If your 1420 MHz rectangular waveguide feed has a probe with a bulbous, paddle-shaped, or capacitive end, that feature is usually intended to broaden the impedance match rather than dramatically widen the waveguide’s fundamental operating range of frequencies/wavelengths.
The actual frequency range depends mainly on the waveguide dimensions, not the probe shape.
For hydrogen-line work at 1420 MHz, most commercial feeds use something close to WR-650 waveguide:
For WR-650:
- Broad wall dimension (a) ≈ 165 mm.
- TE₁₀ cutoff frequency ≈ 908 MHz.
- Next mode (TE₂₀) begins around 1817 MHz.
The single-mode operating region is therefore approximately:
908 MHz to 1817 MHz
However, that does not mean the feed is well matched across that entire range.
A probe with a capacitive “hat” or bulbous tip often improves matching over perhaps:
- 10–20% fractional bandwidth for a simple probe.
- Sometimes 20–30% bandwidth if carefully designed.
For a feed optimized at 1420 MHz, a reasonable estimate would be:
- Good match: roughly 1300–1550 MHz
- Usable match: perhaps 1200–1650 MHz
The exact range could be narrower or wider depending on the probe dimensions and position within the guide.
If you have access to a VNA, the easiest way to determine the real bandwidth is to measure S11 (return loss). Many hydrogen-line feeds show:
- Better than 10 dB return loss over 100–300 MHz.
- Better than 15 dB near the design frequency.