SO.....When I got home tonight I decided to see if my FCD ("FUNCube Dongle) would work any better that the $20 RTL dongle I have.
The FCD has a much narrower bandwidth, about 80kHz, than the RTL dongle. The software I use with the RTL will allow you change the sample rate, which cuts down on the amount of spectrum it decodes, but I didn't see any difference in doing that.
I only had a couple of passes of the NOAA birds, so the only software I tried was SDR#.
The first thing I noticed was that it was very easy to set the filter bandwidth to include *only* the signal from the satellite. Doing this cuts out any strong signals that might degrade the desired signal, and it's always worthwhile doing if you can.
The screenshot below shows the received spectrum of NOAA-18 earlier today. If you look at the bottom pane of the display you'll see the "tracks" of the signal, and looking at the upper pane, you see the grey area corresponding to the receiver filter bandwidth, in this case about 50kHz:
All the extra area covered by the grey that doesn't have the "tracks" in it just allows unwanted noise to be recorded along with the signal.
Here's a shot of NOAA-15 a few minutes later after I adjusted the filter bandwidth down to 40kHz:
Notice how the grey area has narrowed, and includes just a bit of spectrum outside the "tracks". This is pretty much what you want, and is why my fellow Ham Operators love narrow filters when receiving weak signals; match the filter bandwidth to the incoming signal, and the extraneous crud gets cut off.
You can also clearly see the Doppler Shift causing the "tracks" to shift left-to-right as the satellite passes over. There's a single, bright, blue line running under 137.630 MHz, which is internally generated in the little receiver, and it doesn't change frequency, so it's a good fixed reference to compare the actual signal to, and observe the Doppler shift.
Now compare this to the next screenshot which I made over the weekend. This one was using the RTL dongle, with approximately 800 kHz of bandwidth in the display:
Quite a bit easier to determine what filter bandwidth to use when the display isn't crammed together so much! If I hadn't adjusted the bandwidth of the RTL unit for that particular pass, the display would have been about three times as crowded!
ANYWAY....today's experiment was to see if the FCD makes it easier to tune in the NOAA satellites (it does, although SDR# will let you adjust the bandwidth of the RTL unit), and to make an attempt to determine what a good filter bandwidth would be.
40 kHz looks about right, and that falls in line with all the things I've read about what filter bandwidth works best for receiving the NOAA APT satellites.
As far as using WXtoImg to decode the received signals.......well, still no joy. I'm going to take all the files I've recorded so far and run them through "xwxapt" on my Linux machine and see if I have any better luck. From listening to recorded APT signals at various places around The Web, mine sound at least as clean, and I'm pretty sure they should decode. I've done this before, with a Hamtronics R-139 receiver, a receiver specifically designed to receive APT transmissions, and it worked quite well with a homebrew turnstile antenna.
I just downloaded the older program I used to use "WXsat", and I'm going to give it a try, too.
And it looks like I might be cutting some PVC pipe this weekend to make another turnstile antenna........