I built a UHF satellite ground station. It has a yagi with G=16.1dBi and an LNA with G=20.1dB. It’s a 430 - 450 LNA with NF=0.68. The RF cable, rg174/u has I think close to a 1dB/m, which would mean around 5dB loss.
The output of the yagi is connected to an SDR receiver by a thin 5-meter UG174/U RF coax cable as follows:
Antenna —> 5-m cheap RG174/U coax —> LNA —> SDR —> Rx Software
The Challenge is poor packet reception from CubeSats, which could mean either my signal processing or poor link budget is to blame. I tested this hypothesis by first transmitting a playback of recorded satellite signals from 400-meters away. The receiver was able to the get packets without any problems. This test was of course done without considering Doppler shift. However, I do Doppler correction when I track satellites. I use TLEs to calculate the shift in real time. I have verified that my calculated Doppler shift is similar to GPredict.
Before I dissect the DSP functionalities, I have decided to rule out other issues such as link budget first. In particular, I’m worried that the cheap and not-so-well-shielded 5-meter cable from the antenna to the SDR might be gathering too much noise/interference (Friis formula). What do you think of this hypothesis? I welcome all ideas.
I moved the LNA closer to the antenna. I replaced the cheap 5-m RG174/U coax with a 16-cm coax of the same type. The LNA is connected to the SDR through a relatively better RG213 with a 0.65 dB loss. The connection now looks as follows:
Antenna --> 16-cm cheap RG174/U coax --> LNA --> 5-m RG213 --> SDR --> RX Software
The new setup shows significant improvement. The SDR software can demodulate and decode packets (with a few misses here and there). I have a few more ideas/questions to further improve the link budget.
- I still use a cheap 16-cm coax between the antenna and the LNA. Will there be any significant change if I replace this with a better cable?
- The LNA has an internal 10-MHz bandpass filter mentioned in the first answer, which I can't replace without desoldering the LNA casing. Suppose I get a narrower bandpass filter, say 3 MHz bandwidth, and put it at the LNA input. Will this add any improvement in case I don't remove the internal 20 MHz bandpass filter (thinking about Friis' formula)?