Signal Strength

Why does Signal strength vary over a range of 10 dB on a 24 hour cycle?

I have an Outernet patch antenna and preamp mounted outdoors in a fixed position accurately pointed at Inmarsat-4 98W from Columbus, Ohio. The receivers (RTL-SDR and an Ettus USRP) are indoors. I’m seeing signal strength vary from -115 dB to -125 dB (as indicated by RSSI) on a 24 hour cycle. At night the signal is strongest and low in mid morning. The SNR is following the variation in signal strength over a range of 13 dB to 3 dB. Atmospheric water vapor attenuation should be negligible at 1.5 GHz. And, with a 30 plus degree beam width antenna satellite orbital movement should not matter. The timing of the variation makes me think it is caused by sky noise. Can there be that much noise at this frequency?

Does anyone have a good explanation for this signal variation?

oh, wanted to link you the SNR logger thread but looks like you found it! :slight_smile:
that’s the best what you can do understanding what happen (possibly interference near by) variation of 10 sounds like too much.

Thanks for the reply. I am using a RbPi3 with rxos latest version. I don’t think I can use the “SNR logger” script. I tried it previously but no joy. After I implemented the Outernet receiver with the RbPi3 I learned that outernet has phased it out and gone with the CHIP. I’m thinking about buying a CHIP but an not sure which versions outenet is going to support. Meanwhile I have hooked up my Ettus USRP (SDR) with GNU Radio to listen and am getting the same 10 dB daily variation. I’m pretty well equipped with RF test equipment. When I get time I will connect another, higher gain antenna and see if I get the same.
– Rich @ wc8j

Yeah, RPi 3 is fading out due to lack of resources now so CHIP is a good idea to acquire.

@donde posted a good link that describes things that might affect L band reception. The effect you are seeing may be “scintillation” which occurs during certain times of day. I see it here, on the order of a drop of about 2dB SNR and a variation of 2 to 4dB RSSI, always starts late afternoon/early evening, and lasts til late evening.

I have looked further into the fading issue. The RSSI measurement from the RTL-SDR is just a relative indication of signal strength. I think it’s pretty arbitrary. For example I double terminated the feed with a 50 ohm DC isolated termination and didn’t see any change in the RSSI or S/N numbers. So, the 10 dB variation I mentioned in my original post isn’t really 10 dB. It is some other number. I going to set up an HP spectrum analyzer and make some calibrated measurements when I get some time.

Rich @ wc8j

I’d be very curious to see how much energy is actually hitting the antenna.

My equipment setup is as follows:
1 Outernet patch antenna. 8 dB Gain (est)
2 Preamp 34 dB gain (est)
3 25ft RG-58/U Cable 24 dB loss (from chart)
4 Signal strength at spectrum analyzer -113.87 dBm

Equals received signal strength at antenna -131.87 dBm.

Do you know the specs on the Inmarsat transmit signal at the satellite. (EIRP, Antenna pattern, polarization, etc?)

– Rich @ wc8j

The Outernet signal power is varying at the satellite. If you compare the spectrum I posted earlier with this one you will see the following.
1 The noise floor is nearly the same
2 The modulated carrier on the right side is approximately the same power level (about 2dB lower)
3 The Outernet modulated carrier in the center is several dB stronger. To new tissue indicates that the satellite is transmitting more power on the Outernet carrier.

I don’t know much about the uplink or the satellite. So, I’ll keep my theories to myself for now.

Anyone have any thoughts?

Rich @ wc8j

-109 dBm on the ground. @zoltan @Abhishek

I assume you have no LNA on the line? What type of antenna?

His equipment specs are below the first spectrum analyzer picture…

Syed, It would be very helpful for link-budget calculations if you can provide transmitter antenna details please.
EIRP, antenna gain, pattern and polarization, please.

@pe1chl Thanks for pointing that out. The link I receive in my emails to thread updates jumps to the last post, so I previously missed that first picture.

@Sheppo Unfortunately, does not like me making any statements about their satellite, although there is a lot of information that is publicly available. Polarization is RHCP.

Certainly they can object to you telling us the effective radiated power of your own RF carrier. That’s basic information for doing link calculations. It would be nice to have the antenna pattern too.

Rich @ wc8j

We’re a unique customer for Inmarsat, so I always need to check with them regarding what I can and can not state. To answer your previous question, we have a carrier of 18 dBW EIRP.

I have found that the Raspberry Pi3 work as a receiver module much better then the CHIP (or as my location in Alaska). I could not receive anything above 1.25db with the CHIP. pushing 3.45db with the Raspberry Pi3 and MUCH better wi-fi.


I wann ask you about LNA you used in spectrum analyzer, is that LNA from Outernet package or buy from the other source? Because i used the LNA from Outernet Package in spectrum analyzer and the led can’t blink and the LNA doesn’t worked
What i have to do Sir?

Thank You

Best signal I get

James, your signal strengths - - SNR of 10.63 dB and RSSI of -106.56 dBm - - are typical of a working system. I have gotten values like that under Americas 98W and Alphasat 25E in my travels with Pis, CHIPS, and Dreamcatchers (using the new active antenna).

Sometimes weather and local interference play havoc on signal reception, but you are seeing nominal levels. When IMARSAT satellite problems occur, this Forum fills up with lots of complaints which Syed addresses and fixes fast,

Oh yes, don’t forget @pradeeka Pradeeka’s experience several years ago when his bannana trees grew too fast, and blocked his Ku Band reception. :blush: Ken

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Kinda figured it was in normal range. The way I have my patch mounted is ugly but it works. I cut a piece of 2x4 to the correct angle double face taped the patch to it and it is double face taped to the window ledge with a 1 gallon zippy bag pulled over it…

Anyone who knows enough about anything coming from satellite knows it quits working when the weather sucks.