A bit of obscure knowledge.....

[Andrew Thornett]

WR-650 is the ideal rectangular waveguide for hydrogen line work.

WR stands for Waveguide, Rectangular, and the number refers to the internal broad-wall dimension in hundredths of an inch.

Specifically:

• WR-650 → 6.50 inches
• That 6.50 in is the internal width of the broad wall (the “horizontal” dimension)

Converted to metric:

• 6.50 in ≈ 165.1 mm

For completeness, WR-650 internal dimensions are approximately:

• a (broad wall): 6.50 in ≈ 165.1 mm
• b (narrow wall): 3.25 in ≈ 82.6 mm

(The 2:1 ratio is typical for standard rectangular waveguide.)

Why the confusion happens

It’s very common to assume the “650” means 65 mm, but WR designations long pre-date metric standardisation and are firmly rooted in inches.

Given your 1420 MHz hydrogen line work, WR-650 makes sense — it supports TE₁₀ mode comfortably from roughly 1.12 to 1.7 GHz, right in the sweet spot.