[engolling]

But what I already thought about - if I install a bigger funnel over the fine offset gauge i can make it more precise.

Yes, I have experimented with a 15 cms diameter funnel to increase the FO resolution to about 0.1 mm/tip.  First I tried a plastic funnel but the raindrops appeared to "hang" on the plastic so it did increase the resolution, but did NOT advance the warning that "it's raining".  However, recently I have tried a "Stainless Steel" funnel from ebay (only a few Pounds/Euros, locally sourced or from China), which does appear to significantly improve the "Rain" warning.

This was only a thought how to rise the resolution of the sensor, but it will probably not increase the reactivity as you said.

I like the RG-11 sensor
I think in comparison with mine and the one tobyportugal proposed it will do a much better job. So I will try to get such a device.

I think the integration in the TX module is pretty easy. You can supply it with the 12V the regulator of the TX needs and then solder the impulse output directly on the pads of the PCB. But it only works if it is raining and not when there is snow.


Unfortunately shipping costs to Germany and tax will cost round about how much the device itself costs.   
But thanks for the great idea.

Regards,
engolling