Data Transmissions in the CB band

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chrismyers2000

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No, no, I'm not trying to get internet over CB radio. I had a good laugh while googling the topic. I was listening to the 10m band with my rtl-sdr and noticed some strange signals just below 28.000Mhz. I saw the same signal pretty much all over the upper portion of the CB band. For the most part, the signals are prefaced with a short tone burst, - - - similar to the letter O in morse code. After that an 8 khz wide data burst starts and lasts for only about 4 seconds. They don't seem to repeat with any consistency and they seem to be slightly different every time.
Any ideas as to what they are or where they could be coming from? My only conclusive thought was maybe they're just ghosts or resonant frequencies from a nearby transmission, but I have yet to see them anywhere else between 24mhz and 1600mhz.But I know these dongles don't have the best rf filters (if any).

side note:
I've shielded my sdr dongle the best I can with tinfoil and placed that inside an aluminum project case. The USB cable also has 4 snap-on RF chokes.
 

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chrismyers2000

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Token

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First, these signals are not always OTHRs, sometimes they are atmospheric sounders (also called atmospheric radar). The short duration is more indicative of an OTHR, and your assessment is probably correct, I am just adding details here and indicating it is not certain that is an OTHR in your images. Since sounders and OTHRs can use identical waveforms it is sometimes impossible to tell if it is a radar or a sounder. For the purposes of this discussion lets call anything intended to track man-made objects, such as ships, aircraft, or missiles, an OTHR, anything intended to track faster changing natural targets, such as meteors, a radar, and anything intended to track changes in the atmosphere a sounder (it is almost never wrong to call an atmospheric radar a radar, it can confuse some people though, they hear “radar” and they think military or something like that).

The OTHRs and sounders do not intentionally send the letter O in CW before transmission. They can send three pulses, which a person used to hearing CW takes as O. Other transmissions, even from the same OTHR or sounder, may not include the three pulses, for example they may have only a single pulse or no leading pulse at all.

So, how to tell an OTHR from a sounder or atmospheric radar? There is no concrete way, but as a general rule OTHRs that hop in frequency (not all OTHRs move very often at all) hit a given frequency range often, because their intended targets move, requiring frequent updates. Sounders or atmospheric radars hit a given frequency range less often, because their intended target changes more slowly. For my loggings, unless I can positively ID what the signal is from some specific unique parameter or combination of parameters, I tend to call it a radar if I see it near a specific frequency more often than once every few minutes. If I see it once in 5 or more minutes I call it a “probable sounder or atmospheric radar” in my logs.

Both of these items (OTHRs and sounders) typically hop around in frequency. Sounders often take larger hops, radars typically hop around more often and in a narrower total frequency range.

An example of a frequency hopping radar from my Youtube channel, note you can see the pattern to the hops, also note each of these bursts started with a single pulse, could be taken for the letter T in CW:
Radar, Frequency Hopping, LFMCW, 11113 kHz, 11139 kHz, 12490 kHz, May 12, 2013, 1356 UTC - YouTube


This video is an example of 2 radars (the British PLUTO II radar and the Russian 29B6 radar) that do not hop in frequency (although they do shift freqs as needed to keep up with propagation). Also in the video is the sounder probably associated with the Russian radar.
Radar, new Russian Radar with Sounder, 19712 and 19477 kHz, September 08, 2013, 1631 UTC - YouTube

T!

(edit) this thread might need to be in the HF Utility forum instead of the SDR forum.
 
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RC286

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I have never heard OTH radar on 11m, though I have heard CW the odd time. Once I heard MFSK and even an SSTV signal.
 

chrismyers2000

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Thank you Token for the extra info. And yes, I am using HDSDR which you can download free here: HDSDR Homepage

I also use SDR# and GNURadio but I like the look of HDSDR better.
 

Token

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I have never heard OTH radar on 11m, though I have heard CW the odd time. Once I heard MFSK and even an SSTV signal.
I don’t think I have ever noticed OTHR inside 11 meters (26965 to 27405 kHz), but I see it all the time on either side of 11 meters. For example right now I am watching at least 3, if not more, different radars hopping around in the 27500 to 28000 kHz area.

T!
 

chrismyers2000

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Which RTL are you using? I didn't think any of them went much below 50MHz
Im using the DVB-T variety which has the R820T tuner. This one here: Amazon.com : RTL-SDR, FM+DAB, DVB-T USB Stick Set with RTL2832U & R820T. Great SDR for SDR#, HDSDR, and Other Popular SDR Software Packages! : Electronics

It will tune down to around 24Mhz before it drops out. I've successfully tuned 12m and 10m in the HF spectrum without an upconverter,which is in the mail. ordered it 8 days ago :(

I don’t think I have ever noticed OTHR inside 11 meters (26965 to 27405 kHz), but I see it all the time on either side of 11 meters. For example right now I am watching at least 3, if not more, different radars hopping around in the 27500 to 28000 kHz area.

T!
Yeah I was watching it again last night just before sunset EST. They were very faint. They probably are just outside the 11 meter band. They seemed to be in between voice transmissions so I assumed it was in band but so many people on CB these days operate out-of-band and even bleed into 10meters with their dirty amplifiers, really annoying... Also I've never calibrated my SDR because I have no good reference until I get my upconverter and can tune WWV or something similar, so i could be 500khz off for all I know.
 

chrismyers2000

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Noticed this in the 27.450 - 28.275 range just a little while ago (21:02 UTC 2/14). Not sure what it was.


Those appear to be similar, just much stronger than what I was receiving. I wasn't able to capture a good one. By the time I set up my record file the band had dropped considerably.
 

br0adband

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Funny that I just noticed this thread shortly after finding something in the "CB Band" down low. Heard it a few days ago and forgot about it, maybe someone else knows what it might be. It's consistent, it's not inside my apartment (verified by taking the laptop and antenna to the other side of my complex and still got the same signal, same frequency, about the same strength too). I also used a second RTL stick with a different SDR app (SDR# is my preferred one, then SDR-Radio v2 as the secondary) and looked at the VHF-Hi paging areas just in case I was getting harmonics of some kind and never noticed a correlation between the two transmissions based on timing; when a pager transmission happened there was either nothing noted at this frequency (27.145 MHz) or there was a transmission already in progress at the frequency when the pager frequencies were silent.

No idea what it is, and I tried various modes (everything possible from basic AM through all the possible variants), FM (all variants), SSB, DSSB, CW, etc. I captured a short clip (about 10 seconds) as a WAV but transcoded the sample to a FLAC for sharing and a smaller file size, can capture some baseband if anyone cares to attempt to figure out what it is or might be. I used CW mode because that provides the best "audible" version compared to the others.

Who knows... ;)

File sample: https://dl.dropboxusercontent.com/u/19019338/Unknown_Data_Bursts.flac (470.5 KB)

 

chrismyers2000

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Wireless mouse maybe?? Thats the first thing that comes to mind when listening to it. Something about the way it changes rate erratically. I know some wireless mice operate in the 27Mhz range.

Edit: I just checked a wireless mouse of mine, it operates at 27.195 Mhz and behaves similarly, no distinguishable audio except in cw mode.
 
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chrismyers2000

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Haha awesome. I've been having similar fun figuring out phantom signals within my apartment. In fact when I first got my SDR my mouse was causing a lot of issues. I ended up having to use a USB mouse :) I haven't used a corded mouse in......I can't even remember!
 

br0adband

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That's all I use myself, I can't tolerate wireless hardware (aside from Wi-Fi). I have a Bluetooth card in this laptop of mine but I have yet to find a Bluetooth mouse worth purchasing, ever, and that irks me to no end. Not one decent Bluetooth mouse on the market in a decade (at least for my needs and requirements), kinda sucks. Wish I could retrofit my Microsoft Intellimouse Optical with Bluetooth, then it truly would be perfect. :D

If it's electronic it's putting something out, for sure. Just got a new microwave today that's 1KW (compared to our older one at 700W) and although I use an RTL stick that can't receive that high (the 2450 MHz area, it even lists that on the label on the back, go figure), it sure does make a noticeable "hit" (when it's operating) to our Wi-Fi which is in the same general area.

But anyway, as I recover from my "fail" there's more fun to be had...
 

chrismyers2000

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Funny you should mention that (wifi and microwaves). I just did a short test to see how much RF noise my microwave creates in my apartment and the results are pretty cool (and bad at the same time).

The first image is of my normal wifi environment, the signal on the right is my router on channel 11 and the rest are my neighbors on channels 3 and 6.

The second image is my wifi environment with my microwave turned on for 1 minute, the spectrum analyzer is about 3 meters from the microwave in a normal spot you might find a computer, at the kitchen table.

As you can see the microwave gives off quite a wide bandwidth and the signal strength is enough to overpower my router on channel 11.

Its crazy to think theres a 1KW microwave transmitter sitting just inches from my face when I watch my food cook haha. It might explain a lot of whats wrong with me :D
 

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RC286

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Yep my old microwave would literally wipe out my wifi signal. Microwave goes on. Signal strength on the computer would drop to nothing and become intermittent. If I was in the kitchen, no signal at all.

As for wireless mice. My issue was the opposite. If I keyed up my CB, the mouse would loose connection and I would have to reset it. I never noticed interference on the radio. though the antenna is outside.
 
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