Concept radio for amateur satellite communication

This Listed platform might also be useful to serve as an idea and project log. In a way, I can pretty much just set some of my personal notes to publish on Listed, with only some minor additional context provided for the reader. So here goes...

You may or may not know that there are satellites orbiting the earth carrying amateur radio equipment, allowing people to make contacts across much larger distances than they normally would be able to. Working these satellites is fun because it is a little challenging, but it can be done with inexpensive and even home-made equipment.

Here's a picture of one being assembled:
The D-Star One amateur radio satellite being assembled by a worker on a lab bench.

Look how small these guys are. And this is, comparatively, a big one.

There are two main types of amateur radio satellites, simple FM repeaters and linear transponders. The FM repeaters are very easy to use and the regular Joe with his inexpensive handheld can make acceptable contacts without much special equipment. The linear transponders require a lot more fiddling and more specialized equipment. This is mostly because the equipment is not available in a handheld form factor.

So I thought, why not? If I can build a radio to make working the linear transponder satellites almost as easy as the FM satellites, well there might be a market there. And it at least seems like it would be an interesting project.

So, I have the education to design an analog circuit to do the job, but it would certainly be a lot of work (likely much of it trial-and-error). But in this day and age where there is a breakout board available for almost everything, it seems like I should be able to string together a pile of circuit boards to do this task. That doesn't seem to be the case for analog. But SDR is the cool thing now, and that's where we might be able to make this work.

Fast forward to a couple days ago after snooping around the internet off and on for a few weeks and I think I've settled on a concept. I've set aside some better but more complicated ideas for the moment and have what I believe to be, essentially, the makings of a minimum viable prototype. (Not yet a minimum viable product.)

As my English teacher used to say, let me amplify.

To receive the signals, the RTL-SDR dongle should be fine, especially with some filtering and preamplification. We can feed that into a Raspberry Pi running some SDR software, with the audio going out to the user via the built-in pins, some sort of "hat," or even a USB sound card. For transmitting, there is a software package called Rpitx that toggles a pin on the Raspberry Pi fast enough to make it a radio transmitter. Run that through some filters and amplifiers and you've got yourself a full-duplex radio ready for satellite operations. I'd want to add a few more bits and pieces like a screen and some buttons and encoders, but we're pretty much good to go with just the handful of components I describe above. (Well, an astute reader might have noticed that I didn't explicitly mention audio in, but you get my drift.)

I suppose my next step is to make a list of the parts to see if this thing can really go together like I want. I see a few issues on the RF input and output side, particularly how I want to multiplex the filtering and amplification since we will be working with two frequencies on two different bands which can either be used as transmit or receive frequencies depending on the satellite.

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