so ive been messing with this for probably three weekends now and figured id write up what i found since there wasnt a ton of practical info when i was searching around

started with the standard approach, cut three dipoles for each band and hung them all from the same center insulator fanned out at maybe 30 degrees or so between each element. the 40m one is about 66 feet total, 20m around 33, and i cut the 15m one a bit long intentionally to tune it down

first issue was the feedpoint — i was using a really cheap SO-239 chassis mount and the solder joint let go after the first rainstorm. switched to a proper weatherproofed center insulator i got from a local club member and that fixed the mechanical side of things

SWR on 40 came in nice, around 1.4:1 at the bottom of the band where i mostly operate. 20m was trickier because the elements from 40 were interacting and i ended up having to spread the fan angle a bit more to get it below 2:1 across the whole band. 15m honestly just kinda worked, maybe got lucky with the cut length

feeding it all with RG-8X through a 1:1 choke balun wound on a FT-240-43 toroid, about 10 turns. common mode noise dropped noticeably after adding that, which honestly surprised me a bit even though i knew it should help

overall pretty happy with it. not a beam obviously but for a wire antenna its been solid on 20m into europe in the evenings. anyone else run a fan dipole and have tips on reducing the inter-element interaction on the middle bands

the interaction thing is just kind of unavoidable with fan dipoles, the elements are close enough that they do couple to each other especially on harmonically related bands like 40 and 15. spreading the angle helps up to a point but past maybe 45 degrees youre basically just fighting the geometry

one thing that helped me was making the elements slightly different lengths than the textbook numbers and then trimming from there rather than trying to nail it on paper first. also what height are you at? mine behaved pretty differently once i got the apex above 35 feet, the 20m element in particular seemed to settle down SWR-wise

your choke balun choice is solid btw, FT-240-43 with that many turns is probably good through at least 10m. some guys use two cores stacked but i havent found that necessary below 20m

i built almost the same thing last year except i only did 40 and 20, skipped 15 because i was lazy honestly. had the same swr issue on 20 and never fully resolved it, just accepted 1.8:1 and called it good since my rig handles that fine with the internal tuner

yours sounds like it came out better than mine did. might try adding the 15m element now that you mention it worked out