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Trying to triangulate position of rogue radio transmitter.

by tCnEvermore
Tags: position, radio, rogue, transmitter, triangulate
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Dec9-13, 05:12 AM
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sophiecentaur's Avatar
P: 12,157
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.
Dec12-13, 06:31 PM
tfr000's Avatar
P: 127
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.
Dec13-13, 12:51 AM
P: 588
Quote Quote by tCnEvermore View Post
... 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.
What you need to do is see if the latency through the repeater is constant. Key the repeater yourself and use scope to measure the delay. Do this multiple times, it doesn't matter if the latency is long, what matters is how stable it is. If it is stable to within, say, 1 microsecond then you can calibrate it out and place the rogue on a ~1000 foot "thick" hyperbolic path.

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