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Imaging the Milky Way in Neutral Hydrogen with an RTL-SDR Part 2

Last month we shared information about Job Geheniau's success with using an RTL-SDR dongle to image our galaxy in neutral Hydrogen. Our galaxy is full of neutral Hydrogen, and lots of neutral Hydrogen together results in a detectable radio peak at 1.42 GHz. This peak is called the Hydrogen line. By scanning the galaxy at the Hydrogen line frequency with a 1.5 meter dish on a motorized mount, an RTL-SDR, and a few filters and LNAs, Job is able to create a radio image of our galaxy.

In Job's previous attempt he created an image by pointing the dish antenna at 168 predefined grids calculated to cover the Milky Way, resulting in 168 points of exposure data. In his latest work Job has created an even higher resolution image by taking 903 points of exposure data. Each exposure took 150s and the total 903 exposures took 8 nights to record. Once all data was collected he uses the same process as before, which is to input all the Hydrogen line data into a standard 2D excel sheet, then use conditional formatting to create a heatmap which reveals the image. He then applies a blur and stretches the image into the Mollweide Cartographic which can represent the entire Universe in one image.

Job has shared with us his PDF where he documented his process and shares his images (note 16 MB PDF file). We also share his full resolution images below, just click to open. We think that these images are quite amazing and an excellent achievement for a backyard radio astronomer.

If you're interested in Hydrogen line radio astronomy we have a tutorial that will help you observe the Hydrogen line peak on a budget. The tutorial could be improved upon by motorizing the dish, allowing you to create images like the ones above. You might also be interested in a similar project by Marcus Leech who took 5 months of hydrogen line observations with an RTL-SDR in order to create an even higher resolution image.

An Introduction to Radio Trunking Systems with a Focus on P25 and SDR Trunk

Thank you to Carl Makin (VK1KCM) for submitting a video that he produced for his local ham radio club in Australia. In the video Carl first gives an overview on radio trunking systems and explains why they are used to improve spectrum efficiency.

He goes on to focus solely on P25 digital voice trunking networks. Carl is based in NSW, Australia so he talks a bit about what P25 services are available in his area and which ones are unencrypted. Finally he demonstrates the SDR Trunk software decoding one of his local P25 networks with two RTL-SDR dongles, and explains what information we can see in the software.

Carl VK1KCM on P25 Trunked

A Simple Guide to Setting up a DIY NOAA Weather Satellite Ground Station

A few weeks ago we posted about Sophie Dyer and Sasha Engelmann’s work in creating an artistic performance based on weather satellite reception with SDRs. More recently they have uploaded their own tutorial showing how they receive NOAA APT weather satellite images with an SDR, turnstile antenna and computer. Sasha and Sophie note that they are attempting to create visually rich guides that don’t assume any prior knowledge of radio, science or engineering.

From Sasha’s Twitter feed we note that they are also working on upcoming public workshops in the UK and Germany on the topic of reflections on what it means to bring an intersectional feminist ethos to satellite image decoding + weather sensing, & new creative collaborations in 2020. If you are interested in their work please follow @sashacakes and @sophiecdyer on Twitter.

Receiving NOAA weather satellites
Receiving NOAA weather satellites

Guest Post: So you’ve bought an RTL-SDR, now what?

Thank you to Merlin from the SignalsEverywhere discord channel for writing this guest post.

A package arrives at the door, a world of signal exploration awaits you. Ripping the package open you quickly install the drivers and begin exploring the beautiful waterfall of signals only to realize… Listening to the police pull someone over or your neighbors baby monitor they forgot to unplug is getting kind of boring. Where do you go from here?

We will show you that there is a whole lot more in the world of radio, ready for YOU to discover out there. 

Broadcast FM DX (yes, really!)

For some out there, the broadcast band (88-108 MHz) is just a droll block to be skipped over, I will say this about that: 7 times out of 10, you’d be right. However there is a lot more there than you realise. On a normal evening, it’s not unusual to hear Billy-Bob and Tina’s evening drive home show on your commute home, this is referred to as local programming and is perfectly normal. However during lifts in band conditions it’s entirely possible to hear radio from all over the world. From my [Merlin of Signals Everywhere] QTH in SE England, I regularly receive broadcast FM from France, Belgium and the Netherlands (music in Dutch is certainly an acquired taste…). 

Hidden amongst the signals is a service called RDS (essentially give you station information such as station name, current song and so on). Oona Räisänen has done some amazing work digging about and documenting some of the things around RDS check out her blog post for more details. 

On a slightly nefarious note; there are surveillance devices which transmit on low power onto the BFM bands, these are a lot more common than you realise and are available for meer $ on amazon and ebay. 

Licence free wireless devices

All over the bands you’ll see a little blip here, a little blip there and you’ll sometimes be thinking to yourself “What was that?” “I wonder if it’s a regular pattern or…”. Upon looking up you’ll notice that you’re sitting around one of the various licence free allocations. As it stands right now, you’ll regularly encounter things around 13 MHz (RFID) 26-27 MHz (remote control cars and the like) 40 MHz (same as previous) 433 MHz (This should be a section in and of itself, we’ll be circling back to this in a future article) 863-869MHz (All different kinds of things again) and 900 MHz. 

There are a few tools that will help you with a lot of this such as rtl_433 or to even break it down further URH. In the 433 MHz area, you’ll find everything from wireless weather stations to tyre pressure monitoring and a lot more in between. As I said previously, this will need an article in and of itself so keep an eye out for it.

Foreign signals

If you’re like me (Merlin) you’ll absolutely love to hunt for signals from other countries. Being closer to continental Europe than I am to London comes with advantages in that I regularly receive signals from the French and Belgian coast (I am still in fact trying to identify a signal from Belgium that appears quite often and I still have no clue as to what it is, if you’re Belgian and can help leave a comment below). I have received everything from the Dutch P2000 network to an unidentified Belgian trunked system to an unidentified French FSK system that no one has been able to identify

You will also encounter (depending on your location) niche systems such as France’s channel hopping UHF pagers, legacy systems such as Motorola type 2 trunks all the way up to the latest and greatest in digital trunked technology. It’s a case of recognising how the weather effects propagation specifically in your location and tuning in to the right places at the right time. Broadcast FM is a fantastic metric of a “lift” in propagation. 


The range of an RTL-SDR is pretty impressive, however when you start looking into things such as satellite communications or Amateur Television you might find that you just can’t reach the frequency ranges you need. This is where downconverters come into play. To keep it simple, a down converter is essentially just a frequency mixer and some filtering. Many of these down converters use active mixers meaning that we must feed them power, the bias-tee of the RTL certainly comes in handy for this although some downconverters (such as a satellite LNB) may require a bit more power than the 5v the RTL provides.

So what does the down converter do? Essentially, it takes the input signal from your antenna and mixes it with it’s own local oscillator frequency, the mixer then spits out the sum and difference of those signals. So, let’s say you want to listen to a 10GHz signal on your RTL, you could use a satellite LNB with a 12v power injector (since it does use an active mixer and LNA) with a local oscillator frequency of 9.75GHz and your 10GHz input signal would come out of the mixer at 250MHz, Perfectly within the range of your RTL-SDR!

Step-by-step Guide to Creating a GNU Radio Based QO-100 SSB Receiver

Thank you to M Khanfar for submitting his video that shows a step-by-step tutorial on building your own SSB receiver in Windows GNU Radio for QO-100 satellite reception.  His tutorial includes adding several tuning sliders in the GNU Radio GUI as well.

QO-100 / Es'hail-2 is a geostationary satellite at at 25.5°E (covering Africa, Europe, the Middle East, India, eastern Brazil and the west half of Russia/Asia) providing broadcasting services. However, as a bonus it has allowed amateur radio operators to use a spare transponder. Uplink is at 2.4 GHz and downlink is at 10.5 GHz. We note that we are selling a "bullseye" LNB in our store which allows most SDR dongles to be able to receive the signal with high frequency accuracy.

GNU-RADIO QO-100 SSB Receiver

Frugal Radio: 2020 SDR Guide Ep 1 – The Incredible World of Software Defined Radio (RTL-SDR, Airspy, SDRPlay etc.)

Frugal Radio has begun his YouTube channel a few weeks ago, and we've already posted about his YouLoop and SDR-Kits L-band patch antenna reviews as well as his results with the YouLoop on LF & VLF.

He's now begun a new series on his channel where he will be exploring the world of software defined radio in more depth. The first video that he's uploaded today is an overview where he overviews EMS communications, aircraft signals, military air signals, maritime signals, space signals, as well as other interesting signals he's received like wireless earpieces for musicians at concerts and  TV studio talkback links. He writes:

The 2020 SDR Guide Episode 1 has just been released.  It serves as an introduction to the incredible world of Software Defined Radio and will be of interest to both beginners and more advanced users.

Over the next few weeks, Frugal Radio will be exploring various aspects of using SDRs within the  hobby.  These include :
  • Using Free online SDRs
  • Comparison videos (eg Budget dongle shootout - Generic / RTL-SDR V3 / Nooelec SMARt v4)
  • "Must have" software guide
  • Antenna options and more

Users can view the channel at or visit to subscribe directly.

If you're getting started with SDR, or are looking for new projects this might be a series to follow.

2020 SDR Guide Ep 1 : The Incredible World of Software Defined Radio (RTL-SDR, Airspy, SDRPlay etc.)

SDRA2020 Online Conference Videos

The Software Defined Radio Academy is an organization that holds a conference within the yearly HAMRADIO fair in Friedrichshafen, Germany. This year due to the pandemic the conference was held online, and recently videos from the various talks have begun to slowly get uploaded to their YouTube channel.

The talks are typically very technical in nature, but if you're interested in cutting edge SDR research and applications then these are good talks to get caught up on. Currently there are seven videos that have been uploaded, but we are expecting that there are more to come since there are more talks listed in their programme. They appear to be uploading one video per day at the moment so get subscribed to their YouTube channel for the upcoming videos.

The currently uploaded talks include:

  • A Keynote interview with N1UL Dr. Ulrich Rohde
  • Laurence Barker G8NJJ: Using Xilinx Vivado for SDR Development
  • Edwin Richter DC9OE, Crt Valentincic S56GYK: Usage of higher order Nyquist Zones with Direct Sampling Devices
  • Prof. Dr. Michael Hartje DK5HH: Signalprocessing in the man made noise measurement system ENAMS
  • Bart Somers PE1RIK: Long term spectrum monitoring using GNUradio and Python

We are looking forward to the upcoming talks like the one by Dr. Bastian Bloessl DF1BBL that discusses the GNU Radio on Android implementation.

SDRA2020 - 03/04 - Laurence Barker: Using Xilinx Vivado for SDR Development

Characterizing Yagi Antenna Directionality via ADS-B Reception

Over on his blog Alex Krotz has been investigating whether adding more passive director elements actually affects the directionality of his home made Yagi-Uda antenna. Instead of using modelling software, Alex wanted a more accurate result that took into account all the imperfections of his antenna.

His idea was to receive ADS-B signals with his Yagi and a dipole antenna, then compare the data received in order to determine in which directions the Yagi receives better than the dipole. To do this he first creates a standard 2D map of plane tracks collected over a 24hr period for both the dipole and Yagi. A gaussian blur is applied to the two maps in order to fill in blank space and the data is normalized. Then he simply subtracts the dipole plot from the Yagi-Uda plot. The resulting difference plot reveals a mapping of where the Yagi receives better or worse compared to the dipole in a 2D plane.

Directivity of the Yagi revealed by comparing against a dipole
Directivity of the Yagi revealed by comparing against a dipole

TechMinds: How to Track Weather Balloons Using SDR

In his latest video Tech Minds has uploaded a video showing how to use an SDR to receive transmissions from radiosondes carried by weather balloons. Every day meteorological agencies around the world launch weather balloons several times a day. Each balloon carries a device called a radiosonde which continuously transmits weather telemetry to a ground station. With an SDR, antenna and free software it's possible to decode these radiosonde signals yourself from home.

In the video Tech Minds uses an RSPdx, SDRuno, VBCable and the RS41 Tracker software to receive telemetry from an RS-41 radiosonde launched in his area. We note that an RTL-SDR and SDR# could also be used. He shows the various bits of weather information available from the telemetry including information like temperature, pressure, humidity and the dew point. GPS and hardware status data is also available. Finally he shows how to view the balloon's flight path in Google Earth.

How To Track Weather Balloons Using SDR

Metal Case Upgrade for the SDRplay RSP1A back in Stock

Our metal case upgrade kit for the SDRplay RSP1A software defined radio is now back in stock in our store and will be ready to ship out within the next couple of days. This is a premium aluminum metal upgrade enclosure for the SDRplay RSP1A. Helps block RF interference and protects the RSP inside the sturdy enclosure. This will be the final batch made of this product, so if you are interested please order before stock runs out for good.

The kit includes 1x black aluminum metal enclosure with two labelled side panels, 1x black semi-hardshell carry case, 1x thermal pad to keep the RSP1A cool and mechanically stable inside the enclosure, 1x accessory set including enclosure screws, GND lug bolt set and 3M anti-slip rubber feet.

RSP1A Metal Case Upgrade
RSP1A Metal Case Upgrade
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