Trying to triangulate position of rogue radio transmitter.

  1. First off, this is my first post here. If this is in the wrong section I apologize and ask that it be moved.

    I am a amateur radio operator. We have experienced issues with a rogue radio operator recently, who is maliciously transmitting noise. I think it is possible to determine his position geometrically, but its been a long time since I have done this sort of thing.

    To begin with Ill describe the system as much as possible. I am receiving the signal directly from the rogue transmitter, and also the signal as re-transmitted through a repeater.(At a different frequency) This means that if we describe the positions of myself, the rogue transmitter, and the repeater as a triangle, I am receiving the signal directly through a straight line, and through the other two lines as transmitted through the repeater. Therefore, there is a time gap between the two signals I am receiving. With an oscilloscope I can measure the difference in time between the two paths, and since we can assume speed of the signal to be c, I know the difference in total distance travelled.

    We have multiple observers who can take these measurements from as many locations as necessary, but thanks to my rusty geometry skills I am hitting a brick wall trying to solve this. I am almost 100% sure this solvable, but I am not sure I can do it.

    Thanks for any help you might be able to provide, let me know if my description of the situation is confusing or inadequate.
     
    Last edited: Dec 5, 2013
  2. jcsd
  3. berkeman

    Staff: Mentor

    Do you and the other operators have T-hunting experience? What band is the interference on? Do you have portable Yagi antennas that you can use for the T-hunting?

    If you have operators at known GPS locations, and each has a good directional Yagi and a compass, then you can just draw the several lines on a map to see where they intersect.
     
  4. The repeater is 2m, the offset is 600 hz. I thought of the directional yagi solution, problem is he broadcasts for short periods of time so I'm not sure we could get a direction on it. I don't have any experience doing this, do you think measuring the time-gap with an oscilloscope then would be an effective solution? I'm not sure how quickly the repeater transmits.
     
    Last edited: Dec 5, 2013
  5. berkeman

    Staff: Mentor

    I think you mean 600kHz offset... :smile:

    A hand-held directional Yagi antenna is a very good tool for T-hunting. It's pretty hard to hunt for transmitters without one. You can make them pretty easily -- just do a Google search on 2m hand-held Yagi antennas, or look in the ARRL Antenna Book. I've seen them made from metal tape measures and other improvised materials.

    There are certainly more sophisticated T-hunting setups, including the multi-monopole arrays that are on the car roofs of some of the more serious T-hunters around here. But if you have your portable hand-held 2m Yagi ready, you can get a pretty quick fix on a direction. If you and several of your friends have such antennas, you should be able to pretty quickly start narrowing down the rogue transmitter's location. Do you have a local FCC office that you can report him to once you narrow down his location?
     
    Last edited: Dec 5, 2013
  6. berkeman

    Staff: Mentor

  7. I'm off to class, I'll take a look through this when I get back. Thanks for all the help, I'll get back in touch afterwords.
     
  8. berkeman

    Staff: Mentor

  9. Google "handi-finder".
    It's just a circuit that alternates between two antennas at an audio rate. You plug it into your radio's antenna input. You hear the audio tone in your radio and it varies in volume based on direction.
    I made a homemade version some years ago - it worked.
    de N1OOQ
     
  10. davenn

    davenn 3,682
    Science Advisor
    Gold Member
    2014 Award

    back up a minute

    I assume its a legal 2M repeater but there is some clown causing problems on that repeater.
    Forget the repeater and its offset, the repeater doesn't enter into how to find him

    you need to know the input freq to that 2M repeater. That will be the freq he is transmitting on
    if he's only TX'ing in short bursts , then its going to take some time and patience to track him down
    you will need to observe if he does it more often at certain times of the day and have you and a mate, both of you with a 2M receiver on the freq ( repeater input) and each of you with a yagi like Berkeman showed you

    Then the 2 of you will need to get out to a couple of locations and wait patiently till he transmits and do some direction finding with your yagis ... ie. see which direction his signal is strongest
    Then do as Berkeman suggested by drawing those direction lines on a map of the area

    cheers
    Dave
     
  11. Time Difference of Arrival is a pretty good system. tfr000 described this. One thing that could be tried in order to get the guy to transmit for longer intervals is to disable the repeater transmitter when he goes active. He will notice this and key up longer in an attempt to bring the repeater up. If this person is a ham or former ham it is likely they will do this mobile, so good luck. Something else I have seen is a handheld in a pawn shop set on a repeater frequency. People walk through and try it. The fix is to search pawn shops and find it. Then go find out how to do the software reset on it and come back and reset it. 99.9% chance it will not get reprogrammed.
     
  12. Baluncore

    Baluncore 2,716
    Science Advisor

    If you place an FM doppler DF antenna at the repeater you can get a bearing on each transmission received. That bearing can be transmitted by the repeater as a tone at the end of every burst that you can record. You will then know the culprit's bearing and if it is a mobile or fixed source.

    Once you have that information a second system will give you a cross bearing.
     
  13. The problem with many DF methods is that they are highly specialized. For instance, Yagi antennas - you have to buy or build one. Building is not that difficult, but it's non-trivial. Also, a Yagi is basically single frequency or at least single frequency band.
    The "handi-finder" method is pretty small, cheap (something like $35 if you buy the circuit board), and can be used on any freq for which you have a radio. The two antennas can be almost anything, as long as they are identical - they need not even be particularly well tuned to the freq band you are DFing.
    Note that both "handi-finder" and Doppler DF methods are implementing a practical version of the time delay method mentioned in the original post.
    By the way, timing the jammer's transmission vs the output of the repeater is pretty hopeless - there is always some delay in the electronics between receive and transmit in anything like a repeater. Unless you have a way of measuring that delay and subtracting it from what you see on the scope, the results will be meaningless.
     
  14. sophiecentaur

    sophiecentaur 13,700
    Science Advisor
    Gold Member

    To get an accurate Direction for a radio signal, it is usually best to go for a 'null' rather than a maximum. The null from a loop or from two close-spaced dipoles will be much sharper than the broad main beam of even the largest yagi you can afford. You need two or preferably, three people to get direction fixes at the same time and then, three lines on a map will produce a triangle (a 'cocked hat') inside which the rogue transmission will probably be found. You need to be careful about getting the right bearing and not the reciprocal because there will be two nulls on your antenna - one at the front and one at the back. This should resolve itself once you get plotting your three sets of lines on the map.
    Magnetic Variation needs to be taken into account if you are taking a magnetic compass bearing and want to plot a true bearing on a map. If you are trying to get a fix whilst in a car, there will also be Magnetic Deviation, due to the steel in the car, so you may be better to do your DFing a few metres away from the vehicle.
    You need to calibrate the actual direction of the null from your antenna with a source in a known position.
     
  15. Why don't you, if you haven't already, contact your amateur frequency coordinator and tell him what you told us. Besides you working on it, it should also be his responsibility to track down the transmitter. Besides he will know the location of the repeater on that frequency. You may be able to enlist other hams to listen for the noise and help you track it down. If the rogue transmitter is a mobile, you may never be able to track it down.
     
  16. sophiecentaur

    sophiecentaur 13,700
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    Gold Member

    There was a time when men in vans would drive round the country, locating and sorting out rogue transmissions. Nowadays, it costs too much and no one really cares about a bit of inconvenience to amateur radio enthusiasts. any coordination is very loose and the RSGB has no teeth.
     
  17. davenn

    davenn 3,682
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    Gold Member
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    unfortunately, that is so true, and not just in the UK

    tCnEvermore, if you want to put an end to the crap, you and a couple of fellow hams will have to spend the time to do the tracking down.
    a couple of 3 element yagis for DF'ing are really easy to build. Hell they don't even need to be accurately tuned !! you are only using them for receiving !
    You could build us a couple of them in 2 or 3 hours from a couple of old ( redundant) low band TV antennas

    forget about signal path timing etc too difficult and you wont get the accuracy
    do as said above with a couple of 3 ele yagis

    cheers
    Dave
     
  18. sophiecentaur

    sophiecentaur 13,700
    Science Advisor
    Gold Member

    That's an interesting response and I have to say, I see a lot of examples where it's done that way. However, it's a common principle in measurement that detecting a null is a more precise way of establishing angle or position than looking for a maximum. The use of ferrite rods is common (used to be, at least) in DFing at lower frequencies and, at any frequency, a null finding antenna is going to be much more compact and easy to handle. We'v all seen the WW2 films when the Nazis are chasing the secret agent's radio transmitter, using a loop antenna on top of their van. I believe the TV detector vans have (or had?) a pair of directional yagis, connected in anti phase, to give a good, narrow null with which they could locate the TV local oscillator in a particular room, despite that fact that the basic beam width of the antennae would have probably contained the whole building.
    In the case of a detector van, of course, they need to eliminate all the other LO's in adjacent houses, so they really need a narrow beam, in addition to angular precision.
    I cannot think of a way of incorporating 'timing' into any system without a lot of extra kit. The three receivers would need to be phase locked together,using an external reference (GPS?) and then the relative arrival times of the rogue signal would need to be identified unambiguously (phase isn't good enough at 140MHz / 2m ). That, in itself could be an interesting project, of course.

    You could always use local knowledge of the area and, assuming the guy is mobile, he (it's just got to be a bloke, hasn't it?) will probably be using a good transmitting site. You might even get away without triangulation at all and look for a suitable bit of elevated land, somewhere along the direction you detect him. He is unlikely to be changing his location frequently; after all, what he's doing is hardly a high profile bit of civil disobedience like graffiti by a railway line. He;s only affecting a few dozen HAMs at the most.
     
  19. Baluncore

    Baluncore 2,716
    Science Advisor

    Professional DF systems have rigorous protocols to ensure their local field is not significantly distorted. While there is no question that deep nulls are way more precise than main lobes for DF, the problem in built environments is that there are many parasitic structures to distort the local field. Deep nulls are critically susceptible to multipath interference so the accuracy needed to find a good intercept bearing will probably just not be there. There is often insufficient time during short transmissions to find the null.

    The advantage of 3 element yagis for an active mobile search is that they quickly give a crude estimate of direction. As you progressively refine the probable location you can follow the roads and pathways that provide access. Your zigzag path will effectively spiral in on the source of the interference.

    There is a psychological problem that will have to be confronted. In my experience the person who jams a repeater is normally brighter than most and will not be easy to prosecute. They are almost certainly rebels looking for attention. You are better making friends with them and turning their unrealised ability in a constructive direction. In the long run it is better to empower them than to prosecute them. The Government can only prosecute, you can convert.
     
  20. sophiecentaur

    sophiecentaur 13,700
    Science Advisor
    Gold Member

    Some good points there, Baluncore. Local fields will make a big difference to the null and it would always be best to do your measurement well out in the open and away from a vehicle. A directional antenna is certainly more immune from multi path effects. Interestingly, my experience of DFing (even that is second hand) is on small boats and muiltipath tends to be less of a problem out at sea (nothing behind you, most of the time). Steel rigging and spars are the only local problems. Fact is that, despite what they try to tell you about possible accuracy in navigation, you are lucky to get visual bearings down to even 5 degrees and the only really good lines you can rely on, on a chart are transit lines. Radio DFing, on a small boat, must be a real nightmare if you really do need that information.

    I must say, I like your description of how to use a yagi in practice. Very much an iterative process.

    You are also totally correct about the sort of person who creates nuisances like repeater jamming. It's along the same lines as the writers of computer malware. A difficult problem to solve, even if you can actually find them.
     
  21. I didn't find that "null" DFing was all that difficult due to multipath. Yes, you would get an occasional reflection, but after a few fixes it becomes obvious which of them are bogus. I imagine this is the same with any DF system. There is plenty of time to find the null. You swing the thing around, pretty much as fast as you can, and the drop out is very obvious.
    The big difference between "null" and "maximum" methods is that as you get close in to the transmitter, the Yagi loses all directionality - every signal is full scale, no matter which way you point. For this, you need an attenuator between the antenna and the radio.
     
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