding and transmitter location

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Old 02-23-2015, 4:32 AM
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Question How does the Navy do it?

I was watching the show The Last Ship last year and season two comes out this summer. Well, they were able to find the direction and location of a Russian ship. How do they do that with a single transmission? The source of the Russian ship was plotted on a monitor.

In Ham radio direction finding you have to go all over and find the direction and narrow it down with signal strength.

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Old 02-23-2015, 4:33 AM
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Old 02-23-2015, 6:36 AM
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Real world maritime direction finding is done using Doppler Directional techniques, same as used for LoJack tracking. Sometimes if you look closely at some vessels including Coast Guard boats you will notice an antenna array consisting of 4 antennas arranged around a hub.

I have used a marine DDF to fox hunt with great success, all you need is a couple of seconds of carrier from a transmitter and the unit indicates the bearing to the tx. When I lived in the Dallas area we would have monthly fox hunts and the guys with Doppler units would almost always beat the guys using Yagis to the target, the big advantage you didn't need to stop to take a reading.

Ramsey kits sell a DDF-1 kit you can build your own, all you need is audio output from any suitable radio to feed to the unit.
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Old 02-23-2015, 9:14 AM
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That Ramsey kit is great. Used it in a previous job often. Even managed to get some Navy contracted submarine tenders to program dead spots into their gear when they were jamming us due to this.

They claimed their equipment was not causing the problem. Well being able to give a direction and distance, of vessels that are not coming up on AIS (due to jamming and electronic countermeasures) using doppler receivers, worked well.
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Old 02-23-2015, 9:34 AM
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Another technique used in Navy equipment is a small spinning loop receive antenna. Antenna position is slaved to a trace on a CRT. When a signal is received you will see a figure 8 pattern on the CRT. Transmitter will be on either side of the figure 8. Pressing a button unbalances the loop and bends the shape of the figure 8. That bending indicates the direction which is read off a ring on the edge of the CRT.

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Originally Posted by poltergeisty View Post
I was watching the show The Last Ship last year and season two comes out this summer. Well, they were able to find the direction and location of a Russian ship. How do they do that with a single transmission? The source of the Russian ship was plotted on a monitor.

In Ham radio direction finding you have to go all over and find the direction and narrow it down with signal strength.

TIA!
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Old 02-23-2015, 9:13 PM
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Real world maritime direction finding is done using Doppler Directional techniques, same as used for LoJack tracking. Sometimes if you look closely at some vessels including Coast Guard boats you will notice an antenna array consisting of 4 antennas arranged around a hub.
It's called an Adcock Array.
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Old 02-25-2015, 4:33 AM
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Another technique used in Navy equipment is a small spinning loop receive antenna. Antenna position is slaved to a trace on a CRT. When a signal is received you will see a figure 8 pattern on the CRT. Transmitter will be on either side of the figure 8. Pressing a button unbalances the loop and bends the shape of the figure 8. That bending indicates the direction which is read off a ring on the edge of the CRT.
Now that sounds really neat. Is there a way to build something like that?
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Old 02-25-2015, 4:34 AM
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It's called an Adcock Array.
Well how be damned. I knew about these antennas used in aviation, but never knew the proper name.
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Old 02-25-2015, 6:38 AM
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On the military surplus market look for a AN/BRD-6 in the scrap submarine area. If you are really lucky and the antenna dome is still mounted to the mast you will have a hydraulically operated 2 or 3 section telescoping mast. I forget how many section.

Actually I think the only tricky part was syncing the spinning antenna to the display. Mostly because of the convoluted path from dome to receiver display unit. Receiver and CRT display very typical for the times, built like a tank.

I became in involved with the repair of this piece of equipment at one of my duty stations. It seems that the office of the con overlooked lowering the BRD and the attack periscope and goosed it as they dived. Bent the scope to about 15 degrees from straight and snapped the mast of the BRD which pass thru the screw, which was a very very bad thing.

Likely can be built today, but mechanical challenge to spin antenna and keep the display synced. Todays electronics make it easier, but Adcock Array is the way to go.

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Now that sounds really neat. Is there a way to build something like that?
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Old 02-25-2015, 10:26 PM
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Quote:
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...It seems that the office of the con overlooked lowering the BRD and the attack periscope and goosed it as they dived. Bent the scope to about 15 degrees from straight and snapped the mast of the BRD which pass thru the screw, which was a very very bad thing.
That description makes this skimmer squid shudder.

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Old 02-27-2015, 3:38 AM
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Quote:
Originally Posted by mcscanner View Post
On the military surplus market look for a AN/BRD-6 in the scrap submarine area. If you are really lucky and the antenna dome is still mounted to the mast you will have a hydraulically operated 2 or 3 section telescoping mast. I forget how many section.

Actually I think the only tricky part was syncing the spinning antenna to the display. Mostly because of the convoluted path from dome to receiver display unit. Receiver and CRT display very typical for the times, built like a tank.

I became in involved with the repair of this piece of equipment at one of my duty stations. It seems that the office of the con overlooked lowering the BRD and the attack periscope and goosed it as they dived. Bent the scope to about 15 degrees from straight and snapped the mast of the BRD which pass thru the screw, which was a very very bad thing.

Likely can be built today, but mechanical challenge to spin antenna and keep the display synced. Todays electronics make it easier, but Adcock Array is the way to go.

Cool. I'm not too keen on military auctions because you need a membership. I checked ebay for the hell of it and found nothing. Is it difficult to syc the antenna to the display?
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Old 02-27-2015, 5:43 PM
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Special equipment on special ships. May never show up on surplus market. My original comment was me being a smarta$$. My bad.

I don't recall with certainty what the method was used to keep that antenna and display locked. I can say that the common device, a synchros receiver and transmitter were used in cases where angular position was important between two distant locations on the ship. Radar set, fire control system, ship's gyro compass are all typical applications. A synchro looks like a small dc motor, but has more electrical contacts like a stepper motor. They are both energized by an AC signal not unlike plugging a fixture into a wall socket. In the transmit motor magnetic fields are created in the stator based on the position of the rotor. The stator's magnetic field are carried by the interconnecting wires as voltage to the receivers stator. The rotor in the receive motor assumes the same position of the transmit motor because of the transferred voltages are setting up like fields.

Easy to do if you have the right devices. They are expensive and not sure they are in used today. Stepper motors and encoders might be a better workable approach. My strength is repair, I know enough to get into deep trouble trying to design 'stuff'. I keep to the shallow end of the pool.
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Cool. I'm not too keen on military auctions because you need a membership. I checked ebay for the hell of it and found nothing. Is it difficult to syc the antenna to the display?
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Old 02-27-2015, 5:49 PM
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Never did hear what the fall out was for the officers. Likely a serious show stopper. Admiral Rickover ruled the nuke power program with an iron fist, make a mistake I will crush you hand.
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Old 02-28-2015, 11:34 AM
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You gave my hopes up man. I was going to find and buy that sucker and mount it on the freaking roof! BAHAHAHA!

Well, there's that TSCM Doppler unit. If you seen that post you will know what I had in mind. I just don't know if the resolution will be there though.
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Old 02-28-2015, 12:57 PM
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I looked up the thread and then found the manual for the DF2020T. I have done a little Fox hunting with my 2m HT and a tape measure yagi I made.

Being mobile is the key to finding the transmitter. Originally the plan was to lay down bearings on a paper map moving from location to location to establish a localized area on the map. To do that and create something like a equal sided triangle require moving outside the area of interest some distance around the transmitter. That is a time waster. Best tactic was to drive in the direction of the strongest signal. Once the signal was very strong then I switch to using the back of the antenna. The antenna was designed with a broad forward beam and a very deep and narrow notch on the back good for close in tracking. I also had an off-set attenuator to use too which helps when you get really close and the backside notch overloads the radio.

I didn't see anything in the DF2020T that addressed beam width or accuracy. I have to believe it was better and clearly faster than what I was using. I had to stop, get out of the car and move some distance away. Then wave a white and yellow antenna around to decide the next course correction to make.

For the frequency range the equipment operates in, the signal well be local so mobile operation is possible and the key. Anything that tells you the direction of the strongest signal will work. The faster and easier it can do that the better. DF also depends on how long or often the signal of interest is up.

I need to stop writing now. I am slowing talking myself into buy another toy.
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Old 03-02-2015, 1:26 PM
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There is Hollywood, and then there is real life. On that particular show the data shown was not possible passively in the scenario shown with a non-cooperative target, although naturally it could have been acquired with active means. By passive I mean your platform not transmitting a signal, and only listening, active would be involving a transmission from your platform, like radar.

Sure, Navy ships can get a direction on many different types of emitted signals by passive means. But direction and range, a real position plot, is a different story.

Passive systems:
Passively you can get an instantaneous (meaning as it is right now) direction of arrival from a single stationary platform. Over time you can start to develop a plot of possible location via Target Motion Analysis. If the source of the emitted signal is well understood you can make calculations on range. i.e I am monitoring the surface search radar (or some other emission) of a target, and I know the ERP of that specific model of emitter, I can start to estimate range. So now I have a direction to the target, and a range window that the target can be in, no closer than X, no further than Y.

If I can combine two platforms, say my ship and a drone or helicopter with DF capability, I can get a positional fix. I’ll leave how that might be done to your imagination.

Active systems:
Radar and LIDAR, and combinations of IRST and laser range finders. Of course these can give a 3D fix, but you also must give away your position to do it.

This thread has discussed scanning antennas and Doppler detection (simulated scans), but there is also none moving or simulated movement angle of arrival detection. Systems like TDOA (Time Difference Of Arrival) and PDOA (Phase Difference Of Arrival). Typically the moving antennas and the simulated motion systems are “old school”, while current technology leans more towards non-moving and no simulation of movement.

The ship in the series The Last Ship is a fictional ship, DDG-151, USS Nathan James. The exterior shots of a real US Navy ship, when required, are typically of the USS Halsey, DDG-97, an Arleigh Burke destroyer. Decidedly not “old school”.

Old school RF detection systems (think of it as your cars radar detector, with a Department of Defense budget):
Things like the AN/WLR-1, AN/WLR-8, and AN/WLR-11 used a combination of fixed omni antennas and moving (spinning) antennas. The signals were detected on the omnis (such as AS-1174/1175/66131) and then the systems switched over to the spinning antennas (such as AS-571/616/899) to find direction of arrival. These spinning antennas often were quite bulky, but still spun at 150 to 300 PRM.

New School RF, no moving parts:
Systems like the AN/SLQ-32 (primarily a missile defense system, but also with other RF capabilities) use fixed, none moving, antennas for both omni and DF work. One of the techniques used is a Rotman lens array.

And lets not forget that SONAR is a valid passive tool, although of VERY limited use to a surface ship in port.

T!
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Old 03-02-2015, 2:45 PM
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Quote:
Originally Posted by poltergeisty View Post
In Ham radio direction finding you have to go all over and find the direction and narrow it down with signal strength.

TIA!
It's a safe bet that no RF "emitter of interest" to the military is being DF'd by a bearing from a single station.
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Old 03-03-2015, 12:02 AM
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Sent from my drone using encrypted SSH, L2TP, P25, Astro, Provoice-ESK, Tetra, VSLEP, DECT 6.0, WPA2-PSK with a dash of Tapatalk


Great information, thanks!
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Old 03-03-2015, 10:25 AM
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I'm sure the navy has something equivalent to the Russian Tselina satellites as well:

"Tselina (rus. Целина) is a Russian, formerly Soviet, military space-based radio surveillance system. It is capable of determining the exact location of radio-emitting objects and also their type, modes of operation, and how active they are."
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Old 03-03-2015, 11:20 AM
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I have heard that Phased Array antennas are the quickest way to locate and FIX locations of transmitting equipment. Also - the military has been working on RDF systems since RADAR came about, so the capability of quick analysis and reporting of an enemy signal should not be thrown out as "Hollywood" only option.
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