[amsat-bb] RTL-SDR downlink

Robert McGwier rwmcgwier at gmail.com
Fri Jun 17 18:00:42 UTC 2016

Outside of my back window is Poor Mountain.  I can see two television
towers,  two FM radio stations, three paging transmitters, .....

The RTL-SDR is absolutely useless, period, even on a paper clip antenna
without filtering.


On Thu, Jun 16, 2016 at 4:22 PM, Patrick STODDARD (WD9EWK/VA7EWK) <
amsat-bb at wd9ewk.net> wrote:

> 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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Bob McGwier
Founder, Federated Wireless, Inc
Founder and Technical Advisor, HawkEye 360, Inc
Research Professor Virginia Tech
Chief Scientist:  The Ted and Karyn Hume Center for National Security and
Senior Member IEEE, Facebook: N4HYBob, ARS: N4HY
Faculty Advisor Virginia Tech Amateur Radio Assn. (K4KDJ)
Director of AMSAT

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