eHam

eHam Forums => FoxHunting => Topic started by: KC6RWI on April 28, 2022, 07:55:22 PM

Title: How the Feds direction find.
Post by: KC6RWI on April 28, 2022, 07:55:22 PM
I was looking up active antennas on the web. I came across the Rohde and Schwartz antennas, $2K and very much more for a hand held direction finding antenna.
Then I saw these portable receiving radios, 8khz 8 gig. The radios have a huge screen with spectrum analyzer and more. The price for these items is probably $25k or more.
There is a video I'll post here where they find a signal by placing a dome on the car and driving until enough bearings are compiled to triangulate and its all shown on a colored computer map.  I'm pretty sure this is the equipment some agencies use,

https://www.youtube.com/watch?v=fa7aiZ0amN8
Title: Re: How the Feds direction find.
Post by: G8FXC on April 28, 2022, 11:48:53 PM
I was looking up active antennas on the web. I came across the Rohde and Schwartz antennas, $2K and very much more for a hand held direction finding antenna.
Then I saw these portable receiving radios, 8khz 8 gig. The radios have a huge screen with spectrum analyzer and more. The price for these items is probably $25k or more.
There is a video I'll post here where they find a signal by placing a dome on the car and driving until enough bearings are compiled to triangulate and its all shown on a colored computer map.  I'm pretty sure this is the equipment some agencies use,

https://www.youtube.com/watch?v=fa7aiZ0amN8

I'm sure that they have such things available, but I doubt that they get used very often. Most direction finding these days will focus on locating the target's mobile phone - we all very obligingly carry one pretty much all the time - and the cell phone network operators have very obligingly installed a matrix of receiving stations spaced out by a few miles all across the coutry! There is plenty of information in the network operator's computer systems to locate any active cell phone to an accuracy of a few hundred yards in seconds and a lot more tightly with a few minutes more data...

Martin (G8FXC)
Title: Re: How the Feds direction find.
Post by: K5LXP on April 29, 2022, 05:58:00 AM
There's more of a market for direction finding, HF through microwave, than you might believe.  Maybe not for retail/commercial but the outfit I work for builds RDF systems and there is high demand.  Think "bad actor" and "foreign country" and the utility of these becomes more apparent.

Mark K5LXP
Albuquerque, NM
Title: Re: How the Feds direction find.
Post by: KT0DD on April 29, 2022, 06:02:36 AM
Most likely by now, if the offense warrants it, it's done by satellite and probably only takes a few seconds if a "bird" is nearby.
Title: Re: How the Feds direction find.
Post by: WB4SPT on April 29, 2022, 06:11:16 AM
I was looking up active antennas on the web. I came across the Rohde and Schwartz antennas, $2K and very much more for a hand held direction finding antenna.
Then I saw these portable receiving radios, 8khz 8 gig. The radios have a huge screen with spectrum analyzer and more. The price for these items is probably $25k or more.
There is a video I'll post here where they find a signal by placing a dome on the car and driving until enough bearings are compiled to triangulate and its all shown on a colored computer map.  I'm pretty sure this is the equipment some agencies use,

https://www.youtube.com/watch?v=fa7aiZ0amN8

very nice;   and, that was 8 years ago!
Title: Re: How the Feds direction find.
Post by: KC6RWI on April 29, 2022, 07:59:05 AM
I want one, but I'd buy a car before I spent that much money, it would be the most expensive rf noise finder you could buy. It didn't show that it
had audio output but it must have.
This model was older there is a new model same size and function,
Title: Re: How the Feds direction find.
Post by: WB6BYU on April 29, 2022, 11:31:55 AM
There are many different methods, and different antenna systems.

There are several software solutions for a Watson-Watt system,
originally developed during WWII.  It uses fixed antennas feeding
a dual-receiver system, originally drawing a instantaneous trace
on a CRT screen in the direction of the incoming signal.

These days, however, we have the computational power to
measure the phase and amplitude at multiple antennas and
calculate the bearing accordingly.  For mobile use, especially
at VHF and above, reflections are still the major issue, especially
for mobile units at ground level, so multiple readings need
to be processed to choose the right ones.

The FCC vehicles used slot antennas cut into the roof, so there
was no visible antenna on the vehicle.  You often don't want others
to know that there may be DF equipment in the vehicle.

For HF, they use automated fixed receiving sites and correlate
the signals for time and phase, then run it through a model of
the current ionospheric conditions to solve the location based
on transit time (among other things).  That, in conjunction with
azimuth and vertical angle data, gives a pretty accurate location
over long distances, even for high angle signals.  It certainly is
close enough to get a ground wave signal to DF if greater
accuracy is required.

Title: Re: How the Feds direction find.
Post by: KC6RWI on April 29, 2022, 08:03:28 PM
I showed this video to a non ham, and his question was what is the use for this? I couldn't come up with a good answer, I mean it could be used to find a cell phone signal, but there must be other ways to do this.
So lets say its for the criminal element, but they aren't using 2 way radio or are they?
Title: Re: How the Feds direction find.
Post by: WB6BYU on April 30, 2022, 07:35:40 AM
Quote from: KC6RWI

... what is the use for this? ...



I’ve used RDF to search for downed aircraft, and to track
down interference (like noise sources, stuck microphones,
a cat sleeping on someone’s CW key, jammers, etc.).

I’ve nearly had to do it a couple times to find people or
vehicles who got lost, but had a radio with them.
Climbers on Mt. Hood often carry radio beacons so they
can be located in an emergency.

Mostly I do it for fun.  But it takes skill, regardless of your
equipment, and if you don’t do it regularly, then you may
not be ready to do it when needed for some other reason.


Quote

So lets say its for the criminal element, but they aren't using 2 way radio or are they?



Some of them certainly are.

Cell phones leave an identifiable trail.  If you have
a couple lookouts outside while doing something
nefarious in a building, that’s a common way to
keep in touch.  Or keeping in touch with a rural
hide-out, or coordinating activities over a wide
area.  Sure, internet is handy when it is available,
but mobile service still usually depends on a
connection to a cellular network, which leaves
the same trail as a cell phone does.

I knew one game warden who was tracking
teams of poachers.  In inland parts of the country,
marine HTs were popular, as the radios were
easily available (at Radio Shack, for example)
and the frequencies weren’t otherwise used locally.
These days, wideband radios are relatively cheap
(even for HF), and folks can set themselves up
on a quiet frequency where they aren’t likely
to be noticed.
Title: Re: How the Feds direction find.
Post by: KC6RWI on April 30, 2022, 11:42:36 AM
Thanks for the informative response. I just though of one use for the df instrument, for drones that
might be around an airport.
A few decades back when most signal were in the clear, I was always scanning around finding stuff, even the fbi
had bank robberies descriptions in the clear.
So I found a frequency that had a signal people talking and the sound of clothing moving, I took my hand held as
I recognized I was close and found that it was coming from a tax preparer's shop on the street.
I could take the antenna off and still get the signal, I just zipped my jacket and went back to work  168. mhz.