Tagged: hydrogen line

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.

Helping to Raise Funds for the Canadian Centre for Experimental Radio Astronomy (CCERA)

Patchvonbraun (aka Marcus Leech) is one of the pioneers in using low cost SDR dongles for amateur radio astronomy experiments. In the past he’s shown us how to receive things like the hydrogen line,  detect meteors and observe solar transits using an RTL-SDR. He’s also given a good overview and introduction to amateur radio astronomy in this slide show.

Now Marcus and others are starting up a new project called the “Canadian Centre for Experimental Radio Astronomy (CCERA)”. They write that this will be an amateur radio astronomy research facility that will produce open source software and hardware designs for small scale amateur radio astronomers. Currently they also already have a hydrogen line telescope set up, which is producing live graphs and data. From their recent posts it also looks like they’re working on building antennas for pulsar detection. They also have a GitHub available for any software they produce at https://github.com/ccera-astro.

Currently CCERA is looking for donations over at gofundme, and they are hoping to eventually raise $25k. They write:

About CCERA:

Radio astronomy is one of the most important ways to observe the cosmos. It is how we learned about the existence of the afterglow of the big bang (the cosmic microwave background), it is how we observe huge swaths of the universe that are otherwise obscured by dust. Most of what’s going on out there can’t be seen with visible light.

Astronomy has traditionally been one of the areas in science where dedicated non-professionals have continued to make an enormous contribution to the field. Optical astronomy requires little more than a telescope and knowledge.

Radio astronomy has, up until recently, required a lot more skill and resources. However, technology has advanced enough that small groups could be making serious contributions to radio astronomy. With the right sorts of software and information, many dedicated non-professionals could be doing good work in the area, and CCERA intends to help make that a reality.

CCERA will be producing open source software and hardware designs to help non-professional and professional radio astronomers alike, documenting them, and helping people get up to speed so that they can use these powerful tools themselves. Our GitHub repository is: https://github.com/ccera-astro

CCERA will also be operating its own radio astronomy facilities, initially in Ontario, Canada. These will serve as a test-bed for our own designs, as a place for us to train interested people in the operation of low cost radio astronomy equipment, and will also be used for real radio astronomy work. All our data will be publically-available.

About us:

Roughly 10 years ago, I and a number of others started a project to restore a large, historic, satellite earth station antenna at Shirleys Bay in Ottawa. Our goal was to bring the dish back on-line for use in amateur radio astronomy, research, and importantly, educational outreach about science, and radio astronomy.

The project came to a sudden end back in 2013/14 when the owner of the dish (The Canadian Space Agency) needed to dismantle it to make way for other occupants of the site.

However, during that period, we became fascinated with the possibilities that opening up radio astronomy to skilled non-professionals could bring.

Since then, our group has been working on another far lower cost project to build our own a specialized radio telescope somewhere in the Rideau Valley area. Many of our group live in the area, and Marcus lives in Smiths Falls. With good attention to the usability of our designs and open publication of our tools under appropriate open source licenses, our work should be replicable by others. We thus hope to kick off a new era in non-professional radio astronomy.

What we need the money for:

We’ve secured a small office in the Gallipeau Center outside of Smiths Falls, and will be able to erect our specialized antenna arrays over the coming year.

While we have a lot of the equipment we’ll need, we’ll have more equipment to buy, and on-going expenses to cover, including rent, insurance, miscellaneous mechanical construction materials (lumber, metal, etc). We also need to cover expenses relating to incorporation as a not-for-profit.

Our goal is to provide a test facility for small-scale radio astronomy research, and to develop techniques that allow small organizations and educational institutions to run their own small-scale radio astronomy observing programs.

If we are successful, in addition to making our designs and software available under open source licenses, we’ll be holding regular public lectures, host training seminars, host school groups, etc. We will also produce videos of our work for those who cannot visit us directly in Ottawa. We want to make some of the techniques of “big science” accessible and understandable.

We can’t do it without the help of the public, who, we hope, will become our students, collaborators, and ongoing supporters.

We will also make all of our data available to the public without fee or restrictions. We believe in openness in scientific endeavours, even small ones such as ours.

Marcus Leech
(tentative) Director
Canadian Centre for Experimental Radio Astronomy
www.ccera.ca

If you have even a passing interest in radio astronomy please consider donating, as CCERA’s work may open up exciting new possibilities for amateur radio astronomers with low cost SDR dongles.

The pulsar antenna being built at CCERA.
The pulsar antenna being built at CCERA.

IF Average SDR# Plugin Updated

The IF Average tool is a RTL-SDR compatible plugin for SDR# which allows you to plot an average of the current spectrum shown in SDR#. This is especially useful for radio astronomers who often need to average the spectrum for a long time in order to get a good plot of the Hydrogen Line. Recently the plugin was updated to support newer versions of SDR# and to upgrade some features. Daniel Kaminski, the author of the plugin writes:

I used ultrafast FFT which works on 4k to 512k bit space. With this plugin it is possible to average up to 64000000 samples in real time. XNA allows to shows the calculation results in real time.

To install the plugin you will need to install the XNA Framework 4.0 Redistributable first. Then copy the plugin files over to the SDR# folder and add the “magicline” to the SDR# Plugins.xml file.

The IF Average SDR# Plugin
The IF Average SDR# Plugin

Hydrogen Line Observation with an RTL-SDR

The RTL-SDR can be used for many interesting radio astronomy applications such as observing the Hydrogen line. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will 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.

On his website Steve Olney has been writing about his experiments and results with using an RTL-SDR to observe the hydrogen line. On his website he writes that he uses a 3M dish, with an LNA at the antenna to reduce the system NF, a hydrogen line tuned bandpass filter to remove out of band noise, 2 line amps to overcome coax loss, and finally a second LNA just before the RTL-SDR dongle to optimize the signal strength for the ADC. The dongle he uses has been modified to use a TCXO, and is aircooled via a PC fan. He also uses a modified version of the rtlsdr.exe IQ file recorder and his own custom GUI for controlling the RTL-SDR and antenna tracking mechanism.

His results show that he was able to detect the Hydrogen in the Large and Small Magellanic clouds. He also shows a method for converting the 8-bit IQ data down to 1-bit to save disk space, and shows that while some noise is added, the overall result is preserved.

See the related posts for other hydrogen line experiments with the RTL-SDR.

The 3M dish used for hydrogen line detection.
The 3M dish used for hydrogen line detection.
The fan cooled RTL-SDR used to detect the Hydrogen line.
The fan cooled RTL-SDR used to detect the Hydrogen line.

Some new RF filters from Adam 9A4QV

Adam 9A4QAV is mostly known as the manufacturer of the popular LNA4ALL, a low cost low noise amplifier which is often used together with the RTL-SDR to improve reception of weak signals. He also sells an ADS-B bandpass filter and an ADS-B antenna, the latter of which we reviewed in a previous post.

Now Adam has come out with two new RF bandpass filters which are for sale. RF filters are used to block unwanted interference from other strong signals which can cause trouble, especially with low cost receivers such as the RTL-SDR. 

The first new filter that he has developed is for FLARM (FLight Alarm System). FLARM broadcasts at 868 MHz and is a protocol similar to ADS-B. It is used by Gliders and some Helicopters for collision avoidance. It is possible to decode FLARM with an RTL-SDR which allows you to track gliders on a map, as discussed in one of our previous posts.

Characteristics of Adam's FLARM Filter.
Characteristics of Adam’s FLARM Filter.

The second filter is for amateur radio astronomers who wish to detect the Hydrogen Line at 1420 MHz. Hydrogen molecules in space occasionally emit a photon at 1420 MHz. A single emission can’t be easily detected, but space and the galaxy is full of Hydrogen and the net result is an observable RF power spike at 1420 MHz. This can be detected with a high gain antenna, LNA, RF filter and radio like the RTL-SDR. The Hydrogen line can be used to measure things like the rotation and number of arms in our galaxy. Filters are very important for radio astronomy work as man made interference can easily drown out the relatively weak cosmic signals.

Characteristics of Adam's Hydrogen Line Filter.
Characteristics of Adam’s Hydrogen Line Filter.

Adam sells all his fully assembled filters for 20 euros, plus 5 euros worldwide shipping.

One of the ADS-B/FLARM/HLine Filters by Adam 9A4QAV.
One of the ADS-B/FLARM/HLine Filters by Adam 9A4QAV.

Observing the 21cm Hydrogen Line with Linrad and an RTL-SDR

Over on YouTube user S53RM has uploaded a video showing his and S53MM’s observation of the 1420 MHz galactic hydrogen line with an RTL-SDR. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will 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, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot.

In the video they rotate their 3.6m parabolic mesh antenna dish along the Milky Way. As the dish rotates doppler shifted hydrogen line peaks can be observed on Linrad, each peak representing a different arm of the galaxy. The galaxy consists of several spinning arms, some spinning faster than others which causes the hydrogen line peaks produced by the arms to be doppler shifted by different amounts.

They used Linrad to plot the RF spectrum as they were able to use it together with a pulse generator to calibrate the RTL-SDR for a flatter frequency response.

More information about their project can be found at http://lea.hamradio.si/~s53rm/Radio%20Astronomy.htm.

Linrad showing Galactic Arm Hydrogen Line Peaks
Linrad showing Galactic Arm Hydrogen Line Peaks

Low Cost Hydrogen Line Telescope using the RTL-SDR

Amateur radio astronomer Y1PWE has uploaded a pdf document describing how he created a low cost hydrogen line telescope using an RTL-SDR dongle. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). Normally a single hydrogen atom will 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, a power spike indicating the hydrogen line can be observed in a frequency spectrum plot.

Y1PWE created a radio telescope using a quad 22 element yagi antenna, several LNA’s and filters and an RTL-SDR dongle and laptop. Using this setup he can capture some raw IQ data from the RTL-SDR and then use an FFT averaging program to produce some plots. In his plots the hydrogen line is clearly visible.

Radio Telescope Overview
Radio Telescope Overview
Hydrogen Line Plots
Hydrogen Line Plots
Quad Yagi Array
Quad Yagi Array

Horn Antenna for RTL-SDR Radio Astronomy

Rishi Patel has written on his blog about his radio astronomy project which involves an RTL-SDR and an impressive homemade horn antenna.

The horn antenna is constructed from cardboard foam and aluminium foil and is mounted on a telescopic mount. The horn then connects to a waveguide feed which is constructed out of a large metal can. The antennas waveguide then connects to a microstrip filter and LNA before finally connecting to an RTL-SDR. Rishi then uses a simple python program to gather the IQ samples from the RTL-SDR, and then draw the power spectral density plots.

Horn Antenna
Homemade Horn Antenna for Radio Astronomy

With this setup Rishi was able to detect the Hydrogen line. Rishi also used a second even larger horn with a different design to plot the spectrum shown below of the Cygnus (red), Cassiopeia (green), and Cepheus (blue) constellations.

largeHornSpectra
Spectral Plots of Cygnus (red), Cassiopeia (green), and Cepheus (blue) constellations.