This is good info to know.
I did test the existing antenna with the LMR-400 and got an SWR of 1.2.
I haven’t had a chance to go up into the attic yet, it’s too hot here at the moment.
Where is here?
I saw your measurement in meters and assumed Europe.
I’m in Phoenix, AZ. So yeah - I know all too well when an attic gets hot.
If you are under 50 ft… LMR-400 is ok… but beyond that point… you may need LMR-600
Accorting to Pasternac… at 900 Mhz, attenduation is 3.9 dB / 100 ft compare to LMR-600 (2.5 dB / 10ft)
Don’t forget, if an aerial is installed in an attic, depending on the roofing material it is possible to lose a lare amount of the signal. My roof material is made of thick interlocking concrete tiles and i have a loss measured around anything from 6 to 12 db.