Tagged: radio telescope

Building a Hydrogen Line Front End on a Budget with RTL-SDR and 2x LNA4ALL

Adam 9A4QV is the manufacturer of the LNA4ALL, a high quality low noise amplifier popular with RTL-SDR users. He also sells filters, one of which is useful for hydrogen line detection. Recently he’s uploaded a tutorial document showing how to use 2x LNA4ALL, with a filter and RTL-SDR for Hydrogen Line detection (pdf warning). 

Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will only very rarely emit a photon, but since space and the galaxy is filled with many hydrogen atoms the average effect is an observable RF power spike at 1420.4058 MHz. By pointing a radio telescope at the night sky and integrating the RF power over time, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot. This can be used for some interesting experiments, for example you could measure the size and shape of our galaxy. Thicker areas of the galaxy will have more hydrogen and thus a larger spike.

In his tutorial Adam discusses important technical points such as noise figure and filtering. Essentially, when trying to receive the hydrogen line you need a system with a low noise figure and good filtering. The RTL-SDR has a fairly poor noise figure of about 6dB at 1420MHz. But it turns out that the first amplifier element in the receive chain is the one that dominates the noise figure value. So by placing an LNA with a low noise figure right by the antenna, the system noise figure can be brought down to about 1dB, and losses in coax and filters become negligible as well. At the end of the tutorial he also discusses some supplementary points such as ESD protection, bias tees and IP3.

One note from us is that Adam writes that the RTL-SDR V3 bias tee can only provide 50mA, but it can actually provide up to 200mA continuously assuming the host can provide it (keep the dongle in a cool shaded area though). Most modern USB 2.0 and USB3.0 ports on PCs should have no problem providing up to 1A or more. We’ve also tested the LP5907 based Airspy bias tee at up to 150mA without trouble, so the 50mA rating is probably quite conservative. So these bias tee options should be okay for powering 2xLNA4ALL.

Finally Adam writes that in the future he will write a paper discussing homebrew hydrogen line antennas which should complete the tutorial allowing anyone to build a cheap hydrogen line radio telescope.

One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.
One configuration with 2xLNA4ALL, 1x interstage filter, and 1x recceiver side filter with bias tee.

Receiving Jupiter Noise Bursts with an SDRplay RSP1

Over on YouTube user MaskitolSAE has uploaded a video showing him receiving some noise bursts from Jupiter with his SDRplay RSP1. The planet Jupiter is known to emit bursts of noise via natural ‘radio lasers’ powered partly by the planets interaction with the electrically conductive gases emitted by Io, one of the the planets moons. When Jupiter is high in the sky and the Earth passes through one of these radio lasers the noise bursts can be received on Earth quite easily with an appropriate antenna 

In his video MaskitolSAE shows the 10 MHz of waterfall and audio from some Jupiter noise bursts received with his SDRplay RSP1 at 22119 kHz. According to the YouTube description, it appears that he is using the UTR-2 radio telescope which is a large Ukrainian radio telescope installation that consists of an array of 2040 dipoles. A professional radio telescope installation is not required to receive the Jupiter bursts (a backyard dipole tuned to ~20 MHz will work), but the professional radio telescope does get some really nice strong bursts as seen in the video.

The UTR-2 Radio Telescope. Photo Attr. Oleksii Tovpyha (Link)

RTL-SDR Spectrometer for a Small Radio Telescope

Marc Higginson-Rollins of the University of Kentucky has published an academic styled paper in conjuction with Dr. Alan E.E. Rogers of the MIT Haystack Observatory showing how they used an RTL-SDR to implement a Small Radio Telescope (SRT).

In the paper they discuss how they dealt with the frequency drifting and offset problems common in the RTL-SDR. They also show how they dealt with the center spur by correcting the bias that it introduced and how they filtered out RFI noise from a nearby radar station and electronics.

Using the RTL-SDR and SRT they were able to measure the spectra of several well known regions of neutral hydrogen emissions, and measure the galactic rotation curve shown below.

Galactic Rotation Measurements with the SRT
Galactic Rotation Measurements with the SRT and RTL-SDR

RTL-SDR for Budget Radio Astronomy

With the right additional hardware, the RTL-SDR software defined radio can be used as a super cheap radio telescope for radio astronomy experiments such as Hydrogen line detection, meteor scatter and Pulsar observing.

Hydrogen Line

Marcus Leech of Science Radio Laboratories, Inc has released a tutorial document titled “A Budget-Conscious Radio Telescope for 21cm“, (doc version) (pdf here) where he shows:

Two slightly-different designs for a simple, small, effective, radio telescope capable of observing the Sun, and the galactic plane in both continuum and spectral modes, easily able to show the hydrogen line in various parts of the galactic plane.

He uses the RTL-SDR as the receiving radio with an LNA (low noise amplifier) and a couple of line amps, a 93cm x 85cm offset satellite dish (potential dish for sale here, and here), and GNU Radio with the simple_ra application. In his results he was able to observe the spectrum of the Galactic Plane, and the Hydrogen Line. Some more information about this project can be found on this Reddit thread.

Here is a link to an interesting gif Marcus made with his RTL-SDR, showing a timelapse of recorded hydrogen emissions over 24 hours. Reddit user patchvonbraun (a.k.a Marcus Leech) writes on this thread an explanation of what is going on in the gif.

Interstellar space is “full” of neutral hydrogen, which occasionally emits at photon at a wavelength of 21cm–1420.4058Mhz.

If you setup a small dish antenna, and point at a fixed declination in the sky, as that part of the sky moves through your beam, you can see the change in spectral signature as different regions, with different doppler velocities move through your beam.

This GIF animation shows 24 hours of those observations packed into a few 10s of seconds.

 Marcus’ setup is shown below.

RTL-SDR Radio Telescope Setup

And here is just one of his many resulting graphs shown in the document showing the Hydrogen line.

RTL-SDR Radio Telescope Hydrogen Line

A similar radio astronomy project has previously been done with the Funcube. More information about that project can be found in this pdf file. In that project they used the Funcube, a 3 meter satellite dish and the Radio Eyes software.

However, in this Reddit post patchvonbraun explains that the Funcube’s much smaller bandwidth is problematic, and so the rtl-sdr may actually be better suited for radio astronomy.

This image is from the Funcube project document.

Funcube Radio Telescope Project

Another related project is the Itty Bitty Telescope (IBT), which does not use SDR, but may be of interest.

Meteor Scatter

Meteor scatter works by receiving a distant but powerful transmitter via reflections off the trails of ionized air that meteors leave behind when they enter the atmosphere. Normally the transmitter would be too far away to receive, but if its able to bounce off the ionized trail in the sky it can reach far over the horizon to your receiver. Typically powerful broadcast FM radio stations, analog TV, and radar signals at around 140 MHz are used. Some amateur radio enthusiasts also use this phenomena as a long range VHF communications tool with their own transmitted signals. See the website www.livemeteors.com for a livestream of a permanently set up RTL-SDR meteor detector.

In Europe typically the Graves radar station can be used for meteor scatter experiments. Graves is a space radar based in France which is designed to track spacecraft and orbital debris. If you are in Europe you can also make use of the Graves radar simply by tuning to its frequency of 143.050 MHz and listening for reflections of its signal bouncing off things like meteors, planes and spacecraft. Since Graves points its signal upwards, it’s unlikely that you’ll directly receive the signal straight from the antenna, instead you’ll only see the reflections from objects.

In other countries old and distant analogue TV stations can be used or FM transmitters can also be used.

To set meteor scatter up, simply use an outdoor antenna to tune to a distant transmitter. It should be far enough away so that you can not be receive the transmitter directly, or the signal should be weak. If you detect a meteor the signal will briefly show up strongly at your receiver. Performance can be enhanced by using a directional antenna like a Yagi to point upwards at the sky in the direction of the transmitter.

We have several post about meteor scatter available on the blog here. Read through them to get a better understanding of the ways in which it can be monitored. You may also be interested in Marcus Leech’s tutorial where he uses the RTL-SDR to detect forward meteor scatter. (doc here) (pdf here)

Pulsar Observing

A pulsar is a rotating neutron star that emits a beam of electromagnetic radiation. If this beam points towards the earth, it can then be observed with a large dish antenna and a radio, like the RTL-SDR. 

Pulsars create weakly detectable noise bursts across a wide frequency range. They create these noise bursts at precise intervals (milliseconds to seconds depending on the pulsar), so they can be detected from within the natural noise by performing some mathematical analysis on the data. Typically a few hours of data needs to be received to be able to analyze it, with more time needed for smaller dishes.

One problem is that pulsar signals can suffer from ‘dispersion’ due to many light years of travel through the interstellar medium. This simply means that higher frequencies of the noise burst tend to arrive before the lower frequencies. Mathematical de-dispersion techniques can be used to eliminate this problem enabling one to take advantage of wideband receivers like the RTL-SDR and other SDRs. The more bandwidth collected and de-dispersed, the smaller the dish required for detection.

Pulsar detection requires some pretty large antennas, and a good understanding of the techniques and math required for data processing so it is not for the beginner. See the previous Pulsar posts on this blog for more information.

If you enjoyed this tutorial you may like our ebook available on Amazon.

The Hobbyist’s Guide to the RTL-SDR: Really Cheap Software Defined radio.