NooElec have recently released a new LNA + filter combo called the "SAWbird+ H1 Barebones" which significantly lowers the entry bar for new amateur radio astronomers. It's designed to be used with RTL-SDR or other SDRs for radio astronomy, and in particular reception of the Hydrogen line.
The filter is centered at 1.42 GHz with a 70 MHz bandpass region. The LNA has a minimum gain of 40dB. For hydrogen line observations it is important that the LNA have very low noise figure, and this LNA fits the bill with a ~0.5dB to ~0.6dB noise figure. An additional feature on the PCB is an RF switch that is electrically controlled via expansion headers. This switch allows you to switch out the LNA for a 50 Ohm reference which is useful for calibration in more serious radio astronomy work.
This LNA draws 120mA of current meaning that it will work with the RTL-SDR V3 and Airspy's bias tee, but probably not with the SDRplay's bias tee which is limited to 100mA and seems to trip a fuse at higher current draws. For an SDRplay you could use external power instead, although you will need an additional DC blocking cap to prevent power from entering the SDR and destroying the ESD diodes.
If you don't know what the Hydrogen line is, we'll explain it here. 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 space and the galaxy is filled with many hydrogen atoms so 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. You can also measure the rotational speed of our galaxy by noting the frequency doppler shift.
Although this LNA lowers the entry bar, in order to receive the Hydrogen line with the SAWBird+ H1 you will still need a ~1m+ satellite dish and a feed tuned to 1.42 GHz or high gain Yagi, horn or helical antenna. Antennas and feeds like this are not yet available off the shelf, but if you search our blog for "hydrogen line" you'll see many project examples.
PICTOR is an open source and open hardware radio telescope that aims to promote radio astronomy on a budget. It consists of a 1.5 meter parabolic dish antenna, 1420 MHz feedhorn, a two stage low noise amplifier (LNA), high pass filter, and from what we gather, an RTL-SDR. Future designs may also use higher bandwidth SDRs. Currently there doesn't seem to be much information about the build and exact components used in their design, but we're hoping that those details will come in time.
The radio telescope allows a user to measure hydrogen line emissions from our galaxy. Hydrogen atoms randomly emit photons at a wavelength of 21cm (1420.4058 MHz). The emissions themselves are very rare, but since our galaxy is full of hydrogen atoms the aggregate effect is that a radio telescope can detect a power spike at 21cm. If the telescope points to within the plane of our galaxy (the milky way), the spike becomes significantly more powerful since our galaxy contains more hydrogen than the space between galaxies. Radio astronomers are able to use this information to determine the shape and rotational speed of our own galaxy.
PICTOR also has a very interesting web based interface which can be used to let users from anywhere in the world access the telescope and log an observation. The first PICTOR telescope is currently online and observations can be created simply by going to their website, and clicking on the "Observe" link. Users can then enter the frequency and other parameters for their observation, and the resulting graph will be emailed to you after the observation. The software source is available on their GitHub page, and is based on a GNU Radio flowgraph and Python plot script.
For more information about PICTOR, logging an observation, and radio astronomy in general, we recommend checking out their PDF guide. We test ran a short observation at the hydrogen line frequency, and we received a graph with the hydrogen line peak clearly visible (spliced in to the photo below). We note that the wavy shape is due the to shape of the filters they used.
Pulsars are known to have very accurate rotational periods which can be measured by the radio pulse period. However, every now and then some pulsars can "glitch", resulting in the rotational period suddenly increasing. Glitches can't be predicted, but Vela is one of the most commonly observed glitching pulsars.
The HawkRAO amateur radio telescope run by Steve Olney is based in NSW, Australia and consists of a 2 x 2 array of 42-element cross Yagi antennas. The antennas feed into three LNAs and then an RTL-SDR radio receiver. He has been observing the Vela pulsar for 20 months.
His observations indicate that Vela glitched and spun up by 2.5PPM at 14:09 UTC on Feb 1, 2019. He claims that this glitch detection is a first for amateur radio astronomy as far as he is aware.
If you're interested in Pulsar detection, check out a few of our previous posts on the topic.
Last week we posted about some videos of talks from the 2018 GNU Radio Conference which had been release on YouTube. This week a few more videos have been released and we display a small selection below. The full collection of videos can be found on their YouTube channel.
RF Ranging with LoRa Leveraging RTL-SDRs and GNU Radio
Wil Myrick discusses the use of RTL-SDRs and GNU Radio to create a low cost LoRa RF ranging prototype, to aid in the localization of IoT transmitters.
GRCon18 - RF Ranging with LoRa Leveraging RTL SDRs and GNU Radio
Using GNU Radio and Red Pitaya for Citizen Science
Robert W McGwier discusses the use of Red Pitaya SDRs and GNU Radio for use in citizen science ionosphere measurement experiments.
GRCon18 - Using GNU Radio and Red Pitaya for Citizen Science
SETI Breakthrough Listen
Steve Croft discusses the Search for Extraterrestrial Intelligence (SETI) project and how software defined radio is being used in the search.
Thank you to Florent for submitting his website which contains a live log of his meteor scatter observations. Meteor scatter occurs when radio signals reflect off the ionized trail left behind by meteors when they enter the atmosphere. This trail is highly RF reflective, so it can allow distant radio stations to be briefly received.
His set up consists of an RTL-SDR dongle running on a Raspberry Pi 3. His antenna is a homemade 6 element Yagi. Florent is based in France and listens for reflections from the Graves radar at 143.05 MHz. His software captures 768 Hz worth of bandwidth every 0.5s, and then uploads and displays the spectrum plot on his website. When the Graves radar signal is visible on the spectrum, it is an indication of a meteor having entered the atmosphere (or possibly an aircraft).
If you are interested in other peoples live meteor scatter streams, then there is another site at livemeteors.com which displays a live video of an SDR# screen looking for meteor echoes.
GNU Radio is a very powerful open source platform for implementing various digital signal processing (DSP) algorithms. It is very commonly used with software defined radios like the RTL-SDR, as well as much higher end units. The community that uses GNU Radio is very large, and so every year they hold a conference that highlights some of the most interesting applications and developments related to GNU Radio. The 2018 GNU Radio conference was held in Las Vegas during September 2018. Recently they have uploaded the talks to YouTube, and below we're posting some of our favorites. The full list can be found on their YouTube channel.
Keynote Talk: SatNOGs
In this keynote talk Manolis Surligas discusses the SatNOGs project. SatNOGs is a non-profit organization creating an open source and volunteer based satellite ground station network.
GRCon18 - Keynote: SatNOGs
Open Source Radio Telescopes
John L. Makous discusses his work in creating low cost and home made horn antenna radio telescopes designed to receive the 21cm hydrogen line and other astronomical objects and phenomena. The idea is to provide a low cost solution and easy to build telescope to use in schools.
GRCon18 - Open Source Radio Telescopes
Enter the Electromagic Spectrum with the USRP
Nate Temple gives us an overview of several signals that have been decoded with GNU Radio flowgraphs.
GRCon18 - Enter the Electromagic Spectrum with the USRP
Software Defined Radar Remote Sensing and Space Physics
Juha Vierinen discusses using a USRP to measure propagation conditions with ionospheric chip sounders, and improvements to chirp sounders by using spread spectrum noise. He also discusses various other radar techniques and applications.
GRCon18 - Software Defined Radar Remote Sensing and Space Physics
A corner reflector antenna is basically a monopole antenna with a metallic 'corner' reflector placed behind it. The reflector helps the monopole collect signals over a wider aperture resulting in signals coming in stronger from the direction that the corner is pointing at. In past posts we've seen a homemade tinfoil corner reflector used to improve reception of the generic stock RTL-SDR monopole antenna, and a larger one was used in a radio astronomy experiment to detect a pulsar with an RTL-SDR.
Recently The Thought Emporium YouTube channel has uploaded a video showing how to build a large 2 meter 3D corner reflector out of readily available metal conduit pipes and chicken wire. While the antenna has not been tested yet, they hope to be able to use it to receive weather satellite images from GOES-16, to receive moon bounce signals, to map the Hydrogen line and to detect pulsars.
Building a Giant 2m Corner Reflector Antenna For Less than $200 (For Goes-16, Pulsars and More!)
Hydrogen tends to emit radio signals in the 21cm (1.4 GHz) region of the frequency spectrum. An emission from a single Hydrogen atom is very rare, but since there is so much Hydrogen in space a bump at 1.4 GHz can be observed on the frequency spectrum if a sensitive radio is used with a directional antenna pointing up at the sky. This is a moderate difficulty experiment that can be performed by amateur radio astronomers today with cheap RTL-SDRs or other SDRs together with some LNAs.
The astronomers in this experiment focus on a distortion in the 21cm line signal that is expected to have been created when the first stars formed. The their paper they write:
After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz.
The results show a successful detection of the expected phenomena at 78 MHz, confirming the age at when the first stars have been predicted to have begun forming. The phenomena is detected at 78 MHz instead of 1.4 GHz because the wavelength of a Hydrogren line signal gets stretched the further the source is from us, due to the redshift doppler effect from the expansion of the Universe. This detection is from some of the furthest (and thus oldest) stars in the Universe, so a big stretch is expected.
The experiment consisted of a broadband blade dipole which was set up in the Australian outback. Since the cosmic signal is expected to be detected right in the middle of the broadcast FM band, a dedicated radio-quiet location is required to stand any chance of detection. The receiving SDR hardware consists of an LNA, line amp, filtering and a 14-bit ADC that is connected to a PC.
It seems possible that this experiment could be repeated by amateur radio astronomers with commercial SDR hardware, but the biggest challenge would probably be finding a very radio-quiet location without broadcast FM radio signals.