21 cm band also known as Hydrogen line is one of the important bands in radio astronomy.
Please read this wikipedia article if you not familiar with Hydrogen line.
Nowadays even radio amateurs can observe this signal from the outer space, thanks to modern hardware.
One of the most important part of the any receiver is antenna and input stage. Typically input stage is some kind of the amplifier with extremely low noise and high gain due to weakness of the space signals.
In this article I want to describe simple and cheap amplifier for the first stage of the receiver.