[amsat-bb] SDR for fm sats

[Ted Krempa]

Povern, the go to guy on the SDR stuff is Patrick Stoddard, WD9EWK. Awile
back, Patrick posted the following info on this board. I'm sure he won't
mind me reposting:

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Since these were originally made as television receivers, they were designed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.

73 and GL

Ted
K7TRK
>
>