Comment on Made some J-poles

fullsquare@awful.systems ⁨2⁩ ⁨weeks⁩ ago

• tasankovasara@sopuli.xyz ⁨2⁩ ⁨weeks⁩ ago

  Very nice! Thank you for sharing 👌 I’d imagine the width of the J will vary with tuning? In any case for 70 cm this design would be the thing to go for.

  source

  • fullsquare@awful.systems ⁨2⁩ ⁨weeks⁩ ago

    No, width is your design parameter. I’ve only used 4mm wire because store ran out of 5mm wire. Were it all in air with no plastic, width is such that impedances of matching section are close to 300 ohm. Either way 4 or 5mm is not thin on 70cm so impedance of dipole will be probably lower than 5000 ohm, and entire band is covered so it just works

    Pick any material you’re comfortable with. Start with 1 wavelength + couple cm of wire, measure out 1/4 and bend it so that 1/4 length point end up at the bottom most point. Tune by trimming (both arms, lengths of shorter and longer should stay 3:1) and match by moving feedpoint lower or higher (lower is lower impedance). That’s why I’ve made them this way, you can see marks from screw near feedpoint because feedpoint was moved a bit both sides during tuning. By the time trimming gets minimum SWR close to say 420MHz adjust feedpoint to get minimum SWR then alternate between trimming and adjusting feedpoint if necessary. Every mm counts, small trims are better done by filing the wire down instead of cutting it. Plastic needs to stay on during measurement. Keep some 1m of free space (without large metal things) around antenna during measurement

    source