The minimum guaranteed working distance for Ronja is 1/15 of range at 4km
visibility (nominal range). Therefore for nominal range 1.4km it's 93m.
If you need to make Ronja run on N-times shorter
link that 1/15 of maximum, change capacitor in 10M Receiver for a series
combination of resistor and capacitor as seen in the schematic.
The maximum distance will be affected by this too, but much less than the minimum distance.