Thanks Mark for trying to find a solution to prevent Paulinchen's house arrest but this doesn't work, I'm afraid.
I have tried it some years ago . The cats got crazy when they had to wear a collar but even worse was the device at the collar I think. The same problem with the invisible fence. Great for dogs and bigger cats maybe that are used to a collar but little Paulinchen is about 5 pounds now, a skinny old lady. My cats all had/have a microchip implanted that if they should get lost and endet up in an animal shelter or at a vet, they could find out where they belong, that is why they all were not used to a collar.
No, the neighbor cats are afraid of little Paulinchen. The main reason is the cornfield straight behind the house this summer. Paulinchen went into the cornfield and has slept there for hours. I couldn't find her and she didn't hear me calling her because she is deaf but she has to drink and eat every two or three hours day and night . To drink very much and often because of her illness and to eat so often because she cannot eat so much at once anymore because of her age and disease and of course she shouldn't lose much weight.
I must admit I am not an exceptionally gifted handyman. ;) Of course there are days like today when Paulinchen walks from one door to the other looking at me very reproachfully but most of the time she is sleeping anyway and as soon as possible she is sitting on my lap . But for the next year I am planing new balconies and one will become a little enclosure.
Paulinchen wanted to go out so much today....Breaks my heart that she has to stay in the house after being an outdoor cat for more than 18 years but most of the time she is doing okay with this situation.