so i finally bit the bullet and built my first dipole from scratch, been putting it off for like two years because i kept telling myself id just buy something but anyway. cut it for 40m, used the standard 468/f formula which should put me around 33 feet per leg at 14 MHz... wait no that's wrong, it's 40m so 7.150 as center frequency, came out to about 32.7 feet per leg. wire is just 14 gauge stranded copper i had lying around from an old project, fed it with some RG-8X i had in a bin.

strung it up inverted-V style with the center at about 30 feet off the ground, legs going down to about 8 feet at the ends. got it all hooked up and the SWR at 7.150 is showing like 3.2:1 which i know isn't great, and when i sweep across the band the minimum SWR is way down around 6.8 MHz which makes no sense to me unless the antenna is way too long but i measured it twice. the legs arent perfectly straight either, they kind of sag a bit in the middle - does that mess with things that much? also not sure if my choke balun situation is adequate, i just wound like 8 turns of the coax around a 4 inch PVC pipe at the feedpoint. wondering if that's even doing anything useful.

using an old MFJ-259B to sweep it, maybe that's part of the issue? idk. any ideas welcome, i'm clearly missing something obvious

the sag will definitely affect it - a lot of people dont realize how much an inverted-V already shortens the effective electrical length compared to a flat top, but a saggy inverted-V is even more complicated. that said, if your resonant point is down at 6.8 MHz the antenna is reading as *longer* than it should be, which is the opposite of what the inverted-V effect would cause. so something else is going on.

first thing i'd check is whether your coax shield is radiating. that PVC coil balun might not be doing much at all depending on how many turns and what the coax impedance is - 8 turns on 4 inch PVC at 40m is usually not enough, you want more inductance than that. a current choke with more turns or a ferrite core will work better. if the shield is radiating it can really throw off where the antenna appears to resonate on the analyzer because the analyzer sees the whole system, not just the dipole.

also double check your physical measurements, not the wire measurements - sometimes guys measure the wire before cutting and dont account for the connections at the feedpoint center insulator eating up an inch or two on each side. probably not enough to shift resonance 350 kHz down but worth checking. what kind of center insulator are you using?

had almost the exact same thing happen to me last year, turned out my coax had a nick in it near the feedpoint where i'd bent it a bit too sharp pulling it through a window and was getting some weird readings because of that. probably not your issue but worth a look at the whole feedline run if nothing else works out.

also the MFJ-259B should be fine for this, i've used mine for years on homebrew stuff and it's plenty accurate enough to find resonance on a dipole. probably not the analyzer causing it.