so ive been running a mag loop on 40m for the past year because i live in an HOA nightmare but finally got permission to put up a wire antenna as long as its "not visible from the street" which basically means i can run something in the backyard between two trees. figured this was finally my chance to just build a dipole from scratch instead of buying one.

i cut the wire to the standard formula, 468 divided by freq in MHz, targeting 7.150 for the center of the band. came out to about 65.7 feet total so roughly 32.8 each leg. used some 14 gauge stranded copper i had lying around from an old project, center insulator is just a piece of PVC i drilled through, and ran 50 ohm coax down to the shack.

problem is when i sweep it with my NanoVNA the resonant point is coming in around 7.05 instead of where i wanted it and the SWR at 7.150 is sitting around 2.1:1 which isnt the end of the world but id rather get it closer. i know the standard answer is just trim a little off each side but before i do that i wanted to ask — is it possible the wire type or the feedpoint height is causing this? its only about 18 feet up at the center which probably makes it kind of an inverted V shape since the ends droop down to maybe 8 feet. does the inverted V geometry change the resonant frequency compared to a flat top?

yeah the inverted V shape definitely affects it. when the legs droop like that the effective electrical length gets longer compared to a flat dipole at the same height, so you end up resonating lower than the formula predicts. the 468 constant is kind of a starting point anyway and real-world installs almost always need trimming. the fact that yours came in a bit low is actually pretty normal for an inverted V config especially at that height.

id just trim maybe half an inch at a time off each leg and keep checking with the nanovva. youre not far off, probably need to lose a foot to a foot and a half total across both sides. also the velocity factor of the wire itself can nudge things slightly but with bare copper its close enough to 1.0 that its probably not your main issue here. the geometry is almost certainly the bigger factor.

2.1:1 at your target freq honestly isnt that bad, most tuners will handle that no problem, but i get wanting it cleaner. i built a 40m dipole last spring and had almost the exact same situation — resonated about 50kHz low of where i aimed. ended up trimming about 8 inches total and got it down to like 1.3:1 at 7.15. just go slow, easier to take wire off than put it back on