NEC-4.2 as an external engine

NEC-4.2 is the Lawrence Livermore National Laboratory code that followed NEC-2 and preceded NEC-5. It is licensed software: every user holds their own licence from LLNL and builds or receives their own binary. antennaknobs therefore ships none of it — not in the pip install, the frozen Windows workbench, the Docker image or the hosted simulator — and drives the binary you supply, the same way it drives NEC-5 and NEC-2: a deck written to a temporary directory, your executable run over it, its printout parsed back into the impedance, current, power-budget and pattern readouts every other engine serves.

Because the licence covers your own use, the NEC-4.2 slot appears only where the machine running the server can find your binary. The hosted simulator never can, so it never shows the tab.

No NEC-4.2 licence? There are two other routes to the same decks:

• On the workbench: open a NEC-4.2 deck with Open…, and it reads as NEC-4. EZNEC’s NEC-4.2 stamp is detected, or you can choose Read as NEC-4 yourself. momwire then solves it.
• In EZNEC: EZNEC’s External NEC-4.2 engine slot can run momwire-nec4.exe from momwire’s free EZNEC drop-in. See momwire in EZNEC.

Pointing at it

One environment variable, naming a console binary that takes the deck and the printout as its two arguments (nec42cl deck.nec deck.out):

export NEC42_EXE=$HOME/nec42/nec42cl
antennaknobs sweep --builder specialty.buried_dipole --engine nec42 --ground finite

or nec42_exe under [engines] in settings.toml, which the workbench and the command line both read. The variable wins over the file.

nec42 joins the engine roster only when the binary runs: antennaknobs writes a one-wire deck, runs it once, and requires a parseable printout, so a variable pointing at the wrong program is an absent tab rather than a tab that fails at the first solve.

What is different from the NEC-2 tab

NEC-4.2 reads NEC-2’s cards and prints NEC-2’s blocks, so the NEC-4.2 engine is the NEC-2 engine with four differences:

• Buried wires are served. A design with wires below the ground plane, which the NEC-2 tab refuses, runs here over the Sommerfeld ground: the deck carries GE -1, NEC-4’s flag for a ground with wires below it.
• No table files. NEC-4.2’s Sommerfeld ground caches its tables to files in the working directory unless told otherwise, so every GN 2 card is written with NOFILE — and each run happens in a fresh temporary directory that is removed afterwards, never the directory you started antennaknobs in.
• A choice of Sommerfeld ground. The slot’s ⚙ offers GN 2 (the default) or GN 3, NEC-4.2’s newer evaluation, which can differ from GN 2 by around an ohm on buried designs. The same choice is [engines] nec42_sommerfeld = 3 in settings.toml, --nec42-sommerfeld 3 on the command line, and NEC42Engine(builder, ground=..., sommerfeld=3) from Python. The chip reads “NEC-4.2 (GN 3)” when it is on.
• The MININEC-type ground is refused. NEC-4.2 accepts the NEC-2 spelling (GN 1 plus a GD card) but prints the perfect-ground pattern to the digit, so the soil the pattern should reflect off is never read. Use the NEC-2 or NEC-5 tab for that ground.

What is refused, and why

Each refusal names itself rather than letting the binary answer:

• a wire that crosses the ground plane mid-span — split it at z = 0. With a segment end on the interface the binary gives the same answer as the split wire; with a segment straddling it, a very different number and no warning;
• a wire lying in the plane;
• buried wires over the perfect or the reflection-coefficient ground, neither of which has anything below the plane;
• a conductor that ends on the plane in a design that also has buried wires. The flag buried wires need gives that end no current there, so the conductor would read as open-circuited. A conductor that continues below the plane — a rise from a buried hub — is served;
• a junction-node port and a series apex feed, for the NEC-2 tab’s reasons;
• under GN 3, a near-field request with a point near the zenith of the structure: stock NEC-4.2’s GN 3 gets the near field wrong there (GN 2 is clean). The workbench asks NEC-4.2 for no near field, so this bites only a deck you write yourself.

The NEC-4 deck

NEC-4.2’s decks come from the same writer as the NEC-2 deck, in its NEC-4 dialect, so the two describe the same antenna and differ only in the cards that spell it:

• Graded meshes are served: a graded wire is written as chained GW cards, one per panel, as the NEC-5 writer does;
• a current source is NEC-4.2’s own EX 6 on the feed segment, not NEC-2’s phantom-wire gyrator (the licensed binary gives the same impedance as the EX 0 twin to every printed digit);
• Sommerfeld cards end NOFILE, and buried wires carry GE -1, as above.

The workbench’s gear menu has Download NEC-4 .nec beside the NEC-2 and NEC-5 downloads, and the command line has antennaknobs export --dialect nec4. Writing a deck needs no NEC-4.2 binary. The download follows the NEC-4.2 slot’s GN 2 / GN 3 choice.

Citing it

Numbers produced with NEC-4.2 should cite the code: G. J. Burke and A. J. Poggio, Numerical Electromagnetics Code (NEC-4.2), Method of Moments, Lawrence Livermore National Laboratory, 2011, LLNL-CODE-491368.