By default the dump1090-rb is set to adaptive-gain (i.e. dump1090-rb automatically changes gain value according to current signal strength).
The file /etc/rbfeeder.ini does not have any provision to change gain setting of dump1090-rb from adaptive-gain to gain value desired by the user.
Today by trial-and-error, I succede in unearthing 3 undocumented parameters, which can be used to manually set gain to a value of user's choice, and manually disable adaptive gain parameters.
Following is the method to do it:
(1) Open file rbfeeder.ini to edit
sudo nano /etc/rbfeeder.ini
(2) In the file opened, add following 3 lines in [client] section of file /etc/rbfeeder.ini
IMPORTANT NOTE: Replace 28.0 by your desired value of gain (any value between 0 and 60)
dump_gain=28.0
dump_adaptive_range=false
dump_adaptive_burst=false
Please see screenshot attached below
(3) Save and close file
(4) Restart rbfeeder
sudo systemctl restart rbfeeder
(5) Check status
sudo systemctl status rbfeeder | sed 's/ / /'
(6) In output of above command, look at this portion of output:
CGroup: /system.slice/rbfeeder.service
├─105976 /usr/bin/rbfeeder
├─105985 /usr/bin/python3 /usr/bin/mlat-client --input-type dump1090 --input-connect 127.0.0.1:32457 --server mlat1.rb24.com:40900 --lat 43.580000 --lon -79.620000 --alt 155 --user EXTRPI000036 --results beast,connect,127.0.0.1:32004
└─105986 /usr/bin/dump1090-rb --write-json /dev/shm/ --net --quiet --mlat --forward-mlat --net-bo-port 30105 --lat 43.580000 --lon -79.620000 --gain 28.00 --modeac --fix --gnss
.