so ive been at this for a few weekends now and I cant figure out what im doing wrong. built a simple center-fed dipole for 40m, cut each leg to roughly 33.3 feet based on the 468/f formula, fed it with 50 ohm coax through a so-239 connector at the center insulator. its hanging inverted-v style off a 25 foot mast in the backyard with the ends maybe 8 feet off the ground.

measured SWR with my antenna analyzer and its sitting around 2.8:1 at 7.150 which is where I want it, dips to about 2.1 somewhere around 7.3 which is weird. the resonant point seems way off from where I expected it. tried trimming a few inches off each leg but it barely moved. using RG-8X and the run is about 65 feet to the shack. wondering if its the coax or the angle of the legs or just the height above ground messing with things.

I know SWR isnt everything and my tuner can probably handle it but id like to understand why its behaving this way before I just slap a tuner on it and call it done.

the end height is probably killing you more than anything else. 8 feet at the ends with an inverted-v means those tips are really coupling into the ground and pulling the resonant point down lower than expected, which would explain why your dip is shifted up toward 7.3 instead of where you want it. the 468 formula is kind of a starting point in free space and real-world installations almost always need tweaking.

what I'd do is try raising the ends if you can, even getting them up to 12-15 feet makes a noticeable difference on 40m. also double check your center insulator and make sure you've got a decent strain relief on the coax so the braid isn't taking mechanical load and creating a weird impedance situation at the feedpoint. 65 feet of RG-8X at 40m isn't gonna be your problem, that's fine.

also the inverted-v feedpoint impedance is gonna be lower than a flat-top anyway, more like 35-45 ohms depending on the apex angle, so 2.8:1 into 50 ohm coax actually makes some sense depending on your angle.

yeah I had almost the exact same thing happen with my first 40m dipole, ends were too low and I couldnt figure out why the swr was terrible at the frequencies I actually wanted to use. ended up adding about 6 inches to each leg actually, not trimming — the ground was loading it and making it act electrically longer or shorter, I forget which way it goes honestly.

one thing worth checking is whether your coax has a choke or anything at the feedpoint. without a current choke the braid can radiate and basically becomes part of your antenna which throws everything off. I just wound about 8-10 turns of coax into a coil right at the feedpoint, nothing fancy, and it cleaned things up a lot. probably not your whole problem but worth doing anyway.