Thank you very much for your replies and suggestions. This is far from my comfort zone, so I appreciate your help.
I understand that better cables will not be enough. I need to amplify the signal, and when amplified the cable type will not really make much difference as the signal is very strong - i.e. RG6 will be perfectly fine.
@wiedehopf, I think you suggest to have a good cable (LMR400) for the first 30 m/100 ft which are outdoors, then have an LNA, and then e.g. RG6 for the remaining 15 m/50 ft. It would certainly be nice to have the LNA indoors. In my understanding this will give ~4dB/60% loss of signal - which may be good enough, but may also disappoint.
@abcd567, In your solution the LNA is installed outdoors, near the antenna, and RG6 can carry the amplified signal for all the 45 m/150 ft. I like the thought of a LNA powered over coax, but I will discuss the idea with an electrician for any concerns about safety of the installation when it will be outdoors. In my understanding this will give nearly zero relevant loss of signal - or at least all air tracks wil reach the receiver as if there were a very short cable.
For the best result I will initially fo for the @abcd567 solution - with a fallback to the @wiedehopf solution if there are any concerns from an electrician.
On a separate note, the mention of LMR400 made me look at our AIS-installation. We will also be moving an AIS antenna next to the ADS-B antenna. The AIS installation uses an RG213 cable which in my understanding has similar qualities as LMR400.
Knowing very little about radio signals, I expect that AIS installation will also benefit from some amplification. Then I wonder if the same cable/LNA can be used for both with some combiner/splitter to carry the two signals to their respective receivers.
Do you have some insight into the idea of combining AIS and ADS-B in this way?