[amsat-bb] RTL-SDR downlink

[Patrick STODDARD (WD9EWK/VA7EWK)]

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!
> ____________________________________________________________
>