I got a J-Pole for VHF/UHF on Craigslist, and now that I have a NanoVNA (highly recommend!), I can see it’s not correctly tuned for 2m. It looks like it’s about 50MHz low for my 146MH ideal center, and while SWR of 3.4 is kinda usable, from what I read it’s not great for the radio nor ideal for transmission.

The antenna is made from ladder line, with a gap near the bottom (the defining characteristic of a J-pole I believe) and a twist in the center. It’s mounted to a 4x4" wooden post on my roof deck, with a small nail at the top and a plastic+nail coax clip near the bottom. There’s a string of party lights near the top (almost always unplugged) which I discovered was severely detuning my HF dipole that ran parallel to it; but the J-pole is perpendicular and I don’t have another better mounting option.

far

side

top

I’ve read advice on tuning J-poles but it was too technical, and I didn’t understand which part to trim. Is there somewhere I can trim without having to re-solder? (I’m comfortable soldering, but would prefer not to in this case since it’s physically awkward to either solder in place or take it down/up.) Or can I do something to raise the resonant frequency non-destructively?

Thanks for any advice!

  • fullsquare@awful.systems
    3·
    23 hours ago

    have you calibrated your nanovna? you have to do that after every change of frequency range. you need S11 smith chart, not S21 smith chart (port 2 is not connected). swr and logmag get you the same info in different formats, pick 1

    1m away from metal things should be okayish but the more the better. these lights will interfere, if you could at least move them away from the corner it would be an improvement. twin lead cannot be close to metal either (is that sheet metal near bottom of antenna?) needs at least couple cm of distance from it. i would also replace that nail with a ziptie

    strictly speaking that’s a slim jim. why does it have a twist in the middle? untwist it, the radiating part (upper approx 1m) should be a smooth transmission line for it to work right (or shorted at both ends)

    If you want to preserve the way it’s made, you’d need to resolder the top bar after adjustment. But you also can leave wires on top unconnected