Amateur radioastronomy: dish suggestions

Question

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!

Answer 1

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 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 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.

Answer 2

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. 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, 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 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 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

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

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 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#

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+ (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 You would then need to think about powering this setup.

Currently I'm looking at Power Over Ethernet (POE) to the RPi 3 B+, possibly using the RPi POE HAT when it's released this year. Then you could take power from the RPi and use boost converter 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, here and here