For a WR-650 waveguide at the 1420 MHz hydrogen line, the internal dimensions need to be about 165 mm × 82.5 mm (6.5″ × 3.25″).
That means you’re looking for a rectangular metal tin with dimensions close to:
- Width: 165 mm
- Height: 82.5 mm
- Length: at least 200–300 mm for a practical feed section
A standard cooking-oil tin is usually too large in one dimension and not the correct aspect ratio. However, some 5-litre rectangular oil tins are surprisingly close. If you find one around:
- 160–170 mm wide
- 80–90 mm deep
it can often be adapted into a WR-650 feed section.
For amateur hydrogen-line work, many builders simply fabricate the waveguide from:
- Aluminium sheet folded into a box,
- Rectangular aluminium ducting,
- Or aluminium rectangular tube with the correct internal dimensions. A radio astronomy builder on Reddit noted that aluminium box section matching WR-650 dimensions can work well.
Since you’re already working on 1420 MHz feeds, I would aim for:
- Internal width (a) = 165.1 mm
- Internal height (b) = 82.55 mm
- Probe approximately 52 mm long
- Backshort approximately 69 mm behind the probe as a starting point for tuning.
Note that these positions are approximate starting points and will need to be varied depending on exact dimensions of can and antenna.