Short answer: maybe — but probably not the way you hope. Putting a smaller 2.5 L can inside a larger 5 L can will change the electromagnetic environment of the cantenna (it will very likely detune or alter the radiation/sensing pattern), and a plain outer can is not a reliable quarter-wave choke unless it is dimensioned & arranged specifically as a choke sleeve.

Below I explain why and give practical steps you can try immediately (with the numbers you need).

Why an outer can will change things

• A cantenna (cylindrical can used as a waveguide) works because the can’s diameter sets the circular-waveguide cutoff frequency for the dominant mode (TE₁₁). For a 150 mm diameter (0.15 m) the TE₁₁ cutoff is about 1.17 GHz, so 1420 MHz propagates fine — that’s why 150 mm looks reasonable for the 21-cm hydrogen line.
• If you put that can inside a larger conductive can, you create a concentric/annular geometry and a nearby conductive boundary. That changes:
  • the impedance seen by the feed,
  • the coupling from the waveguide to free space,
  • possible cavity or annular-waveguide resonances,
  • radiation pattern and gain, and
  • possible additional waveguide paths (outer can may itself support modes).
• A “choke” is a structure that presents a high impedance at the feed at the operating frequency (often implemented as a λ/4 shorted sleeve or a choke flange). A random outer paint can is not dimensioned to be a λ/4 choke, so it’s unlikely to behave as an effective choke by accident — more likely it’ll detune and reduce signal.

Useful numbers (for 1420.405 MHz)

• free-space wavelength λ = c / f ≈ 0.211 m (211 mm).
• quarter-wave λ/4 ≈ 52.8 mm.
• TE₁₁ cutoff frequency for circular waveguide:
  where is radius.
  • For diameter 150 mm → fc ≈ 1.17 GHz (so 1.42 GHz propagates).
  • For larger diameters the cutoff falls (e.g. 190 mm diameter → fc ≈ 0.925 GHz).

What will likely happen if you slip 2.5 L inside a 5 L can

• If the outer can’s inner diameter is still such that it supports modes below 1.42 GHz, the outer can will itself be a waveguide/cavity and energy can couple into it → unpredictable radiation pattern and impedance.
• The close metallic boundary will change the effective backshort and feed matching — your resonant feed spacing (backshort/feed position) designed for the single can will no longer be correct.
• You may get reduced SNR, shifted frequency response, and/or new narrowband resonances.

If you want the outer can to act as a choke: how to implement properly

A proper choke sleeve solution requires specific geometry:

1. Make the outer sleeve a λ/4 shorted sleeve around the feed point:
   • Sleeve length ≈ λ/4 ≈ 53 mm.
   • The sleeve must be electrically connected (shorted) at one end and have a gap at the feed end so it presents a high impedance at the feed.
   • Details depend on whether the choke is outside the can mouth or around the feed coax — implementation matters.
2. Alternatively, use a choke flange or quarter-wave annular groove (standard “waveguide choke”) patterns rather than a big tin simply enclosing the can.
3. If you only want RF shielding, you can use a larger can but introduce an RF absorber (foam or lossy material) between the inner and outer can to suppress unwanted coupling — that may preserve the cantenna behaviour while giving shielding.

Practical experiments you can do right away (no special kit required)

• Measure the system before and after enclosing the smaller can:
  • Record SNR and background on the hydrogen line (or simply measure received power/peak) with your usual receiver.
  • Measure VSWR/S11 at the feed if you have a VNA or return-loss bridge.
• If enclosing detunes the feed, try increasing the axial spacing of the feed/backshort by a few mm and re-measure — small changes can restore matching.
• Try adding a non-conductive spacer so the cans are not touching (air gap), and measure again — coupling drops rapidly with larger gaps.
• If you want to test a choke: fabricate a shorted sleeve ≈ 52–55 mm long concentric with the feed, leaving a small gap at the feed end — that’s the simplest quarter-wave choke to try.

Summary / recommendation

Best approach: test it experimentally (SNR or S11) and if it degrades, either remove the outer can or adapt it into a proper choke sleeve as above.

Don’t expect the outer 5 L paint tin to magically act as a useful choke just by slipping the 2.5 L inside it. It will almost certainly alter matching and the radiation pattern unless you purpose-design the geometry as a choke.

If your goal is a choke: design a λ/4 choke sleeve (≈53 mm) or use absorbers between cans.

If your goal is shielding only: use absorber or keep a reasonably large air gap and check performance empirically.

Below I explain why and give practical steps you can try immediately (with the numbers you need).

Why an outer can will change things

A cantenna (cylindrical can used as a waveguide) works because the