so ive been running a single 40m dipole in my attic for about a year now and i want to add 20m and 15m without putting up more feedline. the obvious answer is a fan dipole but every time i read about them i get a different answer about whether the elements actually interact with each other or not. some guys say you have to retune everything because the elements detune each other, others say just cut them to resonance individually and hang them and youre fine.

my situation is i dont have a lot of room to spread the elements very far apart, maybe 20-25 degrees between them at most because of where my support points are. attic install so im already dealing with some detuning from the roof structure. just wondering if anyone has actually built one of these and whether the swr ended up being manageable without a tuner on each band or if i ended up needing one anyway. at that point i might as well run separate antennas with a switch or just use a tuner on the 40m dipole for all three bands.

built a 3-band fan dipole (80/40/15) a few years back and yeah the interaction thing is real but honestly not as bad as people make it sound. i had to trim each element with the others installed, not individually. started with them cut to resonance in free space then hung them all and went through each one tweaking until i got acceptable swr. took an afternoon with an antenna analyzer but ended up with under 2:1 on all three bands without a tuner. the spacing matters more at the feedpoint than at the tips, so if you can get even 15-20 degrees between them at the center you should be ok. attic is gonna add some weirdness no matter what you do honestly.

i just use an autotuner on a 40m dipole and call it a day. works on everything from 80 to 10 even if the efficiency isnt perfect. fan dipoles are a bit of a rabbit hole in my experience, you spend a weekend trimming and then the humidity changes and youre back at it. might be a different story if you really dont want a tuner in the shack but for attic use id just keep it simple.