Author Topic: RTL_433 apparently confusing 868MHz vs 433MHz??  (Read 178 times)

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Offline src

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RTL_433 apparently confusing 868MHz vs 433MHz??
« on: May 07, 2023, 03:33:17 PM »
tl;dr rtl_433 scanning the 433MHz band is reporting devices that should be found in 868MHz band! Can any rtl_433 users explain this to me!


I have an rtl-sdr and am running SDRSharp with the rtl_433 plugin. I bought it for a non-weather project in 433MHz band (heating oil tank level monitor) but of course have been scanning to see what else I find!

In the 868MHz band I find various non-weather devices, and some weather devices which aren't mine, but I don't find my Ecowitt WH32 or WH40 (These are labelled as 868MHz as is my GW1100 gateway which receives data perfectly well from the WN32 and WH40).

In the 433MHz band there is much less activity generally, but I do sometimes (not very reliably) detect my supposedly 868MHz WH32 and WH40 (remember this is while scanning the 433MHz band) and rtl_433 reports them as active at specific frequencies in 433MHz band. See data below.

NB I know the WH32 data is from my WH32 and not from a neighbour because when I noticed this the first time I heated up my WH32 (I put it in my pocket) and as the temperature on the display increased so did the temperature reported by rtl_433. I didn't do that this time during the recording of the data below.

I might understand if the devices were very close to the rtl-sdr and overwhelming the tuner, and maybe that is the explanation. But while the WH32 was fairly close (perhaps about 5m) the WH40 was 30m away. And if that was the explanation would that also explain why the frequencies were recorded as being specific frequencies in 433MHz band instead of 868.somethingMHz? And why can't I detect them in 868MHz band - I've tried running rtl_433 for a long time (about 48 hours) at multiple different gain settings from very low to highest.

Or might it be, unlikely though it seems, that all three of my Ecowitt devices are 433MHz mislabelled as 868MHz? And how can I check that? (NB When I say "labelled" it is moulded into the case, not just a stuck-on label).

On a point of information, there isn't much radio noise around here - I'm in a rural area and my nearest neighbour is 450m away and have only a few neighbours within 800m, so not so many devices detected from other properties as would be expected in a city.

I'm puzzled!

Code: [Select]
N Mes. Time        Protocol    Model        ID          Battery      Temperature  Humidity    Integrity    Modulation  Freq1        Freq2        RSSI        SNR          Noise       
1 2023-04-29 16:46:12 78 Fineoffset-WH32 108 1 18.0 58 CRC FSK 433.6 MHz 433.3 MHz -1.1 12.0 -13.2
2 2023-04-29 16:53:40 78 Fineoffset-WH32 108 1 18.1 58 CRC FSK 433.6 MHz 433.3 MHz -1.1 12.0 -13.0
3 2023-04-29 16:54:44 78 Fineoffset-WH32 108 1 18.1 58 CRC FSK 433.6 MHz 433.3 MHz -1.1 11.9 -13.0
4 2023-04-29 16:56:52 78 Fineoffset-WH32 108 1 18.0 58 CRC FSK 433.6 MHz 433.3 MHz -1.1 11.4 -12.5
5 2023-04-29 16:57:56 78 Fineoffset-WH32 108 1 18.1 58 CRC FSK 433.6 MHz 433.3 MHz -1.1 11.7 -12.8
6 2023-04-29 17:02:12 78 Fineoffset-WH32 108 1 18.1 58 CRC FSK 433.6 MHz 433.3 MHz -0.9 12.2 -13.1
7 2023-04-29 17:07:32 78 Fineoffset-WH32 108 1 18.1 59 CRC FSK 433.6 MHz 433.3 MHz -1.1 12.3 -13.4
8 2023-04-29 17:12:52 78 Fineoffset-WH32 108 1 18.0 59 CRC FSK 433.8 MHz 433.3 MHz -1.3 11.8 -13.1
9 2023-04-29 17:20:20 78 Fineoffset-WH32 108 1 18.0 59 CRC FSK 433.7 MHz 433.3 MHz -1.1 12.0 -13.1
10 2023-04-29 17:23:32 78 Fineoffset-WH32 108 1 17.9 59 CRC FSK 433.5 MHz 433.3 MHz -0.7 12.7 -13.4
11 2023-04-29 17:28:52 78 Fineoffset-WH32 108 1 17.9 59 CRC FSK 433.8 MHz 433.3 MHz -1.3 11.8 -13.0
12 2023-04-29 17:29:56 78 Fineoffset-WH32 108 1 17.9 59 CRC FSK 433.6 MHz 433.3 MHz -1.3 11.6 -12.9


Code: [Select]
N Mes. Time        Protocol    Model        Id          Total Rain  Extra Data  Integrity    Modulation  Freq1        Freq2        RSSI        SNR          Noise       
1 2023-04-29 16:47:50 113 EcoWitt-WH40 400 11.5 4080000000 CRC FSK 433.6 MHz 433.3 MHz -0.1 12.4 -12.5
2 2023-04-29 16:49:28 113 EcoWitt-WH40 400 11.5 2040000000 CRC FSK 433.7 MHz 433.3 MHz -0.1 12.9 -13.0
3 2023-04-29 16:56:00 113 EcoWitt-WH40 400 11.5 4080000000 CRC FSK 433.7 MHz 433.3 MHz -0.1 13.6 -13.7
4 2023-04-29 16:57:38 113 EcoWitt-WH40 400 11.5 2040000000 CRC FSK 433.6 MHz 433.3 MHz -2.9 15.9 -18.7
5 2023-04-29 17:01:43 113 EcoWitt-WH40 400 11.5 2040000000 CRC FSK 433.9 MHz 433.3 MHz -0.1 13.1 -13.2
6 2023-04-29 17:03:21 113 EcoWitt-WH40 400 11.5 2040000000 CRC FSK 433.6 MHz 433.3 MHz -0.1 13.0 -13.2
7 2023-04-29 17:04:59 113 EcoWitt-WH40 400 11.5 4080000000 CRC FSK 433.7 MHz 433.3 MHz -0.1 13.2 -13.3
8 2023-04-29 17:06:37 113 EcoWitt-WH40 400 11.5 4080000000 CRC FSK 433.8 MHz 433.3 MHz -0.1 13.0 -13.1
9 2023-04-29 17:13:09 113 EcoWitt-WH40 400 11.5 4080000000 CRC FSK 433.9 MHz 433.3 MHz -0.1 13.1 -13.2
10 2023-04-29 17:15:36 113 EcoWitt-WH40 400 11.5 4080000000 CRC FSK 433.7 MHz 433.3 MHz -0.1 13.3 -13.5
11 2023-04-29 17:17:14 113 EcoWitt-WH40 400 11.5 2040000000 CRC FSK 433.7 MHz 433.3 MHz -0.1 13.2 -13.3
12 2023-04-29 17:21:19 113 EcoWitt-WH40 400 11.5 4080000000 CRC FSK 433.8 MHz 433.3 MHz -0.1 12.9 -13.0
13 2023-04-29 17:22:57 113 EcoWitt-WH40 400 11.5 2040000000 CRC FSK 433.6 MHz 433.3 MHz -0.1 13.0 -13.1
14 2023-04-29 17:24:35 113 EcoWitt-WH40 400 11.5 2040000000 CRC FSK 433.8 MHz 433.3 MHz -0.1 12.7 -12.8

Offline sky_watcher

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Re: RTL_433 apparently confusing 868MHz vs 433MHz??
« Reply #1 on: May 07, 2023, 08:07:19 PM »
rtl_433 scanning the 433MHz band is reporting devices that should be found in 868MHz band! Can any rtl_433 users explain this to me!
Software radios like the RTL-433 are simple and cheap. Therein lies the problem. They have relatively little input filtering and are susceptible to out of band signals. Digital radios operate by sampling the input signal, doing signal processing like a Fast Fourier Transform to resolve the frequencies in the signal being received.

In this case, because the input filtering does not cut off signals outside of the desired band, signals above the desired band are subject to what is called "Aliasing" which means these higher frequencies appear to be lower frequencies.
[Added] Digital receiver can either directly sample the input signal, or have an ananlogue or digital down-converter first stage. The basic premise will remain the same.

If you wish to stop this, you will need to attenuate those out of band signals with a filter before the antenna input connector.
« Last Edit: May 07, 2023, 09:29:08 PM by sky_watcher »
The more a man knows, the more willing he is to learn. The less a man knows, the more positive he is that he knows everything... ― Robert G. Ingersoll

Offline src

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Re: RTL_433 apparently confusing 868MHz vs 433MHz??
« Reply #2 on: May 08, 2023, 06:52:46 AM »
Thanks, that makes sense. What I'm still curious about is why I'm not picking up the devices when I scan the 868MHz band itself, over a wide range of gains.

Offline zoomx

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Re: RTL_433 apparently confusing 868MHz vs 433MHz??
« Reply #3 on: May 10, 2023, 11:04:52 AM »
Maybe it's not exactly 868 but close to 869?