<p>The size of your dish determines two things:</p>
<ol>
<li>Along with the temperature of your electronics, determines the <strong>signal-to-noise ratio</strong> of your telescope.</li>
<li>The size of your dish determines the <strong>angular resolution</strong> you can expect. This has an approximate relationship of <span class="math-container">$$ R = \lambda / D$$</span> where <span class="math-container">$R$</span> is your angular resolution, <span class="math-container">$\lambda$</span> is your wavelength of light and <span class="math-container">$D$</span> the width of your aperture (dish).</li>
</ol>
<p>Instead of talking about dish size lets talk about interesting things to look at and see what the requirements are:</p>
<h3>The Sun</h3>
<p>The sun is a good source of radio waves in the 10cm wavelength. Since the sun is around half a degree, we would need a dish at least 11.5 meters before you could expect to see the sun as anything more than a point source.</p>
<h3>Jupiter</h3>
<p>Jupiter has some magnetic field effects that produce radio waves in the 10-100cm wavelength range. Of course Jupiter is 50 arcseconds across, and would need a dish 412 meters in diameter to do any amount of resolution.</p>
<h3>Cosmic Microwave Background</h3>
<p>The CMB was one of the earliest measurements of radio waves from space. The strongest peak of the CMB is in the 1mm wavelength. However it is very weak. The <a href="https://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation" rel="noreferrer">original investigators</a> cooled their radio detector with liquid helium, and I imagine something similar would be required.</p>
<h3>Terrestrial and Man-made sources</h3>
<p>There are plenty of man-made radio sources. Airports have a radio wave emitter in the 10cm range for radar. There are plenty of geosynchronous satellites along the equatorial plane that use the X-band in the ~2-5cm range. These by definition have a strong enough signal to be seen from Earth with even a small dish.</p>
<h3>Summary</h3>
<p>Resolving power is likely out of the reach of an amateur radio astronomer since the dishes have to be either very large, or you must use a much more sophisticated <a href="https://en.wikipedia.org/wiki/Astronomical_interferometer" rel="noreferrer"><em>radio interferometry</em></a> method to achieve any angular resolution. You can however still see things as point sources in the sky if you are so inclined. An excellent example of what is likely accomplished by most people would be this <a href="https://youtu.be/aeah3fFYlnA" rel="noreferrer">project here</a>.</p>

CC BY-SA 4.0
2018-05-02T21:14:35.433Z
I know it is "hard", but to see and study a little bit "pulsars", is it feasible? What about "nearby" black holes or are we limited as amateurs to SGA* and no much more? I am reading stuff from SARA but I am not sure what can be done...
riemannium
CC BY-SA 4.0
2018-05-02T22:56:04.000Z
Pulsars and black holes are even smaller, so you would need an even bigger telescope to look at them.
Phiteros
CC BY-SA 4.0
2018-05-03T15:29:53.900Z
Some people, it seem,succeded to detect pulsars with 3 or 5 meter antennas...Are they bluffing or joking?
riemannium
CC BY-SA 4.0
2018-05-03T18:09:28.407Z
Do you have a source for that? Also, detecting something is rather different than studying it.
Phiteros
CC BY-SA 4.0
2018-06-28T02:09:17.243Z
There is a good discussion of small RA projects using dishes as small as 4 metres detecting pulsars at https://www.rtl-sdr.com/overview-neutron-star-group-pulsar-detection-projects-rtl-sdr/  Requires integration of the signals over time.
IlludiumPu36
CC BY-SA 4.0
2018-06-28T07:24:45.030Z
@Phiteros   Quite true, but detection is the first step to interpretation...there are people who develop the technology to enable detection, without being involved in the study of that detected.
IlludiumPu36
CC BY-SA 4.0
2018-05-02T13:25:04.233Z
2018-05-02T13:32:49.407Z
2020-06-17T09:47:40.653Z
cms
<p>I have a used 2.4 metre mesh C band dish that I picked up for free that I will be converting for observing the <a href="https://en.wikipedia.org/wiki/Hydrogen_line" rel="nofollow noreferrer">21cm hydrogen line @ 1420MHz</a>. I was lucky with this dish as it's in as new condition, but you need to be aware of any rusting and damage to the mesh that will distort your data.  I'm mounting mine in my backyard on a 75mm steel pole 2 metres above the ground pointing straight up.  This is to do <a href="https://www.assa.org.au/resources/radio-astronomy/meridian-drift-scanning/" rel="nofollow noreferrer">meridian drift scans</a>, so you don't need to have a movable dish, the planet's rotation does this for you.</p>

<p>The hydrogen line can be used to observe deep space objects which emit strong hydrogen line signals.  Observing this spectrum from the Milky Way is one example, as well as say, the <a href="https://en.wikipedia.org/wiki/Cygnus_A" rel="nofollow noreferrer">Cygnus A galaxy</a> and others.</p>

<p>The old large (2 to 3 metres or so) C band dishes once used for satellite TV, now largely replaced by smaller Ku band dishes, can be picked up for about AU$100 or even for free from household backyards.  These are used in a number of amatuer radio astronomy projects, especially for the 21 cm line.</p>

<p>Once you have the dish, you will typically need to:</p>

<p><strong>Replacing the Low Noise Block (LNB)</strong>. The <a href="https://en.wikipedia.org/wiki/Low-noise_block_downconverter" rel="nofollow noreferrer">LNB</a> is attached to the top of the dish struts.  You will need to replace this with one made specifically for 1420MHz.  You can make your own LNB with details at <a href="http://www.setileague.org/hardware/feedchok.htm" rel="nofollow noreferrer">http://www.setileague.org/hardware/feedchok.htm</a>.  On that site there is an Excel spreadsheet with variables for adjusting some of the measurements of the LNB.  </p>

<p>The LNB for the hydrogen line basically consists of an aluminium tube (waveguide) capped at one end.  Inside the tube is the antenna probe, which is just a brass rod - length and placement varies according to the projects I have seen, but the SETI guide above should be OK.  The probe is soldered to the centre pin of the coaxial cable connector fitted to the waveguide.  Again, refer to the SETI page above.</p>

<p>You can also purchase one ready made from <a href="https://www.radioastronomysupplies.com/store/p22/1420_MHz._CYLINDRICAL_FEEDHORN_AND_CHOKE.html" rel="nofollow noreferrer">https://www.radioastronomysupplies.com/store/p22/1420_MHz._CYLINDRICAL_FEEDHORN_AND_CHOKE.html</a>  </p>

<p><strong>You will need a Low Noise Amplifier (LNA) for 1420MHz.</strong> The LNA will need a Gain of > 30dB and a Noise Figure (NF) of somewhere around 0.3dB or lower.  The higher the Gain (sensitivity) and the lower the NF the better, though obviously at a price.  The LNA should be mounted on the coaxial cable connected to the LNB antenna probe inside the LNB waveguide. The closer the better. I have no connections with Radio Astronomy Supplies, but they also have what appears to be a decent LNA for the hydrogen line:</p>

<p><a href="https://www.radioastronomysupplies.com/store/p9/1420_MHz._HIGH_PERFORMANCE_LNA.html" rel="nofollow noreferrer">https://www.radioastronomysupplies.com/store/p9/1420_MHz._HIGH_PERFORMANCE_LNA.html</a></p>

<p><a href="https://www.ebay.com.au/itm/LNA-21cm-1420-MHz-N-female-/113010650218?hash=item1a4ff5ac6a%3Ag%3ACuwAAOSwDMNbBXhA&amp;nma=true&amp;si=gILuRaA7xRatFaHflv7C6kI31Pw%253D&amp;orig_cvip=true&amp;rt=nc&amp;_trksid=p2047675.l2557" rel="nofollow noreferrer">Another LNA made for 1420MHz</a></p>

<p><strong>A receiver.</strong>  The receiver allows you to interpret the signal coming down from the LNA.  I have purchased a cheapish (AU$30) Software-defined Radio (SDR) USB dongle for my setup which will act as the receiver.  In particular a <a href="https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/" rel="nofollow noreferrer">RTL-SDR Blog R820T2 RTL2832U 1PPM TCXO SMA Software Defined Radio</a>  </p>

<p>One example of such usage is at <a href="https://www.rtl-sdr.com/hydrogen-line-observation-with-an-rtl-sdr/" rel="nofollow noreferrer">https://www.rtl-sdr.com/hydrogen-line-observation-with-an-rtl-sdr/</a></p>

<p>More discussion on SDR for observing the hydrogen line is at h<a href="https://www.rtl-sdr.com/rtl-sdr-for-budget-radio-astronomy/" rel="nofollow noreferrer">ttps://www.rtl-sdr.com/rtl-sdr-for-budget-radio-astronomy/</a></p>

<p>The SDR dongle connects to the coaxial line from the LNA. You can then plug the SDR dongle into your computer's USB port. Beware of the length of coaxial line as longer lines will lose data.  An alternative is discussed below.</p>

<p><strong>Software to observe the data.</strong>  There are a number of open-source applications for SDR reception.  Possibly the most popular for SDR - radio astronomy is <a href="https://www.rtl-sdr.com/sdrsharp-users-guide/" rel="nofollow noreferrer">SDR#</a></p>

<p><strong>Using a Raspberry Pi 3 B+ as a server from the dish.</strong>  Alternative to using coaxial cable from the LNA to SDR at the computer is to have a <a href="https://www.raspberrypi.org/blog/raspberry-pi-3-model-bplus-sale-now-35/" rel="nofollow noreferrer">Raspberry Pi 3 B+</a> (RPi) act as a server to send the data to computer via Ethernet cable, rather than coaxial cable.  This has a number of possible advantages including much less or no data loss depending on the cable and length.  I will be using Cat6 cable up to around 20 to 30 metres.  The cable plugs into the RPi RJ45 Ethernet port.  The SDR dongle plugs into a RPi USB port.  The LNA attaches to the SDR dongle directly via the coaxial connectors/adapters.  </p>

<p>This setup can be mounted on the mounting pole for the dish contained in a weatherproof and ventilated box, something like <a href="https://www.desertcart.ae/products/23590912-outdoor-poe-interface-enclosure-box-with-cooling-fan-48-volt-dc-hyperlink-poe-passive-injectors-weatherproof-cabinet-box-waterproof" rel="nofollow noreferrer">this</a>  You would then need to think about powering this setup.  </p>

<p>Currently I'm looking at <a href="https://en.wikipedia.org/wiki/Power_over_Ethernet" rel="nofollow noreferrer">Power Over Ethernet</a> (POE) to the RPi 3 B+, possibly using the <a href="https://www.raspberrypi.org/products/poe-hat/" rel="nofollow noreferrer">RPi POE HAT</a> when it's released this year.  Then you could take power from the RPi and use <a href="https://core-electronics.com.au/pololu-9v-step-up-voltage-regulator-u3v12f9.html" rel="nofollow noreferrer">boost converter</a> to 9v or 12v to power the LNA of choice. as well as any 5V cooling fans you have in your box.</p>

<p>Then when connect to the RPi from your computer (e.g. using SSH) you should be set to receive data.  The other advantage to this setup is that since the RPi is acting as a server connected to the internet, you can access your dish from anywhere in the world with an internet connection.  There is some discussion of this <a href="https://roughlea.wordpress.com/raspberry-pi-experiences/configuring-the-raspberry-pi-as-an-ssh-server/" rel="nofollow noreferrer">here</a>, <a href="https://thepi.io/how-to-ssh-into-the-raspberry-pi/" rel="nofollow noreferrer">here</a> and <a href="http://radioadastra.blogspot.com/" rel="nofollow noreferrer">here</a></p>

CC BY-SA 4.0
2018-06-28T11:16:22.700Z
This is a wonderful answer!  just fyi: [When was the Illudium Q-36 Space Modulator first depicted?](https://scifi.stackexchange.com/q/149563/51174) seem's I've mipselled it.
uhoh
CC BY-SA 4.0
2018-06-29T01:48:14.820Z
Thanks uhoh, yeah, fond memories of watching Looney Tunes in the 60's, the Martian is my favourite hence my avatar!  Re uh oh... https://www.urbandictionary.com/define.php?term=Uh%20oh   Now ceasing to hijack this question...
IlludiumPu36
CC BY-SA 4.0
2018-07-14T15:06:17.353Z
[What do these GPS-related emblems represent? Does one of them show "Marvin the Martian"?](https://space.stackexchange.com/q/29434/12102)
uhoh
CC BY-SA 4.0
2018-07-17T02:34:56.677Z
In my non-expert opinion, I was say definitely the character in the green helmet is Marvin  Now I just bought this patch  https://www.ebay.com/itm/USAF-COUNTERSPACE-OPERATIONS-MARVIN-THE-MARTIAN-DOD-ORIGINAL-SPACE-PATCH-/281842667806 and I am now US$24 poorer, thanks uhoh...
IlludiumPu36
CC BY-SA 4.0
2018-07-17T05:24:07.353Z
Oh you lucky martian you! Wear it proudly, wear it often, or not, depending... Anyway, *hurray for Looney Toons!*
uhoh
CC BY-SA 4.0
2018-06-27T09:02:10.547Z
2018-06-29T08:59:46.837Z
2018-06-29T08:59:46.837Z
IlludiumPu36
<p>What kind of amateur radioastronomy can any interested citized do with:</p>

<p>a) A 2 m dish antenna.
b) A 2,5 m antenna.
c) A 3 m antenna.
d) A 5 m antenna. 
e) A >5 m &lt;10 m antenna (likely I can not afford this or the previous but I just want to know...).</p>

<p>Of course, space is sometimes a logistic issue for big stuff, but I wondered what kind of objects or sources can someone do with suitable instruments and with aid enough if not an expert. Projects and links would be welcome!</p>

CC BY-SA 4.0
2018-05-02T16:32:20.523Z
I have a dish antenna as it happens.
John Duffield
<amateur-observing><radio-astronomy><radio-telescope>
Amateur radioastronomy: dish suggestions
riemannium
# Amateur radioastronomy: dish suggestions

(Asked by: username_0 on 2018-05-02)

What kind of amateur radioastronomy can any interested citized do with:

a) A 2 m dish antenna.
b) A 2,5 m antenna.
c) A 3 m antenna.
d) A 5 m antenna.
e) A >5 m <10 m antenna (likely I can not afford this or the previous but I just want to know...).

Of course, space is sometimes a logistic issue for big stuff, but I wondered what kind of objects or sources can someone do with suitable instruments and with aid enough if not an expert. Projects and links would be welcome!

**Tags:** amateur-observing, radio-astronomy, radio-telescope
> **Comments:**
> **username_1**: I have a dish antenna as it happens.  

---

## 2 answers

### Answer by username_2 on 2018-05-02. Score: 6

The size of your dish determines two things:

1. Along with the temperature of your electronics, determines the **signal-to-noise ratio** of your telescope.
2. The size of your dish determines the **angular resolution** you can expect. This has an approximate relationship of $$ R = \lambda / D$$ where $R$ is your angular resolution, $\lambda$ is your wavelength of light and $D$ the width of your aperture (dish).

Instead of talking about dish size lets talk about interesting things to look at and see what the requirements are:

### The Sun

The sun is a good source of radio waves in the 10cm wavelength. Since the sun is around half a degree, we would need a dish at least 11.5 meters before you could expect to see the sun as anything more than a point source.

### Jupiter

Jupiter has some magnetic field effects that produce radio waves in the 10-100cm wavelength range. Of course Jupiter is 50 arcseconds across, and would need a dish 412 meters in diameter to do any amount of resolution.

### Cosmic Microwave Background

The CMB was one of the earliest measurements of radio waves from space. The strongest peak of the CMB is in the 1mm wavelength. However it is very weak. The [original investigators](https://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation) cooled their radio detector with liquid helium, and I imagine something similar would be required.

### Terrestrial and Man-made sources

There are plenty of man-made radio sources. Airports have a radio wave emitter in the 10cm range for radar. There are plenty of geosynchronous satellites along the equatorial plane that use the X-band in the ~2-5cm range. These by definition have a strong enough signal to be seen from Earth with even a small dish.

### Summary

Resolving power is likely out of the reach of an amateur radio astronomer since the dishes have to be either very large, or you must use a much more sophisticated [*radio interferometry*](https://en.wikipedia.org/wiki/Astronomical_interferometer) method to achieve any angular resolution. You can however still see things as point sources in the sky if you are so inclined. An excellent example of what is likely accomplished by most people would be this [project here](https://youtu.be/aeah3fFYlnA).

> **Comments:**
> **username_0**: I know it is "hard", but to see and study a little bit "pulsars", is it feasible? What about "nearby" black holes or are we limited as amateurs to SGA* and no much more? I am reading stuff from SARA but I am not sure what can be done...  
> **username_3**: Pulsars and black holes are even smaller, so you would need an even bigger telescope to look at them.  
> **username_0**: Some people, it seem,succeded to detect pulsars with 3 or 5 meter antennas...Are they bluffing or joking?  
> **username_3**: Do you have a source for that? Also, detecting something is rather different than studying it.  
> **username_4**: There is a good discussion of small RA projects using dishes as small as 4 metres detecting pulsars at https://www.rtl-sdr.com/overview-neutron-star-group-pulsar-detection-projects-rtl-sdr/  Requires integration of the signals over time.  
> **username_4**: @Phiteros   Quite true, but detection is the first step to interpretation...there are people who develop the technology to enable detection, without being involved in the study of that detected.  

---

### Answer by username_4 on 2018-06-27. Score: 5

I have a used 2.4 metre mesh C band dish that I picked up for free that I will be converting for observing the [21cm hydrogen line @ 1420MHz](https://en.wikipedia.org/wiki/Hydrogen_line). I was lucky with this dish as it's in as new condition, but you need to be aware of any rusting and damage to the mesh that will distort your data. I'm mounting mine in my backyard on a 75mm steel pole 2 metres above the ground pointing straight up. This is to do [meridian drift scans](https://www.assa.org.au/resources/radio-astronomy/meridian-drift-scanning/), so you don't need to have a movable dish, the planet's rotation does this for you.

The hydrogen line can be used to observe deep space objects which emit strong hydrogen line signals. Observing this spectrum from the Milky Way is one example, as well as say, the [Cygnus A galaxy](https://en.wikipedia.org/wiki/Cygnus_A) and others.

The old large (2 to 3 metres or so) C band dishes once used for satellite TV, now largely replaced by smaller Ku band dishes, can be picked up for about AU$100 or even for free from household backyards. These are used in a number of amatuer radio astronomy projects, especially for the 21 cm line.

Once you have the dish, you will typically need to:

**Replacing the Low Noise Block (LNB)**. The [LNB](https://en.wikipedia.org/wiki/Low-noise_block_downconverter) is attached to the top of the dish struts. You will need to replace this with one made specifically for 1420MHz. You can make your own LNB with details at <http://www.setileague.org/hardware/feedchok.htm>. On that site there is an Excel spreadsheet with variables for adjusting some of the measurements of the LNB.

The LNB for the hydrogen line basically consists of an aluminium tube (waveguide) capped at one end. Inside the tube is the antenna probe, which is just a brass rod - length and placement varies according to the projects I have seen, but the SETI guide above should be OK. The probe is soldered to the centre pin of the coaxial cable connector fitted to the waveguide. Again, refer to the SETI page above.

You can also purchase one ready made from <https://www.radioastronomysupplies.com/store/p22/1420_MHz._CYLINDRICAL_FEEDHORN_AND_CHOKE.html>

**You will need a Low Noise Amplifier (LNA) for 1420MHz.** The LNA will need a Gain of > 30dB and a Noise Figure (NF) of somewhere around 0.3dB or lower. The higher the Gain (sensitivity) and the lower the NF the better, though obviously at a price. The LNA should be mounted on the coaxial cable connected to the LNB antenna probe inside the LNB waveguide. The closer the better. I have no connections with Radio Astronomy Supplies, but they also have what appears to be a decent LNA for the hydrogen line:

<https://www.radioastronomysupplies.com/store/p9/1420_MHz._HIGH_PERFORMANCE_LNA.html>

[Another LNA made for 1420MHz](https://www.ebay.com.au/itm/LNA-21cm-1420-MHz-N-female-/113010650218?hash=item1a4ff5ac6a%3Ag%3ACuwAAOSwDMNbBXhA&nma=true&si=gILuRaA7xRatFaHflv7C6kI31Pw%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557)

**A receiver.** The receiver allows you to interpret the signal coming down from the LNA. I have purchased a cheapish (AU$30) Software-defined Radio (SDR) USB dongle for my setup which will act as the receiver. In particular a [RTL-SDR Blog R820T2 RTL2832U 1PPM TCXO SMA Software Defined Radio](https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/)

One example of such usage is at <https://www.rtl-sdr.com/hydrogen-line-observation-with-an-rtl-sdr/>

More discussion on SDR for observing the hydrogen line is at h[ttps://www.rtl-sdr.com/rtl-sdr-for-budget-radio-astronomy/](https://www.rtl-sdr.com/rtl-sdr-for-budget-radio-astronomy/)

The SDR dongle connects to the coaxial line from the LNA. You can then plug the SDR dongle into your computer's USB port. Beware of the length of coaxial line as longer lines will lose data. An alternative is discussed below.

**Software to observe the data.** There are a number of open-source applications for SDR reception. Possibly the most popular for SDR - radio astronomy is [SDR#](https://www.rtl-sdr.com/sdrsharp-users-guide/)

**Using a Raspberry Pi 3 B+ as a server from the dish.** Alternative to using coaxial cable from the LNA to SDR at the computer is to have a [Raspberry Pi 3 B+](https://www.raspberrypi.org/blog/raspberry-pi-3-model-bplus-sale-now-35/) (RPi) act as a server to send the data to computer via Ethernet cable, rather than coaxial cable. This has a number of possible advantages including much less or no data loss depending on the cable and length. I will be using Cat6 cable up to around 20 to 30 metres. The cable plugs into the RPi RJ45 Ethernet port. The SDR dongle plugs into a RPi USB port. The LNA attaches to the SDR dongle directly via the coaxial connectors/adapters.

This setup can be mounted on the mounting pole for the dish contained in a weatherproof and ventilated box, something like [this](https://www.desertcart.ae/products/23590912-outdoor-poe-interface-enclosure-box-with-cooling-fan-48-volt-dc-hyperlink-poe-passive-injectors-weatherproof-cabinet-box-waterproof) You would then need to think about powering this setup.

Currently I'm looking at [Power Over Ethernet](https://en.wikipedia.org/wiki/Power_over_Ethernet) (POE) to the RPi 3 B+, possibly using the [RPi POE HAT](https://www.raspberrypi.org/products/poe-hat/) when it's released this year. Then you could take power from the RPi and use [boost converter](https://core-electronics.com.au/pololu-9v-step-up-voltage-regulator-u3v12f9.html) to 9v or 12v to power the LNA of choice. as well as any 5V cooling fans you have in your box.

Then when connect to the RPi from your computer (e.g. using SSH) you should be set to receive data. The other advantage to this setup is that since the RPi is acting as a server connected to the internet, you can access your dish from anywhere in the world with an internet connection. There is some discussion of this [here](https://roughlea.wordpress.com/raspberry-pi-experiences/configuring-the-raspberry-pi-as-an-ssh-server/), [here](https://thepi.io/how-to-ssh-into-the-raspberry-pi/) and [here](http://radioadastra.blogspot.com/)

> **Comments:**
> **username_5**: This is a wonderful answer!  just fyi: [When was the Illudium Q-36 Space Modulator first depicted?](https://scifi.stackexchange.com/q/149563/51174) seem's I've mipselled it.  
> **username_4**: Thanks uhoh, yeah, fond memories of watching Looney Tunes in the 60's, the Martian is my favourite hence my avatar!  Re uh oh... https://www.urbandictionary.com/define.php?term=Uh%20oh   Now ceasing to hijack this question...  
> **username_5**: [What do these GPS-related emblems represent? Does one of them show "Marvin the Martian"?](https://space.stackexchange.com/q/29434/12102)  
> **username_4**: In my non-expert opinion, I was say definitely the character in the green helmet is Marvin  Now I just bought this patch  https://www.ebay.com/itm/USAF-COUNTERSPACE-OPERATIONS-MARVIN-THE-MARTIAN-DOD-ORIGINAL-SPACE-PATCH-/281842667806 and I am now US$24 poorer, thanks uhoh...  
> **username_5**: Oh you lucky martian you! Wear it proudly, wear it often, or not, depending... Anyway, *hurray for Looney Toons!*  

---

