Short answer: a thicker monopole probe looks electrically “fatter,” which lowers its impedance and broadens the match.
Longer, more intuitive version 👇
What’s going on physically
A monopole probe in a waveguide is basically a little antenna sticking into a transmission line. Its impedance is set by how strongly it couples to the E-field of the dominant TE₁₀ mode.
When you change the probe diameter, you’re really changing its capacitance and current distribution.
Effects of increasing probe thickness
1. Lower reactance (more capacitive loading)
A thicker probe has higher capacitance to the waveguide walls. That:
- Reduces inductive reactance for a given length
- Means the probe can be shorter for resonance
So if you keep length fixed and make it thicker, it tends to look more capacitive.
2. Lower radiation (coupling) resistance
For the same insertion depth:
- Thin probe → higher current density → higher effective radiation resistance
- Thick probe → current spread over more area → lower resistance
Practically:
- Thicker probe → lower real part of impedance
- Thinner probe → higher real part
This is exactly why very thin probes often over-couple and give you crazy SWR unless you trim them carefully.
3. Broader bandwidth A big practical win:
- Thicker probes have lower Q
- Match is less sensitive to small length errors
- Much more forgiving mechanically
This is why commercial waveguide probes are rarely “wire thin”.
Rule-of-thumb summary
| Probe thickness | Impedance effect | Practical result |
| Very thin wire | Higher R, higher X sensitivity | Touchy, narrowband |
| Moderate rod | Lower R, smoother X | Easier match |
| Very thick post | Very low R, strongly capacitive | Needs shorter length or offset |
In WR-650 / 1420 MHz terms (your usual playground 😉)
For hydrogen-line feeds:
- A 2–4 mm brass or copper rod is a sweet spot
- Thicker than ~5–6 mm usually needs noticeable shortening
- Position from the back wall matters more once the probe gets thick