Galactic Hydrogen
Using a Radio Receiver to Detect the Hydrogen Line
Equipment
- Antenna
- Nooelec 1420MHz filter
- Software Defined Radio
- Computer running SDR software e.g SDR#
Electrons in an atom are in fixed and stable orbits around the nucleus of the atom much like the planets orbit the Sun. The nucleus of a hydrogen atom is made up of just one proton, which is positively charged. Orbiting the nucleus is an electron which is negatively charged. These opposing charges keep the electron spinning around the nucleus.
The closest orbit to the nucleus is known as the ground state and this is where the electron orbiting a hydrogen nucleus (i.e.a proton) would normally be found. The electron and proton also have a quantum property called the spin which can be thought of like the rotation of the Earth around it's axis. Usually the spin of an electron and the spin of the proton nucleus are in the same direction. However in a very rare quantum event, the spin of the electron can flip over so it is the opposite to that of the proton. When it does so, it gives off a photon of energy which, in the case of atomic hydrogen, has a frequency of 1420.405MHz. You can read more about this in the section on Electromagnetic Radiation.
This frequency corresponds to a wavelength of 21.106cm and is known as the 21cm hydrogen line. The probability of this quantum spin-flip is extremely low; about once in every 10,000,000 years! However because there is a huge abundance of atoms of hydrogen in our galaxy (about 75% of the atomic mass of the Universe is hydrogen), the sum of all these rare events adds up to a bright signal at 1420MHz that can be detected quite easily on Earth.
A hydrogen line detector is actually a very simple device. In fact amateur astronomers often use old paint thinner tins because they have just the right conductive properties and dimensions for a decent 21cm radio antenna.
The basic equipment required is an antenna that is optimised for detecting radiation at 1420MHz, a radio receiver and some means of displaying the results.
For the antenna there are many different types and configurations. The essential point is that it should be optimised to receive waves of wavelength 21cm. A commonly used antenna is a waveguide, which is a rectangular cross section hollow conductive tube, that is used to guide the radio waves into it, directing them towards a simple detecting wire which protrudes into the waveguide.
The dimensions are critical: the waveguide should be of rectangular cross section, with waves entering the open end, and a closed end. There are detailed instructions in the additional information secdtion. The wider side must be able to allow the waves to enter so they must be at least half the wavelength of the hydrogen line. A waveguide formula is provided in the links and shows that the the waveguide should be approximately 16cm wide.
In order to detect the polarity of these waves, we need to make sure the shorter side is less than half a wavelength, so they do not enter the waveguide. This means that the narrower side should be less than 10.5cm. This ensures that only the hydrogen line waves of a certain polarity are being detected. About 5cm from the sealed end of the tin, a single wire is fixed, insulated from the metal of the tin. Usually amateur astronomers use an female N Type panel mount connector as it has the right properties to allow a 5cm long, 2mm wide bare rod of copper to be inserted as the antenna.
Whilst it might sound rather home made, paint thinner tins are usually made of conductive steel and are often of 15cm by 10cm cross section with a length of 21cm, so nicely fitting the design criteria.
In addition to the rectangular waveguide described above, trapezoidal metal panels are usually attached which fan out from the open end, as a means to channel the waves. The apparatus is called a horn antenna, and an illustration is shown below:
The signal received by the horn antenna, focussed onto the copper rod, is fed to a filter. Nooelec have introduced a range of radio frequency filters for various projects and one of these is the Sawbird+ H1 hydrogen line filter. It permits signals at 1420MHz and filters out unwanted noise, so it acts as a fixed frequency radio tuner.
The signal is fed into an SDR which must have a frequency range that includes 1.42GHz so look for something like an RTL-SDR or Nooelec with a range that goes up to 1.7GHz.
