Interactive Moxon Rectangle Calculator¶

This notebook calculates and visualizes practical first-cut dimensions for a Moxon rectangle antenna, with presets for the 2 m and 70 cm amateur radio bands.

The geometry is intended for kit planning and visualization. Final dimensions should be validated by measurement or NEC modeling, especially at VHF/UHF where millimeters matter.

Theory¶

Calculator Model¶

The calculator below uses wavelength-scaled starting proportions that match the typical 2 m and 70 cm Moxon calculator examples in this repository. The conductor diameter control applies a small empirical correction to the end gap and folded tails because thicker conductors are electrically shorter and usually require slightly different coupling geometry.

The Wire type dropdown picks a velocity factor for common insulation types (or Custom to drag the slider directly). These are estimates for antenna-wire dielectric loading, not manufacturer specs — hook-up wire datasheets publish conductor/insulation dimensions but not a "velocity factor," since that's normally a transmission-line concept. Note that these presets only account for the wire's own insulation: if the wire also runs inside the H-frame's PVC/PETG tube, that adds a further small dielectric effect (bigger if the wire sits snug against the tube wall, smaller if there's an air gap) on top of whatever the insulation alone would give you. Treat the presets as a starting point and use Custom to dial in whatever your tuning measurements (see the tuning protocol notebook) actually show for your build.

For the build kit H-frame, edit the global offset constants in the next code cell. The four equal outer pipes are calculated as A / 2 - RADIATOR_PIPE_OFFSET_MM; the center pipe is calculated as E - CENTER_PIPE_OFFSET_MM.

Interactive Calculator¶

Enter the exact frequency and wire diameter. Useful starting points are 145 MHz for 2 m and 435 MHz for 70 cm.

NEC Export for Simulation¶

Export the geometry above as a NEC2 card deck (.nec) for further simulation in nec2c/xnec2c, 4nec2, EZNEC, or similar tools — for example to get the radiation pattern, gain, or front-to-back ratio of the actual antenna you built. EA1FUO's AntennaSim is a free browser-based option that runs a real NEC2 engine (nec2c compiled to WebAssembly) with no install: open it, go to the Editor tab → Tools → Import / Export, and import the .nec file saved below. It also has a built-in .s1p (Touchstone) overlay if you want to plot a NanoVNA/RigExpert measurement against the simulated SWR curve.

By default the export uses the calculator's dimensions. Check Override with measured dimensions to type in your own values instead — useful if you built the frame with different offsets, or if you have as-tuned measurements (e.g. from the tuning protocol notebook's CSV log) that no longer match the calculator's starting numbers. B/D tails can be set independently per side, since hand-tuning rarely stays perfectly symmetric. Sync from calculator refills the fields from the current frequency/diameter/velocity controls at any time.

The diagram below the fields labels every input (A, E, B left/right, D left/right) directly on the geometry, and flags a red overlap warning if the entered values would make the folded tails cross. It always shows the flat rectangle shape regardless of Polarization, since that setting only changes how this same shape is placed in 3D for the export.

Assumptions baked into the export:

Polarization picks how the rectangle is mounted: Horizontal (default) lies it flat, A and E both horizontal, at Height m above ground — the usual mast-mounted H-frame with horizontal elements. Vertical rotates it 90° about the boom, so the elements (A) run vertically and the boom (E) stays horizontal; Height m then means the height of the boom/feed point (the antenna spans Height m ± A/2), matching how height is normally quoted for a vertically mounted Yagi/Moxon. A red warning appears if Height m is too low for the antenna to clear the ground in Vertical mode.
Mounted over average ground (GN 0, εᵣ = 13, σ = 5 mS/m — the standard NEC modeling default for typical soil), not free space, so gain and elevation pattern reflect a real installation. Edit GROUND_RELATIVE_PERMITTIVITY/GROUND_CONDUCTIVITY_S_PER_M in the library cell for different soil, or set Height m higher/lower to see how mounting height changes the elevation pattern.
The radiation pattern (RP) only covers the upper hemisphere (0-90° from zenith), since nothing exists below ground once a ground plane is modeled.
NEC always models a bare conductor. If the calculator's Wire type is set to something other than Bare copper, the dimensions here are shrunk to compensate for real insulated wire — feed that same shrunk geometry into NEC's bare-wire physics and it resonates above the target frequency instead of at it (a red warning below the fields tells you roughly where). Switch Wire type to Bare copper before syncing if you want the exported model's resonance to land on your target frequency; keep the insulated presets only for the physical cut-length table.
The driven element is split into two wires meeting at the feed point; the voltage source (EX) sits on segment 1 of the wire built outward from the feed.
The folded tail gaps (C) are left unmodeled (no wire), since they are open circuits in the real antenna.
Segmentation targets roughly 100 segments per wavelength (minimum 3 per wire). Tighten this in your NEC tool if you need to resolve the small end gaps more precisely.

This export is a first-cut starting point when using the calculated dimensions — check the resulting impedance/SWR/gain and iterate rather than trusting it blindly. When exporting measured/override dimensions, results should match your build far more closely, but the average-ground assumption still applies.

Static Reference Values¶