I realized pretty quickly that this was one project my $20 rabbit ears couldn't handle, so I proceeded to build an L-band (1-2 GHz) antenna.
I first tried a helix, but it barely worked. I don't understand it, but I seem to have a knack for building bad antennas, even when I follow directions, so I was not surprised.
Then I tried a patch antenna. I'd heard that these work well on the L-band. I was expecting a small 1.5-inch square like those little GPS antennas, and I wondered how that could possibly work, but I was willing to try. On researching it, I found that it actually needs to be about 7 inches square with a ~4-inch square in the middle. This particular antenna was designed by Adam 9A4QAV. This RTL-SDR Blog post shows his antenna and its performance with the SDRplay. Notice how the Inmarsat signals look like blowtorches. What's most important is that you don't need an LNA or downconverter to make this happen with the SDRplay; it already comes with an LNA and full L-band support.
The instructions for this antenna can be found on another RTL-SDR Blog post. Finally, you should see killmore231's
I proceeded to build his antenna out of aluminum. I cut everything precisely, down to the millimeter. The only thing I couldn't do was solder to it, what with it being aluminum. I instead put some varnished wire through it, connected to the patch, and used a alligator clip to clip to the back plate. The first few times I hooked it up, it wouldn't show any satellite signals, making me think this was yet another dud.
Today, however, I realized that using varnished wire might leave conductive areas that could touch the back and short it out, so I ran a cut alligator clip to the patch, the plastic insulation providing guaranteed protection against contacting the back plate. I still needed an alligator clip to clip to the back plate. Then I hooked it up, leaned it near a window, and was able to get some strong Iridium signals.
Before, I had only gotten vague blue and green smudges, but these are sharp and have red areas. Below I have zoomed in on some of them:
There's no doubt this is Iridium. All this trouble makes me wonder how an Iridium phone's built-in antenna can possibly work. I watched a YouTube video of an Iridium phone in action and it got all the bars using just its little antenna.
I decided to mount my antenna on a pole sticking out of a second-floor window that's 25 feet above the ground. It gets great reception when it's outdoors like that.
I'm close to decoding Iridium but there are a few things I still don't understand:
1. Why do I only get the bursts? Why not the constant signals like in Adam's pictures?
2. Why doesn't Inmarsat show up? When I tune the Inmarsat band all I see is a strong signal that looks like GSM (see below)
3. What is keeping Python from running the Iridium Toolkit? I managed to install the dependencies it needed, like SciPy, but when I run extractor.py it seems to run but gives constant errors. I don't see any output files, either, so I assume it's not decoding anything.