Below is the simplest and most reliable way to add an effective quarter-wave choke to a hydrogen-line cantenna. This is a standard method used on feedhorns to reduce spillover and improve impedance matching. It’s practical with tin cans, aluminium sheet, or copper sheet.

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✔ What a choke actually is

A choke creates a high-impedance ring around the mouth of the waveguide at the operating frequency. This suppresses currents on the outside surface and reduces unwanted radiation/spillover.

Most commonly it is made as a λ/4 deep groove or sleeve around the waveguide mouth.

At 1420 MHz:

• λ ≈ 211 mm
• λ/4 ≈ 53 mm

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✔ The easiest version you can build

1. Quarter-wave choke sleeve (the “slipped on” cylinder)

This is the simplest for DIY and works well with paint-tin construction.

What you need

• A metal cylinder slightly larger in diameter than your main 150 mm cantenna (e.g., a short section of a second paint can, or rolled aluminium sheet).
• Its length must be ≈ 53–55 mm.
• A way to electrically connect the bottom rim of the choke sleeve to the mouth rim of the cantenna (screws, solder, aluminium tape, or simple metal-to-metal contact).

How it works

• The sleeve is shorted at the bottom (because it touches the cantenna mouth).
• It is open at the top.
• At λ/4 resonance this creates a high impedance boundary at the mouth → acting as a choke.

How to build it

1. Cut the sleeve to 53–55 mm long.
   A leftover paint tin is ideal; cut off a band from its open end.
2. Slip it over the outside of your 150 mm cantenna mouth.
   It should surround the can like a collar.
3. Ensure good electrical contact around the whole circumference:
   • 4–6 self-tapping screws,
   • or conductive foil tape,
   • or solder spot-welds if using tinned steel.
4. Do NOT close the top of the sleeve.
   It must remain an open cylinder.

This construction is extremely forgiving. A few mm error won’t matter.

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✔ The “classic” version

2. Quarter-wave annular groove (the textbook choke)

This is harder to build but excellent electrically.

Structure

• You form a circular trench around the waveguide mouth.
• Depth: 53–55 mm
• Width: ~10–30 mm (not critical).
• The bottom of the trench must be a short-circuit (electrically connected to the cantenna body).

How to fabricate

• Use two nested cans with a 10–20 mm gap between them.
• Join their bottoms together (short).
• Trim the outer can so the gap depth is 53 mm.
• The gap now acts as the λ/4 groove.

This is essentially what commercial feedhorns use.

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✔ Which one should you build?

For your hydrogen-line can-waveguide feed, method 1 (sleeve) is by far the easiest, quickest, and most effective.
Your existing 5-litre paint tin can be cut into a 53 mm-deep ring and slipped over your 150 mm can.

Don’t put the whole can over the inner can — cut a ring off it.

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✔ How to test if the choke is working

• Point at cold sky → measure power.
• Point at ground → measure power.
  A good choke reduces ground pickup and improves the contrast between sky and ground.

If you have a VNA:

• Measure S11 before and after.
  Resonance near 1420 MHz often improves with a proper choke.

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