In the design of a cantenna for 1420 MHz radio astronomy, how far from bottom of can should be monopole be located?
In a cantenna for 1420 MHz, how far from bottom of can should be the monopole?
Short answer: place the monopole about a quarter of the waveguide wavelength (λg/4) up from the closed end (“bottom”) of the can.
How to compute it for your can:
- Free-space wavelength at 1420 MHz: λ₀ ≈ 211 mm
- For a circular can (diameter D in mm), TE₁₁ cutoff wavelength: λc = 1.706 D
- Guide wavelength: λg = λ₀ / √(1 − (λ₀/λc)²) (only valid if λ₀ < λc, i.e., D > ~124 mm)
- Probe height from the bottom: s ≈ λg/4
Examples at 1420 MHz (rounded):
- D = 130 mm: λg/4 ≈ 172 mm (very near cutoff → impractically large spacing)
- D = 150 mm: λg/4 ≈ 93 mm
- D = 160 mm: λg/4 ≈ 83 mm
- D = 180 mm: λg/4 ≈ 73 mm
- D = 200 mm: λg/4 ≈ 67 mm
Tips:
- Avoid cans close to the minimum diameter (~124 mm); λg blows up and the feed point gets unmanageably high. A 160–200 mm diameter paint can works well.
- Probe length (inside the can) is typically about λ₀/4 ≈ 53 mm, often trimmed a bit shorter (≈ 48–52 mm) for best match.
- Keep the N/SMA connector ground bonded well to the can wall; aim the probe tip toward the can centerline.
How does diameter of can affect distance from bottom?
Excellent question — it’s one of the most important aspects of cantenna tuning.
Here’s how it works:
🧠 The underlying physics
The can acts as a circular waveguide, supporting the TE₁₁ mode.
The wavelength inside the can (the guide wavelength, λg) depends on both the free-space wavelength (λ₀) and the can’s diameter (D).
⚙️ Relationship between diameter and probe distance
The feed point (probe distance from the bottom) should be about λg/4.
Now, as diameter increases:
- The cutoff wavelength λc increases (since it’s proportional to D).
- The ratio λ₀/λc decreases, so the denominator √(1 − (λ₀/λc)²) increases.
- Therefore, λg becomes shorter, and so does λg/4.
👉 Larger can → shorter guide wavelength → probe placed closer to the bottom.
Conversely:
- Smaller can → λg grows rapidly → probe must be placed farther from the bottom.
- If D is too small (below cutoff), the wave won’t even propagate — it just becomes an inefficient resonant cavity.
📊 Example values at 1420 MHz (λ₀ = 211 mm)
| Can Diameter (D) | Cutoff λc = 1.706D | λg | λg/4 (probe height) |
| 130 mm | 222 mm | 687 mm | 172 mm |
| 150 mm | 256 mm | 372 mm | 93 mm |
| 160 mm | 273 mm | 332 mm | 83 mm |
| 180 mm | 307 mm | 291 mm | 73 mm |
| 200 mm | 341 mm | 268 mm | 67 mm |
🧩 In practical terms
- If your can is barely above cutoff (≈125–140 mm diameter), the probe has to be very high up, and performance degrades.
- A can around 160–200 mm diameter is ideal for 1420 MHz — good impedance match and manageable geometry.
- Once the can is much larger (>250 mm), other modes start to appear, and the simple monopole feed becomes less clean.