[amsat-bb] RTL-SDR downlink

Thu Jun 16 22:36:34 UTC 2016

Hi Peter!

You are correct. The inexpensive RTL-SDR dongles are a great way to
give SDR a try. With the low prices, there should also be appropriate
expectations. Since these were originally made as television
receivers, they were designed for use with stronger signals than
what we typically see from our satellites. Can they work as a
downlink receiver? Sure. Just like a Baofeng HT can be used to
work FM satellites. Not the best thing, but they can work.

I've tried a few of these devices as downlink receivers for working
satellites. In my case, these receivers need to be able to handle
nearby RF on a different band, which is a problem for some of these
devices. My list of devices I have tried are:

1. RTL-SDR dongles. No front-end filtering, not as sensitive as
other devices, they shut down in the presence of 5W or less when I
am transmitting to a satellite. Even as little as 500mW can be a
problem for these devices. I might have spent more time using these
if I only wanted to receive. Also, I would like to have something that
didn't require an upconverter to cover the HF spectrum above 24
MHz. I keep one plugged into a PC at my office, and use that to
listen to FM stations and occasionally the local fire or state
trooper dispatch channels (still in analog around Phoenix).

2. FUNcube Dongle Pro (not the Pro+). A better receiver than the
RTL-SDRs, but also lacking front-end filtering. No good for me as
part of my satellite station, but could have been fine for receive-only
setups.

3. FUNcube Dongle Pro+. Nice receiver, has front-end filtering
including SAW filters at 2m and 70cm, directly compatible with the
FUNcube Dashboard and FoxTelem programs. Only drawback for me is the
192 kHz maximum bandwidth. The 240-420 MHz frequency gap is not a
big deal, but is for some who like hearing the military aircraft or
satellites in this range.

4. HackRF. This is a wideband transceiver, covering from around 100
kHz to 6 GHz, with a maximum bandwidth of 20 MHz. It is nice to use
one of these to watch the entire FM broadcast band, but its receiver
really isn't up to the task as a downlink receiver for our current
satellites at 10m (AO-7 mode A), 2m, or 70cm. I have the Great Scott
Gadgets HackRF One and the crowdfunded HackRF Blue version. Other
than the crowdfunded version using some different components and
costing about $100 less, the HackRF Blue functions the same as
the HackRF One. The HackRF devices lack front-end filtering,
and come with warnings about using them in the presence of strong RF,
so I have not tried using one of these along with an FT-817 or
HT to work satellites, but its receive-only performance is not
impressive.

5. SDRplay. This is what I have been using for satellite work for
the past year. With 100 kHz-2 GHz unblocked, maximum 8 MHz bandwidth,
and bandpass filtering across that range, it has worked very well for
me - even in Los Angeles. I use this with an 8-inch Windows 10
tablet and HDSDR software, which is a low-overhead program that does
fine on these low-end tablets. FUNcube Dashboard and FoxTelem
cannot directly use an SDRplay, but with something like HDSDR and
a virtual audio cable I can make use of the SDRplay with those
programs. Or I can make RF recordings of the passes in HDSDR, then
play back the recording later through a virtual audio cable into
those programs to decode telemetry.

I have not tried either the Airspy or Airspy Mini. These units do
not cover below 24 MHz, and require an upconverter if you want to use
them for HF reception, but they will cover the bands currently used
for our satellites, starting at 10m and going up. There is a US-based
distributor for Airspy, so you don't have to order these from
overseas.

Of course, your location will determine how much interference you
will have to withstand. I didn't have that to deal with at a recent
demonstration I gave in Long Beach earlier this year, when I used my
SDRplay and tablet as the downlink receiver to work a couple of the
XW-2 satellites. I don't have those issues at my house here in the
Phoenix area, and have yet to run into a place where there is a lot
of interference to deal with. I have used my SDRplay/HDSDR combination
from many locations across the US and Canada in the past year, and
have yet to run into a situation where the local RF swamps my SDRplay.

SDRplays are sold at HRO in the US for $149. FUNcube Dongle Pro+
is sold by its UK manufacturer, and including FedEx shipping to
the US runs around $200 depending on exchange rates. The HackRF One is
available for around $300. I have 3 SDRplays, two that go with me for
demonstrations or presentations, and one as a backup. I also have a
couple of FUNcube Dongle Pro+ receivers, which will eventually
be used for an unattended receive-only setup at home with the FUNcube
Dashboard and FoxTelem programs on a PC.

I would recommend either the FUNcube Dongle Pro+ or SDRplay if you are
wanting to work satellites using one of these receivers for the
downlink side of your station. If you want to see some examples of
what I have received using my SDRplays, I have lots of RF recordings
at http://dropbox.wd9ewk.net/ - look for the folders with YYYYMMDD
dates at the start of the file names, followed by the satellite name
and the grid I operated from. You are welcome to download them and
run them through HDSDR or some other SDR software. And, yes, there
is a lot of data up in that Dropbox space. :-)

73!

Patrick WD9EWK/VA7EWK
http://www.wd9ewk.net/
Twitter: @WD9EWK

On Thu, Jun 16, 2016 at 8:33 PM, Peter Laws <plaws0 at gmail.com> wrote:

>
> The $20 versions are well worth the effort if you've never played with
> an SDR of any sort before.  For satellite downlinks?  Dunno, never
> tried.  Surely as you describe!
>
> Has anyone done any kind of "shoot out" comparing the cheapos to the
> real ones or even between the real ones (FCD, SDRPlay)?  Before I
> plunk down $200, I'd like to see what I'm getting ...  over and above
> what my $20 dongle can do, of course.  :-)  I read what you typed, but
> I'd like to see numbers.
>
>
>