At 1420 MHz (the hydrogen line at 21 cm wavelength Hydrogen line), a steel dish performs essentially as well as any other good conductor—provided the geometry and surface quality are right.
Key points
1. Conductivity (steel vs aluminium) Steel is a poorer conductor than aluminium or copper, but at 1420 MHz the RF currents only penetrate a very thin “skin depth” (microns). Even mild steel is still highly reflective at this frequency, so:
- Losses from conductivity are small
- Aluminium is only marginally better in practice
So electrically, a steel reflector works fine.
2. Surface accuracy (this is the critical factor) For a parabolic reflector, what really matters is how smooth and accurate the surface is compared with the wavelength:
- Wavelength ≈ 21 cm
- Rule of thumb: surface error should be ≲ λ/16 ≈ 1.3 cm RMS
So:
- If the dish is accurately shaped and not warped → excellent performance
- If it has dents, ripples, or panel misalignment → noticeable gain loss and beam distortion
At 1420 MHz, you can get away with surprisingly rough construction compared with microwave bands, but not “visibly wavy” metalwork.
3. Solid sheet vs mesh Steel works in both forms:
- Solid steel dish: electrically excellent, but heavy and can deform under its own weight
- Mesh steel dish: still fine if holes are small enough
Rule for mesh:
- Hole size should be ≲ λ/10 ≈ 2 cm or smaller
If the mesh is coarse (like fencing), it will start to leak energy and reduce gain.
4. Practical considerations Steel introduces some non-electrical issues:
- Heavier → harder to maintain accurate parabolic shape
- Corrosion → rust can roughen the surface over time (increasing scattering loss)
- Wind loading → more structural stress than aluminium
But none of these are fundamental RF limitations—just engineering ones.
Bottom line
A well-constructed steel dish is perfectly suitable for 1420 MHz radio astronomy. Performance is dominated by surface accuracy and structural stability, not the fact that it’s steel.