Why mesh 5-10mm is fine for 21cm hydrogen line dishes

Example at the 21 cm Hydrogen Line

For your 12-ft (3.65 m) dish, assuming ideal design efficiency

η0=0.7\eta_0 = 0.7η0​=0.7:

RMS Surface Error (σ)ﾠσ / λﾠEfficiency (η/η₀)ﾠNet η

1 mmﾠ0.005ﾠ0.999ﾠ0.70

5 mmﾠ0.024ﾠ0.991ﾠ0.69

10 mm (1 cm)ﾠ0.048ﾠ0.963ﾠ0.67

20 mm (2 cm)ﾠ0.095ﾠ0.857ﾠ0.60

50 mm (5 cm)ﾠ0.24ﾠ0.377ﾠ0.26

✅ Key takeaway:

At λ = 21 cm, even a ±1 cm error only drops your efficiency by about 5 %, which is excellent.

That’s why mesh reflectors (with tolerances of 5–10 mm) work so well at these long wavelengths.