What I learned from building a 21cm Hydrogen Radio Telescope

I spent the better part of a year assembling a horn antenna out of foam board, aluminium tape, and a lot of stubbornness. The goal was simple to state and maddening to achieve: detect the 1420 MHz line emitted by neutral hydrogen drifting through the Milky Way.

The setup

The signal chain ended up being embarrassingly humble — a can-tuned horn, a cheap low-noise amplifier, a SAW filter, and an RTL-SDR feeding into a laptop running GNU Radio. Nothing here costs more than a decent pair of headphones.

What actually mattered

• Shielding is everything. My first "detection" was a nearby microwave oven.
• Integration time buys you signal. Averaging over minutes turned noise into a peak.
• The Doppler shift is real and beautiful. Watching the line move as the Earth rotated was the moment it stopped being a project and started being astronomy.

If you have ever wanted proof that the galaxy is spinning, you can measure it from your backyard. That still amazes me.