  Probing the Universe with atomic hydrogen 21 cm emission is a fascinating and
challenging work in astronomy. Radio telescopes play a vital role in detecting
and imaging these faint signals. Powerful radio telescopes are complex to
construct and operate. We have built a simple, low-cost 21 cm radio telescope
primarily for educational training purposes. The design uses a custom horn
antenna, ready-to-use radio-frequency components, and a software-defined radio
module. The telescope operates efficiently from a rooftop in a city
environment. Using this telescope, we have conducted observations and
successfully detected the 21 cm line emissions from the different directions of
our galactic plane. Based on the Doppler-shift observed in these measurements,
we have successfully derived the Galactic rotation velocity (rotation curve) in
those directions. The paper presents the details of the telescope construction,
21 cm observation, and the Galactic rotation curve derivation.
