I took the opportunity for some experiments with gain settings in dump1090-fa while testing a new built antenna for the ADSB receiver. I compared my previously built QFH antenna to a G7RGQ omni directional antenna that I built last week. Second object of test was a 10m active USB cable (from Amazon).
In fact, the difference between these two setups are, the QFH is connected to the RTL-SDR recevier by a 6m long coax cable with roughly 6.5dB attenuation @ 1090MHz (bad solution, to be changed). While the G7RGQ was attached to a temporary bamboos pole, the RTL-SDR receiver directly connected to the antenna feed point and connected to the Raspberry Pi by an 10m active USB cable.
I started with 48dB gain setting that was used for the QFH before. Then stepping down to 25.4dB gain letting the system run for a while between changes.
The following gain setting were applied at time, so you can compare with the above graphs:
|Time||Real Gain RTL-SDR [dB]||Gain setting dump1090-fa|
- A 10m active USB cable works excellent to replace the 6m coax, hence the additional attenuation.
- Highest gain is not giving best results. Here is why:
- Higher the gain -> higher the noise -> lower the signal-to-noise ratio (SNR) -> lower the sensitivity (see signal level graph).
- Lower the sensitivity -> lower the range.
- Higher the gain -> higher the signal mean level -> lower the dynamic range of the RTL-SDR receiver -> lower the sensitivity.
- Again, lower the sensitivity -> lower the range.
- For maximum sensitivity, ergo range, the gain must set to a level where noise is lowest, mean level is not reaching maximum and peak level is above -3dBFS.
To archive the latter, I found that a good indication is the peak level in the signal graph. As long as the peak level is a flat line the gain is to high, also when mean level is reaching the -3dBFS limit. In this case lower the gain until peak level is changing dynamically but still being above the -3dBFS limit. Perform gain tests during a period with high aircraft traffic so you can expect a stable incoming signal. At night you will usually see the peak level dropping almost to noise floor when not aircraft is in range. But that´s normal.
This post is applicable for the ADSB receiver image from https://www.adsbreceiver.net/ where you have the web portal with performance graphs.