<p>The <a href="https://en.wikipedia.org/wiki/Hydrogen_line" rel="nofollow noreferrer">21 cm line</a> is from <em>atomic hydrogen</em> which is going to be free, neutral atoms in vacuum, where the electron is bound only to the proton. </p>

<p>In water, or any other hydrogen containing molecule, the electron's orbit will be dramatically modified (or even "stolen") by the atom to which it is attached, and so the transition will no longer exist as a narrow line.</p>

<p>Certainly there might be all kinds of RF interference from objects encroaching on your antenna's reception pattern, which may have side-lobes as well as the main beam pointing upwards, so like @Kozaky's <a href="https://astronomy.stackexchange.com/questions/26702/hydrogen-line-observation-interference-from-trees#comment46270_26702">comment</a> suggests, you should to some experimentation if possible to understand contributions to background.</p>

<p>Your sensitivity will depend on the bandwidth you choose for your receiver, as well as the thermal and Doppler broadening of of the source that passes by, so make sure you have an understanding of what the bandwidth should be to see a peak, and also if there are significant offsets in frequency due to a <em>Doppler shift</em>.</p>

<p><a href="https://i.sstatic.net/G2GZe.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/G2GZe.png" alt="enter image description here"></a></p>

<p><strong>above:</strong> From <a href="https://en.wikipedia.org/wiki/File:Hydrogen-SpinFlip.svg" rel="nofollow noreferrer">here</a></p>

<blockquote>
  <p>Ground state hyperfine levels of hydrogen (parallel and antiparallel) with the spin-flip transition, emitting radiation at 1420 MHz. The corresponding wavelength is 21 cm. (21-cm line, hydrogen line)</p>
</blockquote>

CC BY-SA 4.0
2018-09-27T11:52:52.403Z
Be cautious to avoid the common misconception that the 21 cm line comes from a flip of the electron spin.  The change is in the total spin of the atom, which as you say goes from F=1 to F=0.  That's not a change in sign-- it's not a "flip."  What's more, the final state of the transition must find the electron with an indeterminate spin direction, and in some cases the initial state also has that.  It's one of those things where you decide if the wrongness of the cartoon is serious or just a kind of oversimplification.  To me, it loses the beauty of what is actually happening there.
Ken G
CC BY-SA 4.0
2018-09-27T12:10:04.897Z
@KenG indeed, noted, thanks! Now someone tell Wikipedia ;-)
uhoh
CC BY-SA 4.0
2018-09-28T10:56:47.660Z
Yes, it's time for someone to fix that up.
Ken G
CC BY-SA 4.0
2018-06-20T12:38:26.240Z
2018-06-20T12:44:27.297Z
2018-06-20T12:44:27.297Z
uhoh
<p>The GPS system uses frequencies either side of the 1420 MHz frequency of neutral hydrogen.  The GPS system works in woods and forests, so my guess is that you should be ok.</p>

CC BY-SA 4.0
2018-06-21T03:00:36.730Z
By analogy (humor): GPS receivers have a tiny antenna of a few square millimeters (chip-type) to a few square centimeters at most, "so my guess is that" radio astronomy can be done with centimeter sized antennas as well. The large dishes and arrays are for appearance only, and in no way suggest that radio sources at galactic and intergalactic distances might be any weaker than those beamed to Earth by amplifiers in Earth orbit.
uhoh
CC BY-SA 4.0
2018-06-20T12:57:45.413Z
2018-06-20T12:57:45.413Z
Dr Chuck
<p>I am setting up a 2.4 metre parabola pointing straight up to observe the 21cm hydrogen line at 1420MHz with meridian drift scans.  </p>

<p>I'm planning to place the dish on a steel pole embedded in the ground and protruding 2 metres from the ground surface.</p>

<p>There is a tree some metres away that has a few branches well overhead of the antenna.  It's a Eucalyptus tree, so not heavy with green leaves.</p>

<p>Question:  Should I be concerned about interference from a few overhanging branches (say about 15 metres above the dish)?  My understanding is that shorter lines in the cm range should be less affected by interference (e.g weather), but I'm not sure about trees.</p>

<radio-astronomy><hydrogen>
Hydrogen Line observation - Interference from trees
IlludiumPu36
# Hydrogen Line observation - Interference from trees

(Asked by: username_0 on 2018-06-20)

I am setting up a 2.4 metre parabola pointing straight up to observe the 21cm hydrogen line at 1420MHz with meridian drift scans.

I'm planning to place the dish on a steel pole embedded in the ground and protruding 2 metres from the ground surface.

There is a tree some metres away that has a few branches well overhead of the antenna. It's a Eucalyptus tree, so not heavy with green leaves.

Question: Should I be concerned about interference from a few overhanging branches (say about 15 metres above the dish)? My understanding is that shorter lines in the cm range should be less affected by interference (e.g weather), but I'm not sure about trees.

**Tags:** radio-astronomy, hydrogen

---

## 2 answers

### Answer by username_1 on 2018-06-20. Score: 3 | Accepted answer

The [21 cm line](https://en.wikipedia.org/wiki/Hydrogen_line) is from *atomic hydrogen* which is going to be free, neutral atoms in vacuum, where the electron is bound only to the proton.

In water, or any other hydrogen containing molecule, the electron's orbit will be dramatically modified (or even "stolen") by the atom to which it is attached, and so the transition will no longer exist as a narrow line.

Certainly there might be all kinds of RF interference from objects encroaching on your antenna's reception pattern, which may have side-lobes as well as the main beam pointing upwards, so like @Kozaky's [comment](https://astronomy.stackexchange.com/questions/26702/hydrogen-line-observation-interference-from-trees#comment46270_26702) suggests, you should to some experimentation if possible to understand contributions to background.

Your sensitivity will depend on the bandwidth you choose for your receiver, as well as the thermal and Doppler broadening of of the source that passes by, so make sure you have an understanding of what the bandwidth should be to see a peak, and also if there are significant offsets in frequency due to a *Doppler shift*.

[![enter image description here](https://i.sstatic.net/G2GZe.png)](https://i.sstatic.net/G2GZe.png)

**above:** From [here](https://en.wikipedia.org/wiki/File:Hydrogen-SpinFlip.svg)

> Ground state hyperfine levels of hydrogen (parallel and antiparallel) with the spin-flip transition, emitting radiation at 1420 MHz. The corresponding wavelength is 21 cm. (21-cm line, hydrogen line)

> **Comments:**
> **username_2**: Be cautious to avoid the common misconception that the 21 cm line comes from a flip of the electron spin.  The change is in the total spin of the atom, which as you say goes from F=1 to F=0.  That's not a change in sign-- it's not a "flip."  What's more, the final state of the transition must find the electron with an indeterminate spin direction, and in some cases the initial state also has that.  It's one of those things where you decide if the wrongness of the cartoon is serious or just a kind of oversimplification.  To me, it loses the beauty of what is actually happening there.  
> **username_1**: @KenG indeed, noted, thanks! Now someone tell Wikipedia ;-)  
> **username_2**: Yes, it's time for someone to fix that up.  

---

### Answer by username_3 on 2018-06-20. Score: 1

The GPS system uses frequencies either side of the 1420 MHz frequency of neutral hydrogen. The GPS system works in woods and forests, so my guess is that you should be ok.

> **Comments:**
> **username_1**: By analogy (humor): GPS receivers have a tiny antenna of a few square millimeters (chip-type) to a few square centimeters at most, "so my guess is that" radio astronomy can be done with centimeter sized antennas as well. The large dishes and arrays are for appearance only, and in no way suggest that radio sources at galactic and intergalactic distances might be any weaker than those beamed to Earth by amplifiers in Earth orbit.  

---

