How poorly would an attic mounted antenna perform? Obviously worse than something outside, I’m guessing, but how much worse?
My office has one of those “side attics.” It has a half sized door. Mounting a long antenna in there is very tempting because of how much easier it would be than doing anything outside. It’s on the second floor as well.
Late to the party; I built and ran both a 10m and 20m window/ladder-line j-pole with phenomenal results. It’s really neat in that it does not require a counterpoise as it has a matching section built in. As such, the radiating element and matching sections do not need to be coaxial, so if you’re mounting in the attic space, you are free to run the radiating element horizontally (performance will be reduced as this antenna’s optimal performance is vertical), and the matching section can have a different orientation.
The 20m version is about 44’ long, so when I only had a 30’ mast, I had to string the matching section horizontally:
On 100W and a 10m tall mast, I was able to reach Indonesia on SSB on 20m. It took some working and good prop, but we made a QSO.
I upgraded to a 15m tall mast and got some marginal improvements, but being in the residential area made it hard to compare since there was always so much EMI:
As for comparison to other antenna types, I have a 7-band OCFD that is quite long, and a massive headache to install (I primarily operate portable), and this j-pole blew it out of the water.
Check out KB9VBR’s video about it: https://youtu.be/RZv94BvNxrQ
Good luck!