I have a used 2.4 metre mesh C band dish that I picked up for free that I will be converting for observing the 21cm hydrogen line @ 1420MHz. I was lucky with this dish as it's in as new condition, but you need to be aware of any rusting and damage to the mesh that will distort your data. I'm mounting mine in my backyard on a 75mm steel pole 2 metres above the ground pointing straight up. This is to do meridian drift scans, so you don't need to have a movable dish, the planet's rotation does this for you.
The hydrogen line can be used to observe deep space objects which emit strong hydrogen line signals. Observing this spectrum from the Milky Way is one example, as well as say, the Cygnus A galaxy and others.
The old large (2 to 3 metres or so) C band dishes once used for satellite TV, now largely replaced by smaller Ku band dishes, can be picked up for about AU$100 or even for free from household backyards. These are used in a number of amatuer radio astronomy projects, especially for the 21 cm line.
Once you have the dish, you will typically need to:
Replacing the Low Noise Block (LNB). The LNB is attached to the top of the dish struts. You will need to replace this with one made specifically for 1420MHz. You can make your own LNB with details at http://www.setileague.org/hardware/feedchok.htm. On that site there is an Excel spreadsheet with variables for adjusting some of the measurements of the LNB.
The LNB for the hydrogen line basically consists of an aluminium tube (waveguide) capped at one end. Inside the tube is the antenna probe, which is just a brass rod - length and placement varies according to the projects I have seen, but the SETI guide above should be OK. The probe is soldered to the centre pin of the coaxial cable connector fitted to the waveguide. Again, refer to the SETI page above.
You can also purchase one ready made from https://www.radioastronomysupplies.com/store/p22/1420_MHz._CYLINDRICAL_FEEDHORN_AND_CHOKE.html
You will need a Low Noise Amplifier (LNA) for 1420MHz. The LNA will need a Gain of > 30dB and a Noise Figure (NF) of somewhere around 0.3dB or lower. The higher the Gain (sensitivity) and the lower the NF the better, though obviously at a price. The LNA should be mounted on the coaxial cable connected to the LNB antenna probe inside the LNB waveguide. The closer the better. I have no connections with Radio Astronomy Supplies, but they also have what appears to be a decent LNA for the hydrogen line:
Another LNA made for 1420MHz
A receiver. The receiver allows you to interpret the signal coming down from the LNA. I have purchased a cheapish (AU$30) Software-defined Radio (SDR) USB dongle for my setup which will act as the receiver. In particular a RTL-SDR Blog R820T2 RTL2832U 1PPM TCXO SMA Software Defined Radio
One example of such usage is at https://www.rtl-sdr.com/hydrogen-line-observation-with-an-rtl-sdr/
More discussion on SDR for observing the hydrogen line is at https://www.rtl-sdr.com/rtl-sdr-for-budget-radio-astronomy/
The SDR dongle connects to the coaxial line from the LNA. You can then plug the SDR dongle into your computer's USB port. Beware of the length of coaxial line as longer lines will lose data. An alternative is discussed below.
Software to observe the data. There are a number of open-source applications for SDR reception. Possibly the most popular for SDR - radio astronomy is SDR#
Using a Raspberry Pi 3 B+ as a server from the dish. Alternative to using coaxial cable from the LNA to SDR at the computer is to have a Raspberry Pi 3 B+ (RPi) act as a server to send the data to computer via Ethernet cable, rather than coaxial cable. This has a number of possible advantages including much less or no data loss depending on the cable and length. I will be using Cat6 cable up to around 20 to 30 metres. The cable plugs into the RPi RJ45 Ethernet port. The SDR dongle plugs into a RPi USB port. The LNA attaches to the SDR dongle directly via the coaxial connectors/adapters.
This setup can be mounted on the mounting pole for the dish contained in a weatherproof and ventilated box, something like this You would then need to think about powering this setup.
Currently I'm looking at Power Over Ethernet (POE) to the RPi 3 B+, possibly using the RPi POE HAT when it's released this year. Then you could take power from the RPi and use boost converter to 9v or 12v to power the LNA of choice. as well as any 5V cooling fans you have in your box.
Then when connect to the RPi from your computer (e.g. using SSH) you should be set to receive data. The other advantage to this setup is that since the RPi is acting as a server connected to the internet, you can access your dish from anywhere in the world with an internet connection. There is some discussion of this here, here and here