Short answer: because different people are talking about different operating regions of the same waveguide, and some descriptions quietly assume single-mode, low-loss, well-matched operation rather than mere propagation.
Here’s the physics behind it.
1. Cutoff frequency vs usable frequency
For a rectangular waveguide, the dominant mode is TE10. Its cutoff frequency is
fc(TE10) = c / (2a)
where
c = 3.0e8 m/sa= broad wall dimension (meters)
For WR-650:
a = 6.50 in = 0.1651 m
b = 3.25 in = 0.08255 m
So
fc(TE10) = 3.0e8 / (2 * 0.1651)
≈ 0.91 GHz
➡ 1420 MHz is well above cutoff, so propagation is absolutely allowed.
2. Why manufacturers quote higher frequencies
Manufacturers usually quote a recommended operating band, not the raw cutoff. This band is chosen to ensure:
- low dispersion
- good impedance match
- predictable field distribution
- reasonable wave impedance
- compatibility with standard transitions and flanges
A common engineering rule is:
f_oper_min ≈ 1.25 * fc
For WR-650:
1.25 * 0.91 ≈ 1.14 GHz
1420 MHz still satisfies this comfortably — for hobby and scientific use.
3. The real reason: higher-order modes
The next mode to worry about is TE20:
fc(TE20) = c / a
For WR-650:
fc(TE20) = 3.0e8 / 0.1651
≈ 1.82 GHz
Above this frequency, multimode propagation becomes possible, which causes:
- pattern distortion
- impedance chaos
- calibration nightmares (especially for test labs)
So vendors often specify something like:
1.45 GHz – 2.20 GHz
to stay well clear of cutoff and to align with common RF bands and hardware ecosystems.
4. Why 1420 MHz scares catalog writers
1420 MHz sits in an awkward place:
- Close-ish to cutoff → higher wave impedance
- Poor match to standard coax-to-waveguide transitions
- Outside common telecom bands
- Mostly used by radio astronomers, not industry
So catalog descriptions often round the lower limit upward for safety, liability, and customer support reasons.
5. Bottom line (important for you)
From a physics and radio astronomy standpoint:
WR-650 at 1420 MHz = perfectly valid
As long as:
- only TE10 is excited
- transitions are well designed
- you’re not demanding metrology-grade S-parameters
This is why hydrogen-line feeds, horns, and homebrew transitions routinely use WR-650 at 1420 MHz — despite what some catalogues imply.