Optimising the feed for 2m dish used for hydrogen line radio astronomy
ChatGPT suggested following April 2025:
Great — optimizing the feed for your 2 m dish at 1420 MHz will ensure you’re getting the most out of your improved collecting area and resolution. Here’s what we need to cover:
1. Focal Length Calculation
For a typical parabolic dish:
- Focal length (f) is often about 0.25 to 0.45 × D depending on the dish shape (deep vs shallow).
- If your dish is prime focus and shaped similarly to your 1.5 m dish, a safe estimate is:
- f/D ≈ 0.35 → f ≈ 0.35 × 2.0 m = 0.70 m
You can confirm focal length by physical measurement or by sunlight burn focus at noon.
2. Feed Type
At 1420 MHz (λ ≈ 21.1 cm), good feed options include:
- Half-wave dipole + reflector at focus
- Sleeve dipole (to broaden beam pattern)
- Small pyramidal horn
- Cantenna (waveguide open-ended cylinder)
- Scalar feedhorn (best, but complex)
If your previous 1.5 m dish used a dipole and reflector, you can scale it or adjust spacing slightly for the new focal length.
3. Dipole + Reflector Feed Example
This simple setup can work well for a 2 m dish:
- Dipole length:
~ 0.49 λ = 10.3 cm (free space, adjust shorter if near metal or for matching) - Reflector:
Flat metal plate (20–25 cm square or round), placed about 0.25 λ (~5.3 cm) behind the dipole - Orientation:
Mount at the focal point, perpendicular to the incoming rays - Balun/matching:
Use a coax balun (e.g., 1:1 choke or quarter-wave transformer) for impedance match to 50 Ω cable
4. Illumination and Spillover
- Your feed should illuminate the dish evenly, with about -10 dB drop at the edge.
- If the beam is too narrow, you’ll under-illuminate (low gain); too wide, and you’ll get spillover and ground noise.
Dipole + reflector setups naturally give ~60° beamwidth, which suits f/D ≈ 0.35–0.4 dishes well.
5. Next Steps
If you like, I can:
- Provide a sketch/schematic of a dipole + reflector feed
- Simulate or model the beam pattern for your f/D
- Help you design a waveguide or horn feed (e.g., WR-650 section or custom)
Absolutely — let’s do both. Here’s a breakdown of the dipole + reflector feed and a WR-650 waveguide feed suitable for your 2 m dish at 1420 MHz. I’ll follow with diagrams for each.
1. Dipole + Reflector Feed (Simple, Effective)
Key Dimensions:
- Frequency: 1420 MHz → λ ≈ 21.1 cm
- Dipole length (L): ~0.49 λ → 10.3 cm total (5.15 cm each arm)
- Reflector spacing (S): ~0.25 λ → 5.3 cm
- Reflector size: ≥ λ × λ → 25 × 25 cm metal plate
- Mounting: Centered at focal point (about 70 cm from dish vertex), dipole horizontal.
Notes:
- Use RG-58 or RG-6 coax with a 1:1 choke balun (e.g., 4–5 turns of coax through ferrite)
- Match carefully to avoid VSWR degradation
2. WR-650 Waveguide Feed (Better pattern, More Directional)
WR-650 Specs:
- Standard waveguide for 1.12–1.7 GHz
- Internal dimensions: 165.1 mm × 82.55 mm
- Cutoff for TE10 ≈ 892 MHz (well within band)
- Good impedance match for 1420 MHz
Feed Design:
- Use a ~15–20 cm section as a waveguide horn
- Place thin λ/4 probe (~5 cm) into the broad wall for coax connection
- For beam shaping: add a conical or rectangular flare (or scalar ring)