Roll-Up J-Pole Antenna
A roll-up J-pole takes the same half-wave-radiator-plus-matching-stub design as our copper pipe 2m J-Pole and builds it from 300 or 450-ohm ladder line instead of tubing, so the whole antenna coils up flat and fits in a jacket pocket. Hang it from a tree branch on a length of paracord and you've got a full-performance 2m vertical without carrying a mast anywhere. This guide covers cutting the ladder line, finding the feed tap point by SWR sweep rather than a fixed measurement, and packing it back up without ruining the line.
A half-wave radiator with its own built-in matching section
A J-pole is a half-wave radiator fed through a quarter-wave matching stub running parallel to its bottom section — the "J" shape the design is named for. Unlike a ground-plane vertical, it needs no radial system or counterpoise, since the stub itself handles the impedance transformation down to something a 50-ohm feedline can match.
Stub (in) = (2808 / f MHz) × VF
2m (146.52 MHz), VF ≈ 0.97: radiator ≈ 37.2 in, stub ≈ 18.6 in
Why ladder line instead of tubing
Twin-lead/ladder line does the same electrical job as two parallel tubes, but it's flexible, cheap, and light — the whole antenna rolls into a coil small enough for a pack. The tradeoff is that it needs a lightweight support like paracord rather than standing on its own the way rigid tubing does.
Why there's no fixed feed tap measurement
Impedance along the matching stub runs from a dead short at the bottom to a high impedance further up, with 50 ohms landing somewhere in between depending on your exact conductor spacing and line type. Every J-pole design finds that point empirically with an SWR sweep rather than citing one universal measurement — the numbers below are calculated starting lengths, not a substitute for that step.
- Start low: begin your test tap near the bottom of the stub.
- Move up gradually: SWR should improve as you approach the 50-ohm point, then get worse again past it.
Why deployment position matters for tuning
A J-pole hanging near a tree trunk, branches, or other objects will show a different match than the same antenna in the clear, because nearby conductive or lossy material couples into the antenna's near field. Always do your final tuning in the same position you plan to actually operate from.
| Section | Length (inches) | Length (mm) | Notes |
|---|---|---|---|
| Radiator (long leg) | 37.25 in | 946 mm | Calculated half-wave for 146.52 MHz at VF ≈ 0.97 |
| Matching stub | 18.5 in | 470 mm | Calculated quarter-wave; runs parallel to the radiator's bottom section |
| Shorting bar | at stub base | — | Solders both stub conductors together at the very bottom |
| Feed tap starting point | 1-2 in above short | 25-51 mm | Starting point only — find the actual 50-ohm point by SWR sweep |
| Conductor spacing (300-ohm line) | ~0.5 in | 13 mm | Fixed by the ladder line product |
| Conductor spacing (450-ohm line) | ~1 in | 25 mm | Alternative line type, adjust tap point accordingly |
Roll-Up J-Pole Antenna Dimension Calculator
Materials for Roll-Up J-Pole Antenna
Building the Roll-Up J-Pole Antenna
Nothing here needs power tools — this is a soldering-iron-and-patience build. The tuning step is where the real work happens, so don't rush it.
Cut the ladder line
Cut a single run of ladder line long enough to cover the radiator length plus the stub length plus a few inches of handling slack at each end.
Install the shorting bar
At the point marking the bottom of the stub, strip both conductors and solder them together to form the shorting bar. This is the electrical bottom of the matching section.
Separate the radiator from the stub above the short
Above the shorting bar, cut or split one of the two conductors so that one side continues up as the radiator (to the full radiator length) while the other stops at the stub length, leaving the two legs of the J visible.
Attach a temporary feed tap
Clip an alligator lead from your coax center conductor to the radiator side at the starting tap point, and the shield side to the stub leg at the same height, ready for testing.
Add the common-mode choke
Install ferrite beads or a wound coax choke on the feedline close to the feedpoint, before it runs off toward your radio.
Hoist it for testing
Hang the antenna from paracord in the same position and orientation you plan to actually operate from — a bench test won't reflect real performance once nearby objects are in play.
Find the feed tap point
Sweep SWR and slide the temporary clip connections up or down together in small steps until you find the lowest SWR point on your target frequency.
Weatherproof the connections
Coat the shorting bar and the feed connection with liquid electrical tape or heat-shrink to protect against moisture.
Roll it up for transport
Coil the ladder line loosely around your hand or a small spool rather than folding it sharply — tight folds crease the line and can eventually crack the conductors.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Can't get low SWR anywhere on the stub | Bad shorting bar connection or a cut length off from the table | Check continuity across the shorting bar and re-measure both legs | Re-solder the short and correct the radiator or stub length |
| Good SWR on the bench, poor SWR once hoisted in the tree | Nearby branches or the trunk detuning the antenna | Compare a bench sweep to a sweep in the actual hung position | Always do final tuning in the real deployed position |
| Feed tap point keeps drifting between setups | Conductor spacing changing as the line flexes | Check that the ladder line isn't stretched or pinched near the tap | Keep consistent spacing near the tap with a small spacer or clamp |
| Hot mic or choppy audio despite good SWR | Missing or insufficient common-mode choke | Check for ferrite beads or a wound choke at the feedpoint | Add or increase the choke at the feed connection |
| Ladder line developing kinks or cracks after repeated packing | Rolling too tightly or folding sharply | Inspect the line for stress marks at fold points | Roll on a wider diameter and avoid sharp folds |
| Antenna twists or won't hang straight in wind | Insufficient support or no weight at the bottom | Check halyard tension and bottom-end stability | Add light tension at the bottom or improve the halyard attachment |
Why twin-lead instead of coax or tubing?
Twin-lead gives you the same two-parallel-conductor structure a J-pole needs, but flexible and light enough to roll up and pack — tubing can't do that, and coax isn't built for this kind of open parallel-line matching section.
Do I need a ground plane or counterpoise?
No — the matching stub handles the impedance transformation, which is exactly what makes the J-pole design useful when you don't have radials or a ground system to deploy.
How do I find the exact feed tap point?
By sliding a temporary connection up the stub while watching an SWR sweep, in the position you'll actually operate from — there's no fixed universal measurement for this step.
Can I make this dual-band (2m/70cm)?
Not with this single-stub design — dual-band J-poles use a different matching arrangement, and that's a separate build from this single-band roll-up version.
How do I pack it without damaging the line?
Coil it loosely around your hand or a wide spool rather than folding it — sharp folds are what eventually crack the conductors inside the line.
Does it need to hang perfectly vertical?
Close to vertical is best for a clean omnidirectional pattern, but a slight lean from wind or an imperfect branch won't meaningfully hurt performance for casual use.