To design an effective waveguide for 1420 MHz (21 cm hydrogen line), you need to consider several factors, including waveguide dimensions, material, and feed design. Here’s a step-by-step guide to help you:
1. Choose Waveguide Type
For 1420 MHz, the rectangular waveguide WR-650 is commonly used because its cutoff frequency and propagation characteristics match well.
- WR-650 (standard choice)
- Internal dimensions: 165.1 mm × 82.55 mm (6.5″ × 3.25″)
- Cutoff frequency: 1.15 GHz
- Upper operational frequency: 1.73 GHz
- Mode: TE10 (dominant mode)
- Circular waveguide (alternative)
- Diameter: ~170 mm (6.7″) for TE11 mode
- More difficult to manufacture but can be used for scalar feedhorns.
2. Material Selection
- Aluminum (lightweight, corrosion-resistant, easy to machine)
- Copper (best conductivity but expensive)
- Brass (good balance but heavier)
- Steel (if cost is an issue, but lower conductivity)
3. Waveguide Length
- Minimum length 1-2 wavelengths (~21-42 cm) to ensure proper mode propagation.
- A longer guide can help impedance matching.
4. Feedhorn Design
- A flared horn (exponential or pyramidal) improves illumination of your dish.
- A scalar feedhorn (corrugated edges) reduces sidelobes and enhances efficiency.
5. Probe or Dipole Feed
- Place a copper probe inside the waveguide at 1/4 wavelength (~52 mm) from the back wall.
- The probe length should be around 35-40 mm.
- Connect this to an N-type or SMA connector.
6. Matching & Optimization
- Use a quarter-wave transformer or impedance matching stub if needed.
- Test with a network analyzer or SDR to optimize VSWR (aim for <1.5:1).
7. Mounting on the Dish
- Position the waveguide at the dish focal point.
- Use a dielectric lens or choke ring to improve beam efficiency.