[amsat-bb] RTL-SDR downlink

Fri Jun 17 07:02:47 UTC 2016

Hi all,

I am using RTL-SDR's for downlink reception for quite some time now. I've
got one from the rtl-sdr blog in aluminum casing with TCXO. I can hear the
downlinks of pretty much all the LEO's (except SO-50) fine with it using
indoor antennas. In my experience it is easier copying the downlinks of
linear birds, as the required SNR for a copyable signal in SSB is lower.

I can also successfully decode ISS APRS packets using a vertical, RTL-SDR,
GQRX and direwolf, so that makes for a very cheap ISS APRS receive setup.
Note: the gnuradio FM demodulator in GQRX works way better than rtl_fm,
using the latter I cannot decode any packets. GQRX is pretty CPU-hungry
though, so a Raspberry Pi solution would be tricky. (/offtopic)

As others said, the RTL-SDR has no front-end filtering and therefore
terrible desense, furthermore it has a pretty low dynamic range, so full
duplex operation is not possible. Nonetheless I've made a couple of QSO's
via AO-85 using the RTL-SDR on the receiving end, just ignoring the audio
when TXing.

I have an AirSpy Mini on order now, I'm curious how it will perform with
nearby TXing. AFAIK it doesn't have any front-end filtering like the
SDRPlay or FunCube dongles have, but the dynamic range is way bigger than
the RTL-SDR's, so hopefully it will still perform a lot better. Otherwise
I'll have to build some filters.

Regards,
Rico PA3RVG

On Fri, Jun 17, 2016 at 1:08 AM, Eduardo PY2RN <py2rn at arrl.net> wrote:

> The cheap RTL dongles are as much sensible (or more) than the others more
> expensive.Lack of filtering is the big issue although there are some
> practical and easy workarounds to improve it.For ham satellites downlink
> frequency stability should not be an big issue, there are some models
> already been sold  with 0.5PPM TCXO option which are still cheap.It is a
> great opportunity to operate full-duplex on amateur satellites with very
> low investment and improving operational capabilities.
> EME (Moon bounce) audible signal RX comparison between TS-2000 / RTL /
> FunCube Pro+ can be seen here:
> https://youtu.be/3OxyO5ylwfs
>
> 73
> ED   PY2RN
>
>
>       From: Patrick STODDARD (WD9EWK/VA7EWK) <amsat-bb at wd9ewk.net>
>  To: "amsat-bb at amsat.org" <amsat-bb at amsat.org>
>  Sent: Thursday, June 16, 2016 5:22 PM
>  Subject: Re: [amsat-bb] RTL-SDR downlink
>
> Hi, (name or call?)!
>
> The inexpensive RTL-SDR dongles are a great way to get started with
> SDR receivers. For amateur satellite work, other than the ISS and maybe
> AO-85, they have a couple of significant drawbacks...
>
> 1. These dongles were designed to be TV receivers, working with
> signals much stronger than we have from our satellites or even the
> ISS ham station.
>
> 2. These dongles lack front-end filtering. This means that there could
> be a strong signal near you that swamps the receiver that wipes out
> what you're trying to hear. If you are trying to work satellites
> full-duplex, it is possible that your transmitter will shut down
> the dongle until you end your transmission. This was a problem I
> experienced early on when I tried using one of these dongles as
> my downlink receiver, and quickly moved on to something else.
>
> Unfortunately there isn't anything in the middle ground between these
> dongles and devices like the SDRplay (sold by HRO in the US for $149)
> or the FUNcube Dongle Pro+ (sold by its UK manufacturer for around
> $200 depending on exchange rates, which includes FedEx next-day
> shipping from England to most addresses in the continental USA). Both
> of these devices do well as the downlink receiver for working
> satellites. Both come with front-end filtering that the RTL-SDR
> dongles lack, and still have sensitive receivers. The SDRplay
> has a low-noise amplifier that is engaged when receiving at VHF or
> higher, but you can reduce the amount of gain from the built-in LNA.
> For my work, I keep that gain reduction value set to 0, so I have
> maximum gain to hear the downlinks.
>
> Good luck, and 73!
>
>
>
>
>
> Patrick WD9EWK/VA7EWK
> http://www.wd9ewk.net/
> Twitter: @WD9EWK
>
>
>
>
>
> On Thu, Jun 16, 2016 at 2:30 PM, bruisedreed at juno.com <
> bruisedreed at juno.com>
> wrote:
>
> > Hello all!  Been lurking a while here and just wanted to say hi and
> thanks
> > to everyone.  Just started working the birds. VERY limited success so far
> > on the linear transponders. Limited budget leads me to either have to run
> > in half duplex (obviously not preferred) or come up with another low cost
> > receiving option. I bought an RTL-SDR to see if it could be any help. I
> > have to say I really like this little receiver. It’s a little buggy
> > figuring things out but it receives really pretty well…UNTIL I try
> > to receive CW and SSB on the transponders. I have no problem receiving FM
> > repeaters and simplex and have monitored a few SO50 passes with it no
> > problem, but for some reason I’m not hearing the same signals I can
> > hear on the receiver of my FT100 with the EXACT same antenna. I A/B them
> > and have nothing on the SDR. Is anyone using one of these? I am probably
> > missing something simple. When I started receiving HF I couldn’t
> make
> > it work until I figured out I had
> >  to change the sampeling in the setup to direct from
> > quadrature…only learned that through a forum and I assume
> something
> > like that will make the thing come to life. Lack of documentation on some
> > of these things is kind of a pain. Thanks in advance for any help you can
> > offer and my apologies to anyone I have frustrated working half duplex! I
> > will figure out what I’m doing!
> > ____________________________________________________________
> >
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> _______________________________________________
> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions
> expressed
> are solely those of the author, and do not reflect the official views of
> AMSAT-NA.
> Not an AMSAT-NA member? Join now to support the amateur satellite program!
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