Moxon dimensions for radio scatter at BAA Radio Beacon and GRAVES
Here are practical, build-ready Moxon dimensions for the two bands. I’m using standard Moxon A-E notation (A = overall width; B = tip gap between element ends; C = feedpoint gap; D = front-to-back depth; E = “tail” length). Conductor diameter slightly shifts tuning, so treat these as starting cuts and trim for lowest SWR at your target frequency.
50 MHz (Six metres)
- Overall span × depth: ~2160 mm × 800 mm (proven 50.0–50.5 MHz design).
- Critical gap at the tips: ~85 mm.
- Original build used 16 mm OD centre sections and 12 mm OD end sections; a simple 1:1 choke (BN43-1020 beads on the coax) at the feedpoint worked well. (oh1tv.fi)
Tip: With 10–16 mm aluminium tubing you’ll likely land a few hundred kHz high; lengthen the elements a touch if your dip is above 50.3 MHz. The 85 mm tip gap is the most sensitive dimension—adjust it in small steps to optimise F/B and SWR. (oh1tv.fi)
143 MHz (2 m, centred at 143.0 MHz)
Starting from DK7ZB’s 2 m Moxon and scaled slightly for 143 MHz (assumes ~2.4–3.2 mm rod/wire):
- A (overall width): ~749 mm
- B (tip gap): ~105 mm
- C (feed gap): ~29.7 mm
- D (depth): ~142.5 mm
- E (tail length): ~277 mm
These are a ~0.7% up-scale from his 144 MHz table; if you use thicker elements (4–5 mm), B grows slightly and A may shorten ~1–2 mm—use as trim guidance. (qsl.net)
Notes you’ll care about
- Conductor diameter matters. If you deviate a lot from the cited diameters, expect a small retune. (That’s why tools like MoxGen exist; they bake in wire size.) (ac6la.com)
- Choking the feed. A few ferrite beads or 5–7 turns of coax at the feed keeps common-mode down and preserves F/B, particularly on 2 m. (qsl.net)
- Fine-tuning order: Set A (width) for resonance → tweak B (tip gap) for F/B → touch C (feed gap) for best match near 50 Ω.