so ive been putting together a 5 element yagi for 2m, nothing fancy just a homebrew job with some aluminum tubing i had laying around and a boom from an old tv antenna. got the element spacing and lengths from an online calculator, double checked everything with calipers, all looks good on paper.

problem is when i hook it up and sweep it with my nano vna the SWR curve is just... off. like the minimum is sitting around 146.5 instead of 144.2 where i was aiming and the whole curve looks a little lumpy. bandwidth seems narrower than expected too. resonant point shifted up maybe 1.5 mhz from where it should be.

someone at the club mentioned that at VHF the coax length can actually affect what you read at the shack end, and now im second guessing whether im even measuring at the right point. i was measuring from the shack end with about 18 feet of rg8x running down from the antenna. should i be measuring right at the feedpoint instead? or is it more likely my elements are just too short and i need to trim/adjust?

also the driven element is a folded dipole if that matters, split feed with a 4:1 balun i wound myself on a binocular core.

yeah coax length can definitely mess with your readings at vhf, the vna is showing you the transformed impedance through that 18 feet of rg8x and if the antenna isnt a perfect 50 ohm match at the feedpoint (and most homebrew yagis arent right off the bat) then what you see at the other end of the coax isnt what's actually happening at the antenna. measure right at the feedpoint if you can, even if its awkward to get up there with the vna. some guys use a short reference cable they've characterized first.

that said, a 1.5 mhz shift high on 2m usually means your elements are a touch too short, not a measurement artifact. with aluminum tubing you also need to account for the diameter when calculating element length, thicker elements resonate shorter. what diameter tubing are you using for the driven element? the folded dipole spacing matters too. id start by checking the physical dimensions first before blaming the coax.

I built a similar yagi a few years back and went through basically the same thing. ended up being my balun honestly. the binocular core wound baluns are fine but if the impedance transformation is off even a little bit it shifts everything and makes the SWR curve do weird stuff. i'd try disconnecting the balun and just connecting coax directly (unbalanced, i know) just as a sanity check to see if the resonance point moves. if it does then the balun is your culprit not the elements.