RTL-SDR for Radio Astronomy – Signal Capture2


Fig.1 Rx Location 52Km North of Toronto
Fig.2 Wide-band Power <<-70dBm

In the previous five posts, I looked at my receiving equipment used for GOES16, to see if I could re-purpose it for reception of the so called hydrogen line at 1420MHz/21cm (Ref.1/5). Over several weeks I tried and experimented with various times/configurations but was not able to receive the hydrogen line. Finally I took a wide-band power measurement and discovered that my downtown location was extremely hot from an RF perspective at -34dBm! I also located an OFDM type signal at 1440MHz which may have been causing intermodulation due to it’s high level and similarity to the comb I was seeing on my background.

I decided to move the equipment to a location far outside the urban environment. Figure 1 shows the test location 52Km north of Toronto in a farming valley, surrounded by hills. The background wide-band level did not even register on the power meter as shown in Figure 2. This proved an ideal location and I was able to clearly see the line. This has been the most challenging project I have done over the past 2 years!!

Dish Alignment & Signal Reception

Fig.3 Stellarium Sky View July 21st at 10:28
Fig.4 SDR# IF Averaging Background Calculation
Fig.5 SDR# IF Averaging Finished Background
Fig.6 SDR# IF Averaging Hydrogen Line Signal

Figure 3 shows the Stellarium view of the Milky Way from the test location. I first aimed the dish directly west away from the MW to calculate the background. Note the two spurious as shown in Figure 4 (the old enemy 1420.8 is back!). Figure 5 shows the receive signal minus the background which is stable and very flat. Then I moved the dish to the south east at almost vertical. The Hydrogen line is clearly seen in Figure 6. I followed the excellent procedure as outlined in Reference 6.

Fig.7 YouTube Video RTL-SDR for Radio Astronomy – Signal Capture2
YouTube Channel
YouTube Channel


#1. – “RTL-SDR for Radio Astronomy – EM Interference”

#2. – “RTL-SDR for Radio Astronomy – Signal Capture1”

#3. – “Radio Astronomy LNA/BPF Testing with NanoVNA”

#4. – “RTL-SDR for Radio Astronomy – LNA/BPF Testing”

#5. – “RTL-SDR for Radio Astronomy – Planning

#6. – “Cheap and Easy Hydrogen Line Radio Astronomy with ….”

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.