In the previous two posts (Ref.1/2) I looked at re-purposing my GOES16 equipment for reception of the so called hydrogen line at 1420MHz/21cm. I did a quick test on the LNA/BPF using an input random noise test. The bandpass was about 65MHz flat and 75MHz half way down the edges as shown in Figure 1. In this post I will do a more detailed gain/response test using the NanoVNA.
Figure 2 shows the test setup. It is critical not to over drive the SAWBIRD as well as not over driving the NanoVNA. I placed 30dB of attenuation in the input to the SAWBIRD, so receive levels are nominally at -30dBm. On the output, I have a 20dB pad so that receive levels into the NanoVNA are nominally -10dBm. I also added a DC Block just in case (only the paranoid survive!). Figure 3 shows the nominal 40dB amplifier gain and the 65MHz response which exactly matches the data sheet. Ref.3 is a great resource for setting up and calibrating the NanoVNA.
#1. – “RTL-SDR for Radio Astronomy – LNA/BPF Testing”
#2. – “RTL-SDR for Radio Astronomy – Planning
#3. – “Absolute Beginner’s Guide to the NanoVNA”