Since you’re building a tuneable waveguide probe (I’m assuming 1420 MHz given your past adventures…), here’s a clean, mechanically stable way to do it without it turning into a wobbly brass fishing rod.

1️⃣ The Probe Itself (Electrical Side)

For 1420 MHz:

• Free-space λ ≈ 211 mm
• Quarter wave ≈ 52–53 mm
• Inside waveguide it’ll be a bit shorter (because of cutoff), so expect optimum around 45–50 mm depending on guide size.

So:

• Inner sliding tube = the radiating probe
• Outer tube = mechanical guide + electrical connection to connector pin (if needed)

You want:

• Smooth sliding
• Solid electrical continuity
• No rotational wobble

2️⃣ The Mechanical Arrangement (The Important Bit)

Here are three good approaches ranked from “quick bench hack” to “proper instrument build”.

🥇 Best Overall: Sliding Tube Through a Bulkhead + Compression Lock

This is the cleanest and most controllable method.

What you do:

1. Drill hole in broad wall of waveguide.
2. Solder or bolt a small brass block or thick plate over hole.
3. Drill that plate to snugly fit outer brass tube.
4. Fix outer tube permanently in place (solder or epoxy).
5. Inner tube slides inside it.
6. Add a tiny:
   • M3 grub screw
   • Or miniature compression screw
   • Or collet-style clamp

That gives:

• Smooth tuning
• Firm locking
• No RF wobble

Very repeatable. Very satisfying.

🥈 SMA/BNC Bulkhead Method (Cleaner RF)

If you’re feeding via coax:

1. Use SMA bulkhead connector through waveguide wall.
2. Remove center pin.
3. Solder your brass outer tube to the center conductor.
4. Inner tube becomes adjustable extension.

Advantage:

• Excellent RF grounding
• Clean impedance transition
• Easily removable

This works beautifully if you’re matching to an LNA directly.

🥉 Simple Friction Fit (Quick Test Setup)

If you just want experimental tuning:

• Outer tube soldered in place.
• Inner tube slides by friction.
• Add tiny O-ring at base for resistance.

Surprisingly stable if tolerances are good.

3️⃣ Preventing RF Leakage

Very important at L-band:

• Make sure outer tube is electrically bonded to waveguide wall.
• Keep hole tight.
• Avoid long gaps between tube and wall.
• If needed, add copper tape around entry point.

You don’t want it behaving like a slot antenna.

4️⃣ Fine Tuning Tricks

Once installed:

📡 Method A – Maximum Noise Method

Point at cold sky. Adjust probe length for maximum background noise rise.

📡 Method B – Impedance Dip

If you’ve got a NanoVNA:

• Look at S11
• Adjust probe length for deepest return loss at 1420.405 MHz

The probe depth and length both matter.

5️⃣ Even Better: Make It Dual Adjustable

If you’re feeling ambitious:

• Inner tube adjusts length.
• Whole tube assembly slides in/out for depth.

Depth often has a surprisingly strong effect on match.

6️⃣ A Small but Crucial Detail

Round the tip of the inner probe slightly.

Sharp square cuts can:

• Concentrate current
• Slightly detune
• Cause tiny arcing if high power (not your case, but still tidy practice)

A gentle dome or chamfer is ideal.

7️⃣ Stability Tip (You’ll Thank Me Later)

Add a small scale:

• Engrave mm marks
• Or use calipers and record optimal setting

Once you find “the magic 47.8 mm”, you’ll want to return to it.

If you tell me:

• What waveguide size you’re using
• Whether this is feed into LNA or SDR directly

I can give you:

• Starting probe length
• Starting insertion depth
• Expected impedance behaviour

And we can get that hydrogen line singing properly 🎯📡