How thin can you go? Wire gauge for POTA

Every portable operator has packed this decision. The 28 AWG magnet-wire dipole weighs nothing and disappears into a tree; the 18 AWG version survives being yanked out of one. Between them sits a real three-way tradeoff — weight vs. SWR bandwidth vs. radiated power — that mostly gets settled by folklore (“thin wire is lossy”, “thick wire is broader”). This example settles it with a solver instead.

One catalog design carries the whole comparison: dipoles.pota_invvee — a half-wave 20 m inverted-V on a 7 m telescoping pole over average ground, where the wire itself is a knob. The wire_type dropdown selects from a wire catalog the same way the station designs pick a feedline from the cable catalog: 28, 22, or 18 AWG copper, bare or PVC-insulated, each entry carrying its real radius, conductivity, jacket, and grams per meter.

What the wire actually does in the model

This isn’t a lookup-table correction. Since momwire’s distributed-loading release, the wire’s material enters the impedance matrix itself as a per-meter series impedance:

• Skin-effect resistance. At 14 MHz current flows in the outer ~17 µm of the copper, so RF resistance runs 5–15× the DC value — about 1 Ω/m for 28 AWG, 0.3 Ω/m for 18 AWG. The solver uses the exact solid-round-conductor law (the same one NEC’s LD 5 card implements), valid from DC through full skin effect.
• Insulation loading. A dielectric jacket stores E-field energy next to the wire, which acts as distributed series inductance — the wave on the wire slows down, and the antenna tunes a few percent lower than bare wire cut to the same length. That’s the velocity factor behind “my cut-to-formula insulated dipole came out long.”

Both engines model both effects. Conductor loss: the momwire loading and PyNEC’s native LD 5 card agree on the added resistance to half a percent. Insulation: NEC-2 has no insulated-wire card, but the same jacket inductance enters as a distributed series load (LD 2, henries per metre), and the two engines agree on the resonance shift to a few percent. Each pairing is a cross-engine oracle in the test suite — two independent implementations of the same physics, kept honest against each other.

The numbers

Stock design, PVC-insulated wire, retuned length per gauge not required — the numbers below are the dropdown flipped with everything else constant (2:1 SWR window from the band-locked sweep; weight for the full ~10 m of wire; loss relative to a perfect conductor):

wire	weight	2:1 SWR window	gain vs lossless wire
28 AWG PVC	17 g	550 kHz	−0.36 dB (92 %)
22 AWG PVC	53 g	605 kHz	−0.18 dB (96 %)
18 AWG PVC	111 g	645 kHz	−0.11 dB (97.5 %)

Three readings worth taking home:

1. The loss argument is real but small. Going from 18 AWG all the way down to 28 AWG costs a quarter of a decibel — nobody on the other end will hear it. The power budget in the workbench shows exactly where it went: a wire loss (I²R) row that grows from ~2.5 % to ~8 % of the input power as the wire thins.
2. Bandwidth favors thick wire, for the geometric reason. Two effects stack here and pull the same way with different price tags: a fatter conductor is intrinsically broader (lower ln(L/a) Q — free), and a lossier conductor is also broader (resistance flattens the SWR curve — paid for in watts). The table shows the honest sum: 18 AWG is ~100 kHz broader than 28 AWG and radiates more. Thin wire’s loss does not buy its bandwidth back.
3. The insulation is a bigger tuning effect than the gauge. Flip any PVC entry to its bare twin and the resonance jumps ~500 kHz — several times the entire gauge-to-gauge bandwidth difference. Cut your antenna for the wire you’ll actually hang.

And the weight column is the one the solver can’t argue with: the 28 AWG antenna is 17 grams. For a summit activation where every antenna is a compromise, −0.36 dB for a 6× lighter pack is a fine trade — now it’s a number instead of a guess.

Try it

Open dipoles.pota_invvee in the simulator, put the sweep on 20 m, and flip wire_type through the catalog. Watch the wire weight row in the Info pane, the wire loss (I²R) row in the power budget, and the SWR trace breathe as the gauge changes. Then flip 22 AWG PVC to bare 22 AWG and watch the whole resonance walk up the band — the velocity factor, live.