Why a C-band scalar ring cannot be used for 1420MHz Hydrogen line radio work
Using a conical C-band ring on a 1420 MHz (L-band) cantenna is generally not advisable because of the large frequency mismatch, but let’s break it down in detail:
📡 Key Concepts
✅ 1. C-band ring (conical, waveguide, or choke ring)
- Typically designed for C-band: 4–8 GHz, often peaking around 5.8 GHz.
- Dimensions are tuned to those higher frequencies — typically much smaller than what would be needed at 1420 MHz.
- Functions like impedance matching, mode suppression, or gain enhancement are frequency-specific.
✅ 2. 1420 MHz (L-band) cantenna
- The hydrogen line frequency, used widely in radio astronomy.
- Corresponds to a wavelength of ~21.1 cm.
- The cantenna’s aperture, waveguide diameter, and feed dimensions should all be designed around this wavelength.
- Any waveguide or choke structure should match this scale.
🚫 Why a C-band ring won’t work well
| Parameter | C-band ring (5.8 GHz) | 1420 MHz needs |
|---|---|---|
| Target wavelength | ~5.2 cm | ~21.1 cm |
| Ring diameter (typical) | ~10–15 mm | Needs ~70–80 mm |
| Impedance match range | Narrow, C-band specific | Too far below design |
| Resonant behaviour | At 5–6 GHz | Will not resonate well |
Using a conical ring sized for C-band would likely introduce mismatch, signal loss, or unwanted reflections at 1420 MHz.
✅ What to do instead
If you’re trying to improve a 1420 MHz cantenna:
- Use a λ/4 choke ring: Design a choke ring specifically for 21 cm wavelength.
- Build a matching structure: A properly scaled conical flare or ridge could be used — designed for L-band.
- Simulate with software: Use 4NEC2 or CST/MATLAB Antenna Toolbox to optimize dimensions.
🔧 Rule of Thumb: Scaling from C-band to L-band
If a structure works well at 5.8 GHz, to adapt it to 1.42 GHz, scale all dimensions by a factor of:
5.81.42≈4.08\frac{5.8}{1.42} \approx 4.08
So a 12 mm C-band ring would need to become roughly 49 mm in radius for L-band use.
🧠 Summary
- ❌ A C-band conical ring will not be effective at 1420 MHz due to frequency scaling.
- ✅ You can use a scaled-up ring or design a quarter-wave choke for 21 cm wavelength.
- 🎯 Always design or adapt elements specifically for L-band if you’re working with the hydrogen line.