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FM Transmitter Finger Printing

Paul Signorelli (W0RW) on March 24, 2017
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Transmitter Hunting by FM Transmitter 'Finger Printing'
By Sherlock W0RW

The signals intelligence method of 'Finger Printing’ has been used for a long time and Is very simple. It consists of an FM Receiver and a discriminator pick off that goes to an oscilloscope.

Here is how it works: When an FM transmitter is keyed up the radiated signal varies depending on what type of radio it is. An old crystal controlled radio will come right up on frequency with little overshoot deviation or drift. There are not many of those on the air today.

All of the other newer radios use a PLL/VCO to generate the transmit frequency and when they start there can be positive or negative overshoot, sometimes up to 1 KHz. All this takes place within a few milliseconds so you don't hear it but you can see it on a scope set to DC.

Some transmitters are so slow starting there is an audible click at the beginning of the transmission. The polarity and amplitude of the overshoot gives you the fingerprint of the transmitter. After the transmitter is on there can be additional drift depending on the length of the transmission. Of course if there is CTCSS/PL on the carrier that adds another fingerprint element. A storage oscilloscope is very helpful recording this data.

Some newer FM receivers use a FM demodulator chip which won’t allow you to have access to the discriminator but any older FM Radio will have access. Pin 4 on the meter socket of an old Motorola is the discriminator output.

Then you compare the ‘finger print’ to your history data base to identify the owner of a transmitter.

If you are unfamiliar with it you can read a paper on Finger Printing at:

http://onlinelibrary.wiley.com/doi/10.1029/2000RS002345/pdf.

Paul, W0RW

Member Comments:
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FM Transmitter Finger Printing  
by K4JJL on March 24, 2017 Mail this to a friend!
I remember old DOS software that did this back in the mid-90s. My old club's repeater manager fingerprinted tons of people, and it helped track down problems like people sitting on their mic in the car.
 
RE: FM Transmitter Finger Printing  
by K6AER on March 24, 2017 Mail this to a friend!
With today's DDS signatures you would be able to identify the radio from the manufacture but not individual radios.
 
FM Transmitter Finger Printing  
by WX4O on March 24, 2017 Mail this to a friend!
When I was in the Army Security Agency in the 60's we DF'd a lot of transmitters, and used what we called Radio Finger Printing.
 
RE: FM Transmitter Finger Printing  
by AF3Y on March 24, 2017 Mail this to a friend!
You sound like an old 056 DF op. heh heh heh

73, Gene
 
FM Transmitter Finger Printing  
by AB1XG on March 24, 2017 Mail this to a friend!
We've been able to ID some "unfriendlies" on our local machine using this method.

Once they realize we've identified them, they go away and leave us alone.
 
RE: FM Transmitter Finger Printing  
by K4PIH on March 24, 2017 Mail this to a friend!
I fingerprinted hundreds of radios during my years in Air Force Electronic Security Command. I could tell you what unit used it, where it was, when it was maintained, all from its rf output. We had the best toys😎
 
FM Transmitter Finger Printing  
by WA0ZZG on March 24, 2017 Mail this to a friend!
I've got a 1978 Collins Radio FM transceiver that will fingerprint like nothing else.
 
RE: FM Transmitter Finger Printing  
by N4KC on March 25, 2017 Mail this to a friend!

Hey, shouldn't this article appear a week from today, on April 1?

Just kidding. I know some transmitters have definite signatures but I'm still not convinced you can learn all that some of you say you can. I was once a hundred pages into writing a science fiction thriller in which a murderer was caught because of a tell-tale spike on his mouse button when accessing the Internet. I never finished the book because I was not sure such a thing was even remotely possible.

Now, if we could just employ such technology on 7.200 or 14.313...

73,

Don N4KC
www.n4kc.com www.donkeith.com



 
RE: FM Transmitter Finger Printing  
by KG4RUL on March 26, 2017 Mail this to a friend!
So, you finally I.D. them and, given the responsiveness of the FCC, you are penetrated by an inclined plane, wrapped helically around an axis.
 
RE: FM Transmitter Finger Printing  
by AA4PB on March 26, 2017 Mail this to a friend!
If you have previously recorded the signature of a particular radio, you can identify it again when it transmits. The rest of the stuff, what unit used it, where it was, when it was maintained, comes from the equipment records that you keep rather from the RF signature. It's just like people's fingerprints. It can only provide information if you have recorded it previously.

 
RE: FM Transmitter Finger Printing  
by ONAIR on March 26, 2017 Mail this to a friend!
Military satellites can now pinpoint the location of any transmission to within inches, in real time! Should make things a lot easier for the FCC.
 
FM Transmitter Finger Printing  
by AF7EC on March 26, 2017 Mail this to a friend!
If you could please left-justify this article, it would be easier to read. Thanks!
 
FM Transmitter Finger Printing  
by KE4ZHN on March 27, 2017 Mail this to a friend!
Playing radio policeman is a fruitless endeavor with today's FCC. There are guys who have fines levied on them of thousands of dollars who still get on the air and jam daily. And have been for years. Some of them jammed for decades with little to no action from the FCC. So what do you think they are going to do about some idiot qrming a repeater? The only effective action against jammers is usually illegal in most localities.
 
RE: FM Transmitter Finger Printing  
by AF6AU on March 27, 2017 Mail this to a friend!
Agreed, FCC seems more interested in making money selling off spectrum, and selling amateur band freq. infringments to companies making toys for the military and police. Nuf said..
 
FM Transmitter Finger Printing  
by KB4QAA on March 28, 2017 Mail this to a friend!
Long ago and far away in the US Navy we called this method "HULTEC" for "Hull To Emitter Correlation".
 
FM Transmitter Finger Printing  
by KL7FZ on March 29, 2017 Mail this to a friend!
MoTron offered the TXID-1 based on transmitter key up waveform. We used it at a taxi company but found it a bit difficult to use when the system was busy. Also we had a lot of radios of the same model and sometimes they looked the same on key up. It did work most of the time but just was not operator friendly. We went to ANI but had to dump that after everyone complained about the darn chirps. Finally we gave all the radios individual PL and DPL codes and used a reader to identify individuals. But that will only work on small systems.
 
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