Scanning with rtl_fm

Alan Corey alancorey at yahoo.com
Sun Apr 28 20:48:46 UTC 2013


Thank you.  QST has talked a good deal about the fact that power companies in some areas were trying to put broadband wireless internet out over power lines.  It didn't work very well and radiated enough to be a major nuisance.  Either they quit on their own or the FCC made them stop, I forget which.  This sounds similar.  I'm ab1jx by the way.  You should be able to see that with sdr sharp.

I don't know whether it depends on frequency or not, but I can scan the AM aircraft frequencies around 120 MHz and I don't have the same problem.

#!/bin/sh
rtl_fm -M -S -l 132 -p 102 -f 123000000 -f 135675000 -f 123750000 -f 122700000 \
-f 125350000 -f 133600000 -f 118350000 -f 134850000 -f 119600000 -f 122950000 \
-f 121500000 -f 126450000 -f 121600000 -f 127800000 -f 126450000 -f 122800000 \
-f 135675000 -f 135875000 -f 119025000 /tmp/`date +"air_%Y-%m-%d_%H%M.raw"`

That doesn't pick up anything odd, just pilots talking.  The other is Massachusetts state police around 850 MHz nbfm.  I tried taking each frequency out of the pool one at a time, raising the squelch level (it's an int by the way).  

I have a couple of pigtails that fit my dongles, so I put a 50 ohm terminator in place of the antenna and I still hear it, even here in a basement.  It could be something from a computer, but it isn't on any one particular frequency.

I was thinking it was some artifact of the demodulation method used.  If that were the case other people would hear it too when they scan.  It isn't a local noise I don't think.

  Alan
-----

Radio Astronomy - the ultimate DX

--- On Sun, 4/28/13, Ron Hayes <ka2rrz at yahoo.com> wrote:

From: Ron Hayes <ka2rrz at yahoo.com>
Subject: Re: Scanning with rtl_fm
To: "Alan Corey" <alancorey at yahoo.com>, "osmocom-sdr at lists.osmocom.org" <osmocom-sdr at lists.osmocom.org>
Date: Sunday, April 28, 2013, 3:01 PM

Last year our power company changed the meter so they can read it from there office and it seems to be in the VHF area, I havent pin pointed if its a fact yet But I am now getting a large burst I didnt have before.
        From: Alan Corey <alancorey at yahoo.com>
 To: osmocom-sdr at lists.osmocom.org 

 Sent: Sunday, April 28, 2013 10:43 AM
 Subject: Scanning with rtl_fm
   I'm using rtl_fm in a scanning (nbfm) situation, and even with the antenna input disconnected and terminated, I get a burst of noise that sounds like a squelch tail maybe once each scan cycle.  They're a little over 1 second apart.  I've tried setting the squelch up to 10000 where I don't get signals anymore, but I still get the noise.

I discovered that if you scan and have the output be a raw audio file, nothing gets recorded when nothing breaks squelch.  This makes time-lapse recordings where all the pauses are taken out.  Very nice,
 except for the noise bursts.

I'm using:
#!/bin/sh
rtl_fm -N -S -l 132 -p 102 -f 854040000 -f 854240000 -f 854315000 -f 854415000 -f \
854490000 -f 854540000 -f 855165000 -f 855240000 -f 858790000 -f 867350000 \
-f 867715000 /tmp/`date +"%Y-%m-%d_%H%M.raw"`

and recorded
 overnight, but I mostly got noise bursts.

  Alan
-----

Radio Astronomy - the ultimate DX    
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