Category: Radio Astronomy

Paper on Building a Low Cost RTL-SDR Based Hydrogen Line Radio Telescope

Back in 2020, we posted a tutorial on how to set up a low-cost Hydrogen Line radio telescope using an RTL-SDR, LNA, and WiFi grid antenna. Since then we have seen similar setups successfully replicated in the community many times.

In a recent Hackaday post, we discovered a paper by Jack Phelps who has written an in-depth technical and scientific description of his attempt at Hydrogen line radio astronomy with similar equipment. His paper goes into deeper scientific explanations and describes the experiment and hardware setup in detail including some signal processing, observation, and calibration equations that might be useful for those looking to understand the science more deeply. 

Jack Phelps Radio Astronomy Setup and some Results
Jack Phelps Radio Astronomy Setup and some Results

Project H Line 3D: YouTube Talk

Last week we posted about Alex Petit Jr's 'Project H Line 3D' which is a collection of documents and programs designed to be a beginner's guide to antenna fabrication, reception, recording, software processing, and graphic display of the 21 cm Hydrogen line. The project makes use of an RTL-SDR and LNA as the radio front end.

This week Alex gave an online talk to the Society of Amateur Radio Astronomers (SARA) discussing the project and giving an overview.

Project H Line 3D' is a collection of documents and programs designed to be a beginner's guide to antenna fabrication, reception, recording, software processing, and graphic display of the 21 cm Hydrogen line. The project makes use of an RTL-SDR and LNA as the radio front end.

The Hydrogen Line is an observable increase in RF power at 1420.MHz that is created by natural hydrogen atoms. The Hydrogen line is most easily detected by pointing a directional antenna toward the Milky Way where neutral hydrogen is abundant. Properties of the hydrogen line curve such as its shape and Doppler shift can be used to measure the shape and properties of our galaxy.

Alex's project H Line build is designed to be inexpensive and easy for students to build and set up for drift scans which involve pointing the antenna towards the sky and letting the Earth's rotation drift the Milky Way into and through the view of the antenna.

The project includes a design for a 13-element circular patch feed Yagi that can be built using common materials available from a hardware store. The 13-element Yagi results in about 15dBi gain and a 30-degree 3dB bandwidth.

The software portion of the instructions uses the SDR# IF Average plugin, and uses that to record log files every few minutes. The log files are then converted by an included Java program by Jamison Adcock into a logarithmic dB scale and a format compatible with Rinearn 2D and 3D graphics packages.

Alex Pettit & Jamison Adcock: H Line 3D project

Project H Line 3D: Beginners Guide to Hydrogen Line Antenna Fabrication, Reception, Software and Graphic Display

Thank you to Alex Petit Jr who wanted to submit 'Project H Line 3D' which is a collection of documents and programs designed to be a beginners guide to antenna fabrication, reception, recording, software processing, and graphic display of the 21 cm Hydrogen line. The project makes use of an RTL-SDR and LNA as the radio front end.

If you were unaware, the Hydrogen Line is an observable increase in RF power at 1420.4058 MHz that is created by natural hydrogen atoms. The Hydrogen line is most easily detected by pointing a directional antenna toward the Milky Way where neutral hydrogen is abundant. Properties of the hydrogen line curve such as its shape and Doppler shift can be used to measure the shape and properties of our galaxy.

Alex's project H Line build is designed to be cheap and easy for students to build and set up for drift scans which involve pointing the antenna towards the sky and letting the Earth's rotation drift the Milky Way into view of the antenna.

The project includes a design for a 13-element circular path feed Yagi that can be built using common materials available from a hardware store. Alex started with a Yagi design using circular director elements but found these difficult to find and fabricate. However, through NEC antenna analysis software he found that replacing the circular elements with more commonly found and easier-to-fabricate square elements had a negligible effect on the antenna's performance, unlocking a cheaper build. The 13-element Yagi results in about 15dBi gain and a 30-degree 3dB bandwidth.

Plate Yagi gives an almost identical Hydrogen line detection as the Disk Yagi
Plate Yagi gives an almost identical Hydrogen line detection as the Disk Yagi

The software portion of the instructions uses the SDR# IF Average plugin, and uses that to record log files every few minutes. The log files are then converted by an included Java program by Jamison Adcock into a logarithmic dB scale and a format compatible with Rinearn 2D and 3D graphics packages.

Project H Line 3D
Project H Line 3D

The Latest Talks from the Society of Amateur Radio Astronomers

Over on YouTube a bunch of new talks from the Society of Amateur Radio Astronomers (SARA) have recently been uploaded from their recent SARA Western Conference that was held in April 2024. The talks typically involve small home-based radio astronomy setups that use small satellite or WiFi dishes and RTL-SDR or similar low-priced SDRs in their setup. Some of the latest talks include:

  • Nathan Butts: A Novice's Guide to Radio Astronomy (Link)
  • Dr Andrew Thornett: Detecting Cosmic Rays & Building your own version of the Large Hadron Collider (Link)
  • Dr Andrew Thornett M6THO: Lichfield Radio Observatory - Mapping Milky Way at 1420.405 MHz (Hydrogen) (Link)
  • Bruce Randall: IBT Eclipse and other Radio astronomy Failures (Link)
  • Felicia Lin: Mapping the Milky Way by Cross Section Data (Link)
  • Kent Britain WA5VJB: Antennas for Radio Astronomy (Link)
  • Charles Osborne: Eclipse Detection using a VLF Receiver (Link)
  • Rob Lucas - Eclipse Research (Link)
  • Dr Wolfgang Herrmann: Lunar Occultation Observation of Radio Sources (Link)
  • Keynote: Dr Linsay King - Gravitational Lensing (Link)

We note that the last talk was uploaded only a few hours ago at the time of this post, so we're not sure if more talks are yet to be uploaded. So please keep an eye on the SARA YouTube videos page.

Nathan Butts: A Novice's Guide to Radio Astronomy

A Tribute to Amateur Radio Astronomer Job Geheniau (Job’s Radio Telescope)

Job Geheniau was someone whose amateur radio astronomy projects were often featured on RTL-SDR Blog (often referred to as Job's Radio Telescope). It with great sadness that we have recently learned that Job Geheniau passed away from cancer in late December 2023. We would like to take the time share this post to highlight some of his achievements in the amateur radio astronomy field.

Back in 2020 Job first surprised us with one of his first radio astronomy results (Part 1, Part 2) where he was able to image the Milky Way in neutral hydrogen by using a 150cm dish, RTL-SDR, LNA and motorized mount. Over eight nights he recorded hydrogen line readings throughout the Milky Way and ended up creating a 2D Excel sheet that showed an image of the Milky Way at the 1420 MHz hydrogen line frequency. 

Job would go on, rapidly evolving and each time showing us that low cost hardware set up in a backyard could be used to unlock many of the secrets of the universe. Using a satellite dishes less than two meters in diameter, RTL-SDRs, LNAs and filters he was able to:

Job's Radio Astronomy website remains up at https://jgeheniau.wixsite.com/radio-astronomy, and many results and writeups of his other experiments can be found there. We will sorely miss posting about Job's achievements, but we hope that his life has inspired you to take a closer look at the amateur radio astronomy hobby.

A tribute to Job will also be published in the next membership journal from the Society of Amateur Radio Astronomers.

Crowd Supply Discovery Dish Teardown Session: Thursday 30 November Noon PST

Crowd Supply is hosting Teardown Session 38 on Thursday 3- November at Noon PST time which will feature the Discovery Dish. Join us for this livestream where I will be talking about and showing the Discovery Dish prototype.

Discovery Dish is currently being crowd funded over on Crowd Supply. It is designed to be an easy entry to the world of L-band weather satellites, hydrogen line radio astronomy, and Inmarsat reception. The Discovery Dish aims to be the start of an ecosystem of hardware designed to get users set up with satellite reception, including a planned companion light-duty antenna rotator.

Remember to click on the “Notify me” button on the YouTube link in order to be reminded about the stream!

Teardown Session 38: Discovery Dish

TotalPower: Windows Program for Hydrogen Line Detection and Analysis with an RTL-SDR

Thank you to Mario A. Natali (I0NAA) who wrote in an wanted to share his Windows software called TotalPower which is designed for mapping the galactic Hydrogen line and works with RTL-SDR dongles.

The Hydrogen Line is an observable increase in RF power at 1420.4058 MHz which is created by Hydrogen atoms. It is most easily detected by pointing a directional antenna towards the Milky Way as there are many hydrogen atoms in our own galaxy. This effect can be used to measure the shape and other properties of our own galaxy.

Mario writes:

[TotalPower] was originally designed to measure total power of received spectrum and that, thanks to the input of many users, is now able to perform many other tasks including the 3D mapping of selected sky areas and HLine detection with the ability to estimate the speed of rotation of galaxy arms ( respect to our position )

TotalPower is available from the downloads section on Mario's website. Mario has uploaded a manual which explains how the program works, which we have mirrored here.

TotalPower measuring the rotational speed of galactic arms
TotalPower measuring the rotational speed of galactic arms

Discovery Dish Now Available for Crowd Funding! A Lightweight Dish and Feed for L-Band Weather Satellites, Hydrogen Line and Inmarsat

Today our Crowd Funding campaign for the Discovery Dish has gone live! Thank you to anyone who supports this project and our goal of bringing affordable products that make getting into various radio projects easier.

Our launch announcement reads:

We decided to develop Discovery Dish because we were disappointed by the lack of ready-to-use, low-cost, lightweight dish antennas on the market that are suitable for software-defined radio projects like receiving L-Band geostationary and polar-orbiting weather satellites, as well as for 1.5 GHz Inmarsat reception and 1.42 GHz hydrogen line radio astronomy. With excellent open source weather satellite decoding software, like SatDump, now available, it’s time for a complementary, easy-to-use hardware solution.

Through testing over several years, we chose 65 cm as the diameter, as we found that 60 cm is close to the minimum diameter required for perfect GOES weather satellite reception at 24° elevation, so this size should be suitable for most of the world that has GOES reception available. For LRPT satellites like GK-2A, and HRPT polar-orbiting satellites, it is more than large enough. We combined the dish with a carefully tuned feed that has a built-in low-noise amplifier (LNA) and dual filtering, which means there is no loss from feed to LNA. This also means we can use thinner and less stiff coax cable, which is a lot easier to handle and route. Finally we ensured that the entire dish and feed system is waterproof.

The only other ready-to-use dish offering we found is based on a modified 2.4 GHz grid Wi-Fi dish, which is still in our opinion too big and heavy. Size and weight is especially the important if you want to be able to use a low-cost, light-duty antenna rotator, which typically can only handle less than 1 kg in weight. We found that the grid Wi-Fi dish offering also has no solution for waterproofing the LNA, so the LNA needs to be placed indoors and very thick and unwieldy coax is used to avoid feed to LNA losses.

Other ways to receive these weather satellites and carry out hydrogen line experiments typically involve modifying a 2.4 GHz Wi-Fi grid antenna, or an old satellite TV dish. But these modifications can be time-consuming and difficult to get right, and even 60 cm satellite TV dishes are too heavy for light-duty antenna rotators.

Finally, we developed Discovery Dish with an eye toward it being used with a low-cost antenna rotator, and we are in the process of prototyping our own rotator design. Our antenna rotator is not ready for crowdfunding yet, as there are still some things to work out and long-term stress testing to be done, but please keep an eye out for it in 2024! An antenna rotator is a great addition if you want to use a dish antenna to decode images from the polar-orbiting HRPT weather satellites.

Note that you don’t need an antenna rotator to receive geostationary satellites like GOES, or to do drift hydrogen line observations. For polar-orbiting HRPT satellites, the lightweight nature of Discovery Dish also makes tracking the satellites by hand a much easier prospect.

Learn more about Discovery Dish on our main campaign page. Thank you to everyone who supports the Discovery Dish project in any way!

Discovery Dish: Simplified system for weather satellite reception and hydrogen line radio astronomy