Result of a calculation which produces the PA wiggle at a nearly merged double notch feature. a) Microbeam intensity (dot-dashed line) and the macroscopic emissivity with a Gaussian cavity at φ = 0 (thick solid). b) Net polarisation fraction Π_⊥ (top thin solid) and net X-mode intensity profile (thick solid). The Π_⊥ is slightly smaller than 1, as shown by the bottom thin line presenting the quantity 100(Π_⊥−0.99). c) Net X-mode PA (ψ_⊥, solid) overplotted on the reference ψ_B (dotted). The central part of ψ_⊥ is thinner to reflect the lack of high-quality data points at the centre of notches in B1821−24A (see Figs. 1d and 2d). The result is for σ_η = 1.53°, ψ_B = −0.6φ, ρ_crv = 5 × 10⁴ cm, and ν = 1 GHz.