A full-scale Ewen (often spelled Ewan) hydrogen horn is a big structure, and most of the confusion comes from the fact that the horn geometry is well defined, but the wooden struts are not historically standardised — they were simply structural members sized for stiffness. That said, there is a commonly accepted “full-scale” geometry used by serious replicas, and from that we can specify practical, conservative timber dimensions that work.
Below is a faithful full-scale build, mechanically sound and radio-correct at 1420 MHz.
1. Electrical horn geometry (fixed – do not scale)
These dimensions define what “full scale” means.
- Total horn length: 6.1 m
- Mouth aperture: 2.4 m × 2.4 m (square)
- Throat aperture: 0.25 m × 0.25 m
- Flare type: Linear (pyramidal)
- Flare angle: ~11.2° per side
- Feed: Half-wave dipole at throat
Everything wooden exists to hold this shape accurately.
2. Structural concept (how the wood is arranged)
The horn is best thought of as:
- 4 longitudinal edge rails (define the horn edges)
- Transverse square frames every ~600 mm
- Diagonal bracing to prevent racking
- Rear support frame holding the throat and feed
3. Recommended timber sizes (this is what you asked for)
A. Longitudinal edge struts (4 pieces)
These run the full horn length and define the flare.
- Quantity: 4
- Length: 6.2 m (allows trimming)
- Cross-section: 75 mm × 50 mm (3″ × 2″)
- Orientation: 75 mm dimension facing outward (stiffer against sag)
Anything smaller will sag over 6 m.
B. Transverse square frames (aperture formers)
Placed at regular intervals to maintain geometry.
- Spacing: Every 600 mm
- Number: 11 frames (including mouth and throat)
- Material: 50 mm × 25 mm (2″ × 1″)
Each frame consists of 4 pieces cut to the horn cross-section at that station.
Example dimensions:
- Frame at throat: ~250 mm internal
- Frame at mouth: ~2400 mm internal
- Intermediate frames scale linearly
C. Diagonal bracing (anti-twist)
Essential outdoors or if the horn is tilted.
- Material: 50 mm × 25 mm
- Length: Varies (typically 800–1200 mm)
- Arrangement:
- Diagonals on every second bay
- Alternate sides to avoid warping
D. Rear throat support structure
This carries the feed and cable strain.
- Vertical posts:
- 2 × 75 mm × 75 mm
- Height ~1.2 m
- Cross-members:
- 75 mm × 50 mm
- Feed mounting plate:
- 18 mm exterior plywood or aluminium
E. Mouth stiffening ring (strongly advised)
Stops the aperture “breathing” in wind.
- Material: 75 mm × 25 mm
- Total length: ~9.6 m (perimeter)
- Can be scarf-jointed from shorter lengths
4. Total timber summary (rough order list)
- 4 × 6.2 m — 75×50 mm
- ~50 m — 50×25 mm (frames + braces)
- 2 × 1.2 m — 75×75 mm
- ~10 m — 75×25 mm
5. Alignment tolerance (important)
For hydrogen line work:
- Aperture squareness: ±5 mm
- Edge straightness: <10 mm over full length
- Feed position: ±3 mm
Wood is fine if sealed — polyurethane or epoxy varnish.