How to count and identify carrier frequencies?

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lsmod
Posts: 7
Joined: Sun Mar 05, 2017 3:32 pm

How to count and identify carrier frequencies?

Post by lsmod » Sun Mar 05, 2017 4:01 pm

Hi,

how it is possible to make an automatic analysis of a frequency band?
I am working in Linux and i want to identify or at least count carriers as shown in the pictures.

Perfect would be a solution that can step through a frequency range and identify carriers with it frequency and level.

One problem is that this carriers seems to be pulsed, but not modulated.
It's something like an array of frequencies switched on and off.
The goal is to detect if this array is present.

Any suggestions how to do this?
Attachments
1.5MHz.jpg
intermittent carriers
1.5MHz.jpg (534 KiB) Viewed 7667 times
62MHz.jpg
special band carriers with low signal
62MHz.jpg (380.3 KiB) Viewed 7667 times

rtlsdrblog
Site Admin
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Re: How to count and identify carrier frequencies?

Post by rtlsdrblog » Sun Mar 05, 2017 10:17 pm

Not sure about automatic, but if you used rtl_power, you'd have all the data there. Then just run a threshold algorithm to detect any peaks over a certain level.

But what you're seeing there is most likely just noise from some nearby electronics. Maybe your LCD screen, a nearby switching power supply. It could also be overload of powerful broadcast FM.

lsmod
Posts: 7
Joined: Sun Mar 05, 2017 3:32 pm

Re: How to count and identify carrier frequencies?

Post by lsmod » Mon Mar 13, 2017 9:09 am

Sorry for my delayed answer.

I already read a little bit about rtl_power, but there are no good examples for using it.
Is there somewhere a better documentation?

I could found this here that is not bad, but it is not really detailed.
http://kmkeen.com/rtl-power/


What do you mean with "run a threshold algorithm" ?

rtlsdrblog
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Re: How to count and identify carrier frequencies?

Post by rtlsdrblog » Mon Mar 13, 2017 12:21 pm

That's probably the most detailed guide to rtl_power, there's plenty of examples of its use on that page.

I also just remembered there's also the SDR# plugin http://www.freqmgrsuite.com/. I think this has a way to monitor and log active signals, but i'm not sure if it would work properly with those random noise signals you are seeing.

Threshold algorithm - by this I mean to write code to record the frequency of signals that go above a certain power level threshold.

lsmod
Posts: 7
Joined: Sun Mar 05, 2017 3:32 pm

Re: How to count and identify carrier frequencies?

Post by lsmod » Mon Mar 13, 2017 8:20 pm

Thank for your answer.

I think i just have to play around with the parameters of rtl_power and make some experiments.

Then i have to find some tools to visualize the results.
The heatmap script seems not to be as flexible as needed.

My hope was that someone already has a solution for such a analyze problem.

rtlsdrblog
Site Admin
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Re: How to count and identify carrier frequencies?

Post by rtlsdrblog » Mon Mar 13, 2017 10:03 pm

What is your end goal?

Are you trying to locate the source of the interference? Or just trying to figure out at what times of the day it shows up?

lsmod
Posts: 7
Joined: Sun Mar 05, 2017 3:32 pm

Re: How to count and identify carrier frequencies?

Post by lsmod » Tue Mar 14, 2017 8:21 am

The source of the interference would be interesting.
Any idea how to locate them with simple tools?

The main goal is to record the count and intensity of this phenomenon over the time.
I would scan every 10 minutes different frequency ranges and report the result into an database.

Then i must think about a good tool to analyze an visualize the result.

rtlsdrblog
Site Admin
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Re: How to count and identify carrier frequencies?

Post by rtlsdrblog » Tue Mar 14, 2017 8:34 am

I think building a heatmap using rtl_power would be the best idea for that application. Then you can see graphically when the interference was visible over time quite easily.

If you want to actually find the interference you could use a directional antenna like a yagi to try and find the general direction that it's coming in. Or just connect the dongle to the laptop and walk around the house, seeing where it gets stronger. Of course that won't work if its external to the house.

For a more sophisticated solution look at Tim Havens "driveby" work http://blog.dxers.info/search?q=driveby. But it is quite a step up in terms of complexity.

snn47
Posts: 215
Joined: Tue Dec 27, 2016 11:00 pm

Locating RFI

Post by snn47 » Tue Mar 14, 2017 4:01 pm

Since you did not specify the frequency range you want to look into here some general comments.

Without receiver selectivity it's difficult to say what is interference and what is generated by a strong RF environment at where you live.

If you add a 20 to 40 dB attenuator between the antenna and the receiver, this should reduce all signals by 20 to 40 dB. If many, previously strong signals are gone your receiver is overloaded.

While having to tune a antenna is inconvenient to many, using a single loop magnetic antenna eliminates receiver overloading at the cost of having to re tune the magnetic antenna. The Q of a antenna can be well above 100 which results similar to a good preselector in a narrow bandwidth,

Diameter for SW frequencies between ~14 to 28 MHz a 40 cm diameter work if your variable capacitor has sufficient tuning range. Below 14 MHz the size for single loop magnetic antennas will become to large to carry, so you have to change to multiple loop designs. For 2 MHz and lower coils on ferrite roads, e.g. from an AM radio are easy to use. Sizes above 30 MHz become increasingly smaller, and require above 600 MHz intricate work, but are still manageable, and the bandwidth increases.

Unless you know if the RFI is horizontally or vertically polarized you have to try it vertically and horizontally. If the RFI source is within a AC-device you should have a wide spectrum RFI like many switching PS or dimable LED or Power Line device it will use the AC wires as antenna, which can make locating the source more diffuicult.

lsmod
Posts: 7
Joined: Sun Mar 05, 2017 3:32 pm

Re: How to count and identify carrier frequencies?

Post by lsmod » Wed Mar 15, 2017 1:52 pm

Thanks - that is very interesting.

I should make the test with the 20 to 40 dB attenuator.
Use a simple resistor devider?

The frequencies are in different ranges, as you can see in the screenshots.
For automatic measurements it is difficult to make a calibration of the antenna.

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