RTL-SDR for Radio Astronomy – Signal Capture1


In the previous three 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. I checked the dish for VSWR (Ref.1), and tested the LNA/BPF (Ref.2/3). The final step was to orient the dish for maximal reception of the Milky Way and perform a signal capture. There are several excellent articles/videos describing the procedures involved (Ref.4). Various authors recommend the Open Source software Stellarium for location of the Milky Way (Ref.5). In particular, there is a step by step tutorial for using an RTL-SDR + Dish + LNA/BPF (Ref.6).

Since I am not an astronomer, and this being a totally new area for me, it was tough trying to identify the H line signal. I spent a whole week orienting my dish in different directions (and all hours!) trying to capture a signal generating portion of the MW. My initial fear was that a spurious product received earlier (1420.8MHz) would overwhelm my reception when I connected the dish. Fortunately this didn’t occur. What did happen was that I received a wide band comb of components which was disconcerting. This happened when I pointed the dish due south 45/90deg and east 45/90deg. When going up/down in frequency by 10MHz the components disappear. I will have to try a different location outside of my urban area to get reception.

The Milky Way in Stellarium

Fig.1 Stellarium Sky View East July 15th_2022 at 06:00

Stellarium is a pretty amazing program, it also comes with a user guide pdf. The first thing I did was set my default location in the location window on the left hand side menu. Following the tutorial, I disabled the day time atmosphere by pressing “a”, then set the Milky Way brightness to “6” in the sky & viewing menu. This clearly shows the current location of the milky way in the sky from your default location. Figure 1 shows the appearance of the Milky Way from my location in Toronto looking south east.

Performance of LNA/BPF + Dish

Fig.2 LNA/BPF Gain Test with Dish
Fig.3 LNA/BPF 1420MHz Mrkr=-83dBm
Fig.4 LNA/BPF+Dish 1420MHz Mrkr=-68dBm

I was curious to see how well the dish was working at 1420MHz. So I did the same random noise test that I did with the LNA/BPF (Fig.2). First I measured the wide band noise performance of the LNA/BPF with an open input (Fig.3), then with the dish attached (Fig.4). You can see a noticeable gain (approx. 20dB).

Signal Capture Attempt on SDR#

Fig.5 Equipment Block Diagram
Fig.6 SDR#_IFAvg July 15th Looking East Showing Spurious
Fig.7 SDR#_IFAvg July 15th Looking East After Averaging Background

During the past two weeks I experimented with various methods of signal capture. I followed the tutorial (Ref.6) and used SDR# with the IF Averaging Plug-In. Figure 6 shows reception centred at 1420.405MHz showing the comb signal. Figure 7 shows the spectrum after averaging out the background. The spurious are so strong that even after averaging the hydrogen line signal is not visible. I am going to move the equipment to a different location preferable out of the city to take another signal capture.

FIg.8 YouTube Video RTL-SDR for Radio Astronomy – Signal Capture1

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

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

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

#4. – “RTL-SDR Blog Articles Hydrogen Line”

#5. – “Stellarium Astronomy Software”

#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.