Smallest antenna that can detect hydrogen in Milky Way

Detecting the 21 cm hydrogen line (the characteristic emission from neutral hydrogen in the Milky Way) requires a radio antenna sensitive to that wavelength. The wavelength of this emission is approximately 21 cm, corresponding to a frequency of 1.42 GHz.

Factors Affecting Antenna Size
• Wavelength and Frequency: The physical size of the antenna is related to the wavelength it is designed to receive. Generally, the minimum size of an effective antenna is about half the wavelength, or around 10.5 cm for the 21 cm line.
• Sensitivity and Gain: While a small antenna can pick up the signal, its sensitivity (ability to detect weak signals) depends on its design, such as being part of an array or having additional electronics to amplify signals.

Practical Small Antennas
A single quarter-wavelength monopole antenna or small horn antenna can detect the 21 cm signal under ideal conditions. However, most amateur or professional setups use larger parabolic dishes (often a few meters in diameter) for better sensitivity and to minimize noise.

Smallest Practical Antennas
• Amateur Radio Telescopes: Hobbyists have detected hydrogen signals using dish antennas as small as 1.2 meters (4 feet) in diameter when paired with low-noise amplifiers and suitable receivers.
• DIY Solutions: Some setups use a small Yagi or patch antenna (about 10–30 cm in size), though detecting the signal becomes more challenging due to noise.
For scientific accuracy and better resolution, larger antennas or arrays are preferred, but small setups can still achieve this detection with the right equipment and conditions.