u/Zealousideal-Pen9091

I've been cross-validating NEC decks on three independent engines — NEC-2 (nec2c), NEC-5, and momwire (an independently written method-of-moments code) — and publishing a per-antenna three-way verdict. The first two community submissions both caught real NEC-2 defects: a 20:1 tapered dipole where NEC-2 reads the reactance 23 Ω high and gets worse as you refine the mesh, and a hentenna where it reads 8 Ω low.

If there's an antenna you don't trust your simulator on, submit the deck here — you get a published verdict with the numbers, attributed to you:

https://github.com/stevenmburns/antenna-problem-decks/issues/new?template=submit-deck.yml

Or send it to me at: smburns47@gmail.com and I'll submit it.

73, Steve, KK7KNB

I've spent the last year building an open-source antenna modeling tool, and it's finally at the point where I'd rather hear strangers' opinions than keep polishing. It's called antennaknobs (antennaknobs.dev, MIT licensed, GitHub).

The core idea: an antenna is Python code, not a table of coordinates. A design is a small builder class — geometry computed from named knobs like length_frac, apex_height, z0_stub — and then you use it two ways:

- In a browser: open it in the workbench and drag the knobs. Pattern, SWR, impedance, and the power budget update live as you drag (solves are 10–40 ms warm, so it genuinely feels like tuning, not batch simulation). The hosted instance is free, and there's nothing to install.

- From a script: sweep any knob against any output, optimize for resonance, compare designs, or export a .nec deck.

What's under the hood: an in-house method-of-moments solver with two independent basis families — a degree-2 B-spline Galerkin basis (the default) and a NEC-2-style sinusoidal basis — plus H-matrix/array accelerators for big structures, and an optional PyNEC backend as a third opinion. Having different discretizations of the same physics in one tool turns out to matter more than I expected: agreement between them is an actual accuracy measurement, not vibes. We just finished a catalog-wide convergence study and wrote the method up here: How many segments? Convergence you can measure. Grounds run from PEC through reflection coefficient to a real Sommerfeld solve.

It imports your existing decks. read_nec handles the 4nec2/EZNEC/xnec2c dialects (SY symbols, gauge shorthand, LD traps, EX 6 current sources...) — the importer is validated against a corpus of ~3,000 wild .nec files scraped from the open web, scored against nec2c on every one.

Matching and feedlines are first-class: transmission lines, tuners, traps, baluns, and loads compose with the wire solution as one circuit, so you get an honest power budget — what the tuner eats, what the termination burns, what actually radiates — instead of a bare feedpoint Z.

There's a catalog of ~90 ready designs to start from (dipoles, verticals, loops, yagis, LPDAs, curtains, a lot of L.B. Cebik's designs re-created with citations) — every one a parametric starting point, not a frozen example.

What it is NOT, so nobody wastes their time: it's thin-wire MoM — wire antennas only. No surface patches, no dielectric volumes, no solid modeling; it will not replace HFSS/CST for your patch array. The hosted instance caps very large solves (run locally via pip install antennaknobs for those). Modeling over real dirt inherits the same caveats NEC users already know. We document them, including one case where we warn that a widely-used reference engine is unreliable.

I'd genuinely value: things that break, decks that import wrong, designs the catalog is missing, and "why doesn't it do X" — the issue tracker gets read, and the wild-deck corpus grows from exactly those reports.

-Steve Burns, KK7KNB