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Author Topic: RF Noise Identification  (Read 3004 times)

WrongwayCorrigan

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RF Noise Identification
« on: July 19, 2010, 0510 UTC »
During the night hours (local time) I have been observing strong interference with a distinctive sound. The interference is repeated on several frequencies anywhere from 2 MHz - 30 MHz. The frequency changes each night.

Anything with a strong carrier will have no problem cutting through the interference. The signal from the interference remains the same strength no matter where I relocate to outdoors.

Is this natural electromagnetic interference from the atmosphere or beyond? I am located in an urban area so perhaps that is another clue. Audio clip is below.

Receiver: Grundig G6 Aviator
Anntenna: Telescoping Rod
Place of Reception: Northeastern United States
Date: 07/19/2010
Time: 0456 UTC
Frequency: 2395 kHz

http://www.zshare.net/audio/7846714599debe10/

« Last Edit: July 19, 2010, 0522 UTC by WrongwayCorrigan »

Offline SW-J

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Re: RF Noise Identification
« Reply #1 on: July 19, 2010, 1505 UTC »
Here are several tests I apply mentally to determine a signal source as local or remote (distant):

Does it sound regular, a repeating pattern or random?
 o Switching power supplies or
 o Conventional TV Horz sweep rates (occurring every 15,734 Hz color, 15,750 Hz B&W) or
   - have a 60 Hz or 59.94 (and harmonics thereof) Hz buzz (more due to conventional TV video)
 o Does have what might be conluded to have a complex waveform based on the sound? (manmade/data)

Does it fade and peak - indicates ionospheric propagation (or, on the much higher frequencies, tropospheric or E-layer skip)

Does it exhibit selective fading - a sure indication of ionospheric propagation (it's like multipath @ VHF/UHF, only involving hundreds of miles instead of just miles)

Does the frequency slowly drift or is it fixed?
 o For instance, switching Power supplies will drift with time, something a 'rock' (crystal) or reference-controlled (e.g. GPS ref'd gear) will usually not ...
   - 28.635 MHz for instance will usually show several carriers, never drift, never fade, from computers/monitors and this rate is xtal controlled (vid card?) ... in fact, a few years back operating 10 Meter mobile while driving I would notice different neighborhoods exhibited various different signals on/about this frequency ...

On your signal (in the mp3 file), I detect some selective fading, so, I would conclude the source is experiencing ionospheric propagation and 'hundreds of miles' distant (could be less, could be more, but it's not just down the street!).
I also detect a very regular rhythm or beat, almost like a Motorola Trunking channel; it sounds like a man-made 'data stream' of some sort ...

I presently have a 'whine', no fade or change in strength, coming from one of the neighbors (I've walked the street and determined a possible soruce) ... sometimes it slightly dips in frequency, a smooth dip, not a hard switch in frequency, causing me to think it's due more to an LC or RC network determined appliance of some sort as a load or loads on the AC circuit (in the house) changes ...

« Last Edit: July 19, 2010, 1937 UTC by SW-J »
o Icom IC-756ProII, ProIII, Alinco DX-70, Kenwood TS-680s
o WinRadio G303e, Degen/Kaito 1103/DE1103, Stoddart NM-25
o 1/2 wave 80m Dipole used with several tuners
o Tuned loops from 2' thru 16' diam. capable of 160m thru 10m

WrongwayCorrigan

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Re: RF Noise Identification
« Reply #2 on: July 20, 2010, 0539 UTC »
The signal is continuous with no breaks. It does exhibit fading as you mentioned as well as a regular 'beat'

Since you mentioned 'data stream' here is the closest example I can find:

MOTOTRBO™ Professional Digital Two-Way Radio System:
http://business.motorola.com/mototrbo/mototrbo.html

And here is unencoded audio of MOTOTRBO:
http://www.nonstopsystems.com/radio/sounds/radio-mototrbo-undecoded.wav

cmradio

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Re: RF Noise Identification
« Reply #3 on: July 20, 2010, 0621 UTC »
I dunno if it's related, but there's been a LOT of spread-spectrum digital comms on the government bands just below 10M lately around Vancouver in the wee hours of the morning :-\

Peace!

Offline SW-J

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Re: RF Noise Identification
« Reply #4 on: July 20, 2010, 1521 UTC »
The signal is continuous with no breaks. It does exhibit fading as you mentioned as well as a regular 'beat'

Since you mentioned 'data stream' here is the closest example I can find:

MOTOTRBO™ Professional Digital Two-Way Radio System:
http://business.motorola.com/mototrbo/mototrbo.html

And here is unencoded audio of MOTOTRBO:
http://www.nonstopsystems.com/radio/sounds/radio-mototrbo-undecoded.wav


They sound remarkably similar don't they?

Reason being, I conjecture, is, continuously sent 'data packets' or streams of this nature are usually encoded in fixed-length packets, and these repetitious packets are usually encoded with a 'sync pattern' to allow initial synchronization to the incoming bit stream ... then additional processing takes place to begin to read the series of bits ... also encoded (depending on the purpose or function of the data) are addresses,  system overhead info (like system ID, maybe a protocol to observe when responding, time/date etc) as well as situation-specific data (like data payload: something that means something to the receiving station) ... as well as some sort of FEC (forward error correction - similar in concept to the Reed-Solomon encoding on a CD for instance) ... I say this, b/c, in past years I made an intensive study of the Moto 3600 baud GMSK-modulated trunking control channel format ... and it too corporates these features/facets I mention above ... plus one or two more features - like a block parity code (used for verifying the FEC corrections - they could be in error!)

o Icom IC-756ProII, ProIII, Alinco DX-70, Kenwood TS-680s
o WinRadio G303e, Degen/Kaito 1103/DE1103, Stoddart NM-25
o 1/2 wave 80m Dipole used with several tuners
o Tuned loops from 2' thru 16' diam. capable of 160m thru 10m

WrongwayCorrigan

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Re: RF Noise Identification
« Reply #5 on: July 23, 2010, 0652 UTC »
Here is my noise observed tonight on the same frequency. The signal is stronger with less fading so you will hear more of its distinctive sound.

It has the characteristic of the continually sent stream of data that you describe. Still curious as to the exact type of data encoding used and what the source may be. I didn't think my MotoTRBO sample was an exact match.

http://www.zshare.net/audio/785961617c164371/

WrongwayCorrigan

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Re: RF Noise Identification
« Reply #6 on: July 03, 2011, 1808 UTC »
Yes, I am digging up a year old topic.

Does anyone think the data mode heard in the sample above is STANAG 4285 ?