After have seen what happens with the original spring antenna, I simulated a 1/4 wave antenna to find out what could happen at my good TwinTalker 3600 if I substitute the little spring with a true antenna.
Respect to the spring antenna simulation, I only substituted the spring with a copper wire 1 millimetre diametre and 16.8 centimetre long, so the final graph of the radio/antenna system could be like this:
We can see that, substituting the poor spring antenna with a whip of 16.8 centimetres long, antenna's gain increases of 2 dB. Considering that, with our little power, all counts, maybe it worth explant the spring and substitute it with a simple straight wire. I'll have to try.
Below, the .nec file of this simulation for 1/4 wave antenna of my TwinTalker 3600 just seen.