Testing a Prototype of the Outernet L-Band Downconverter

Outernet are a startup company that hope to revolutionize the way people in regions with no, poor or censored internet connectivity receive information. Their service is downlink only, and runs on C and L-band satellite signals, beaming up to date news as well as other information like books, educational videos and files daily. To receive it you will need one of their official or homemade versions of the Lighthouse or Lantern receivers (the latter of which is still to be released), or an RTL-SDR or similar SDR. Recently they began test broadcasts of their new 5 kHz 1539.8725 MHz L-band signal on Inmarsat I4F3 located at 98W (covers the Americas), and they hope to begin broadcasts in more regions soon too.

The typical RTL-SDR is known to often have poor or failing performance above 1.5 GHz (though this can be fixed to some extent), so Outernet have been working on an L-band downconverter. A downconverter works by receiving signals, and shifting them down to a lower frequency. This is advantageous because the RTL-SDR is more sensitive and does not fail at lower frequencies, and if used close to the antenna, the lower frequency allows longer runs of cheap coax cable to be used without significant signal loss.

Earlier this week we received in the mail a prototype of their downconverter. The downconverter uses a 1.750 GHz LO signal, so any signal input into it will be subtracted from this frequency. For example the STD-C frequency of 1.541450 GHz will be reduced to 1750 MHz – 1541.450 MHz = 208.55 MHz. This also means that the spectrum will appear reversed, but this can be corrected by selecting “Swap I & Q” in SDR#. The downconverter also amplifies the signal with an LNA, and has a filter to remove interfering out of band signals.

The Outernet downconverter circuit board.
The prototype Outernet downconverter circuit board.
Specsheet for the downconverter.
Specsheet for the downconverter.

We tested the downconverter using their patch antenna which they had sent to us at an earlier date (the patch antenna is used and shown in this Inmarsat STD-C reception tutorial). Our testing found that overall the downconverter works extremely well, giving us much better signal levels. Previously, we had used the patch + LNA4ALL and were able to get reception good enough to decode STD-C and AERO signals, but with the requirement that the patch be carefully pointed at the satellite for maximum signal. With the downconverter the signals come in much stronger, and accurate pointing of the patch is no longer required to get a signal strong enough to decode STD-C or AERO.

The downconverter can be powered by a bias tee connection, and this works well with our bias tee enabled RTL-SDR dongles. We also tested with the bias tee on the Airspy R2 and Mini and had no problems. It can also be powered with a direct 5V connection to a header, and they note that the header will be replaced by a USB connector in the production version.

The release date and exact price that these will be sold at is not confirmed, but we believe that it will be priced similarly to upconverters at around $50 USD or less. A good low cost downconverter should help RTL-SDR and other SDR users receive not only the Outernet signal better, but also other satellite signals such as STD-C and AERO. Although the input is filtered and the RF frequency is specified at 1525 to 1559 MHz, we had no trouble receiving signals up to GPS frequencies of 1575 MHz, and even up to Iridium signals at 1.626 GHz, though reception was much weaker up that high.

Below are some screenshots of reception. Here we used the Outernet patch antenna sitting in a windowsill with the downconverter directly after the antenna, and then 10 meters of RG6 coax cable to the PC and bias tee enabled RTL-SDR. We found that with the downconverted ~200 MHz signal the loss in the RG6 coax was negligible. Better reception could be obtained by putting the patch outdoors. In some screenshots we used Vasilli’s R820T driver with the decimation feature, which allows you to zoom into narrowband signals much more clearly.

Some AERO Signals Zoomed in with the Decimation feature in SDR#.
Some AERO Signals Zoomed in with the Decimation feature in SDR#. Received with the Outernet downconverter and patch antenna.
Some AERO and other Signals Zoomed in with the Decimation feature in SDR#.
Some AERO and other Signals Zoomed in with the Decimation feature in SDR#. Received with the Outernet downconverter and patch antenna.
Signals zoomed out.
Signals zoomed out. Received with the Outernet downconverter and patch antenna.

11 comments

  1. Jeff C

    Just stumbled on to this post while looking for something else. I’m not really interested in Outernet bus was thinking the downconverter could be used to extend the range of the Airspy above 1800 MHz. I’m doing some work up around 2400 MHz and was looking for a DC option, this looks like it would work. The LO frequency is good and presumably a stable PLL, but it probably needs some retuning to the RF and IF filters.

    Does anyone know if the DC is available? It’s not on Outernet’s website. Thanks.

    • Adam

      I just did the test you ask, and looking the photo’s here I see no difference with using just a RTL.SDR dongle and bare LNA4ALL without any filtering involved. I get the 18dB s/n ratio for the same signals and I can see at the same time the AERO signals from 2 birds, IOR and AOR-E. Using indoor patch antenna and the tune is not sharp at all.

      • admin

        It’s possible that when I tested with the patch and LNA4ALL something was suboptimal. Maybe something to do with strong FM/DVB-T blockers interfering. I actually have another prototype here from Outernet that I will write about soon which is a Ceramic Patch -> LNA -> Filter -> LNA, which gives me results about as good as the downconverter + metal air gap patch.

        Though you still need to note that without heatsinking, there are still some RTL-SDR’s that will fail to work properly at 1.5 GHz, so the downconverter guarantees proper operation.

        • Adam

          Yes, agree. The downconverter is better option in this situation where they should guarantee proper operation for the commercial service they provide. As you can see, on the prototype converter you have, there is not much room left regarding the frontend chain and analog signal gain. They use the amplifier at the input, active mixer and another amplifier on the IF side. So they are tight there. I see some bandpass and lowpass filtering using lumped elements and that’s it. it will be interesting to see how the system will work using the ceramic patch antenna designed for the GPS primary. If they want to stick to their initial idea of lantern shape there is not much place for the antenna except the ceramic patch.

          Ceramic patch antenna can work on GPS because they use spread spectrum technology and a lot of processing gain. Outernet can not benefit as they are using other technology and modulation. Will be interesting to see how they will end this project regarding the hardware.

          I will test today a ceramic patch with LNA and also the tandem with 2x LNA with filter in between.
          Have fun..

  2. COMMYWATCH

    According to Khan Academy’s filings with the U.S. Internal Revenue Service, Salman Khan has received over $350,000 in annual compensation from Khan Academy since 2011. In 2015 it was raised to $556,000. In 2013, President and COO Shantanu Sinha also received over $350,000 in compensation.[49] Khan’s perks as Khan Academy’s leader include first-class travel on business flights. In total, 29 employees of Khan Academy make more than $100,000 per year.

    not for profit…yeah right…..

      • terry

        I believe it’s about the guy who runs Outernet and it’s dishonest “not for profit” claims.

        • Syed

          Outernet has no official relationship with Khan Academy. We broadcast their content because most people we’ve talked to have found it useful–and it is available under a Creative Commons license.

          Outernet is not a non-profit. We are definitely profit-oriented. But I don’t make anywhere near what Salman Khan makes.

  3. COMMYWATCH

    Yes but this, “education” is only as good as those providing it and their political agenda

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