Take-Off Angle Terrain Analysis (Boscastle QTH)

Station: M7SQI — Boscastle, Cornwall, UK (IO70pq)
Antenna: 40 m end-fed wire, zig-zagged across an ~8 × 3 m plot, ≈2 m AGL (apex ≈5 m)
Altitude: ~89 m ASL

What this page adds

This page ties the measured take-off angles from TOA_Data.html to the physical environment around my QTH. The west-facing slope toward the Atlantic, plus salty/Conductive ground, explain why a low zig-zag wire still favors those “magic” ~8° DX paths on 15/17/18 m.

Overlay of non-Brewstered elevation patterns (all bands) — Figure A — Non-Brewstered overlay (all bands). `output(10).png`

Site geometry & terrain

The QTH sits on a west-facing shelf (~89 m ASL). Across the first kilometre the ground drops ~60 m to the sea (≈3–4° mean down-slope). That acts like a “tilted mirror”: seaward launch angles are effectively reduced by a few degrees, helping shallow-angle DX. To the S–SW, nearby high ground raises the minimum usable elevation.

Boscastle DEM overlaid with terrain-limited minimum take-off angle contours — Figure B-1 — Combined elevation map and MTA overlay. The shaded relief shows terrain height (greens = low, reds = high), while the orange contours indicate the minimum take-off angle (degrees above local horizontal). Note the broad shallow corridor W–NW toward the Atlantic (~0.5–0.8°), and the rising floor to the S–SW (~2–3°) where nearby high ground partially blocks low-angle paths.

View the MTA azimuth table (HTML)

Azimuth sector	Terrain	MTA (°)	Comment
260–310°	Falls to sea	0.5–0.8	Best DX corridor
330–110°	Gently rising plateau	0.7–1.2	EU / Asia
120–180°	Hills 275–300 m	1.5–2.0	Partial shadow
180–240°	Ridge near Treknow	2.0–3.0	Raised floor S/SW

Clearance-angle geometry. For an obstacle of height h_obst at distance d from the QTH (h_QTH):

α = arctan((h_obst − h_QTH) / d)

If a lobe sits below α, that azimuth is partially shadowed (diffraction or later hop needed). The DEM + MTA overlay show why W–NW supports very low angles, while S–SW is floor-limited by nearby high ground.

Ground conductivity & Brewster reinforcement

Zone	Conductivity σ (S/m)	ε_r	Notes
Inland (slate + clay)	0.015–0.02	13–15	Thin topsoil; moderate loss
Coastal fringe	0.03–0.05	18–22	Salt & moisture enhanced

For H-pol HF over this ground the Brewster minimum is roughly 8–10°. Near that, the reflected and direct fields reinforce (≈+2 to +5 dB) so the effective pattern is flatter and stronger at shallow angles — the Cornish Brewster-lens effect.

Elevation pattern comparison (interactive sliders)

Drag each slider to compare the purely geometric (non-Brewstered) model with the Brewster-biased version that accounts for slope, salinity, and soil conductivity. Differences are subtle — typically a few dB of reinforcement near 8–10°, most noticeable on 15–20 m.

non-Brewstered

Brewster-biased

Non-Brewstered

160 m (output(11) vs output(19))

non-Brewstered

Brewster-biased

Non-Brewstered

80 m (output(12) vs output(20))

non-Brewstered

Brewster-biased

Non-Brewstered

40 m (output(13) vs output(21))

non-Brewstered

Brewster-biased

Non-Brewstered

20 m (output(8) vs output(22)) · alt non-Brewstered: output(14)

non-Brewstered

Brewster-biased

Non-Brewstered

17 m (output(15) vs output(23))

non-Brewstered

Brewster-biased

Non-Brewstered

15 m (output(9) vs output(24)) · alt non-Brewstered: output(16)

non-Brewstered

Brewster-biased

Non-Brewstered

12 m (output(17) vs output(25))

non-Brewstered

Brewster-biased

Non-Brewstered

10 m (output(18) vs output(26))

How the angles were calculated (ham-science summary)

Per-band Es/F split. Short-range distance histograms were smoothed; thresholds picked and clamped to sane ranges. On 6 m, a higher threshold was adopted (paths typically Es).
F single-hop spacing (μ). Distance histograms showed a comb at multiples of an underlying hop length. A seed μ was estimated from that periodicity, refined by minimizing the spread of distance mod μ, and tuned by a short grid search per band and for day/night.
Heights. Virtual reflection heights of ~340 km (F, day), ~280 km (F, night), and ~110 km (Es) were used.
Hop count. For F-mode, n ≈ round(D/μ) with guards against sub-hop artefacts and unrealistic values, assuming ground-touch per hop. For 6 m, multi-hop Es counts were optionally reported using a nominal Es hop length.
Geometry. The take-off angle is the elevation that reaches one hop of D/n at the chosen virtual height on a spherical-Earth approximation: e = atan2(2h′, D/n) (degrees). No path-loss modelling — geometry only.

Reading the results

Typical DX angles. On 20/17/15 m, long paths commonly cluster in the single-digit to low-teens range.
Why 40 m looks “shorter hop”. Higher average launch angles (plus daytime D-layer absorption) shorten per-hop ranges compared with 20/17/15 m.
6 m. Es-dominated at mid-latitudes; F2 at 50 MHz is rare, with multi-hop Es and (seasonally) TEP more common.
Antenna Analysis I ran some antenna modelling analysis to try and validate the data here. The Results can be seen on this page

Limits & uncertainty

We assume single virtual heights, equal hop lengths, and great-circle propagation. Off-great-circle tilts, ducts and full ray-tracing are not modelled. Medians carry a small systematic uncertainty (a few degrees) — acceptable for band-to-band comparison. Implausible distances were capped or excluded to avoid misleading tails.

Bottom line

West-facing slope + coastal conductivity + salty air = a passive shallow-angle booster. That’s why my measured mode around ~8° aligns with frequent JA/VK/NA success on 15/17/18 m — even with a low, scrappy wire.