At 20 kHz you’re really designing a resonant system, not just adding a tuner box. For your kind of radio astronomy / interferometry work, the most practical approach is a high-Q magnetic loop with integral tuning or a short vertical with heavy loading. I’ll outline both, but the loop is almost certainly the better route.

🌀 Option 1 — Tuned magnetic loop (recommended)

This behaves like a sharply tuned band-pass filter + antenna in one.

🔧 Basic layout

        ┌─────────────── Loop (multi-turn) ───────────────┐
        │                                                 │
        │                                                 │
        └───────||───────────────||───────────────────────┘
                C (tuning)       C (split optional)

                    │
                 Link coil
                    │
               Preamp / RX

📐 Design targets (20 kHz)

• Frequency: 20 kHz
• Loop diameter: 1–3 m (bigger = better SNR)
• Turns: 10–50 turns (depending on size)
• Inductance: typically 10–100 mH

🧮 Resonance condition

For 20 kHz, typical combinations:

• L = 50 mH → C ≈ 1.26 nF
• L = 20 mH → C ≈ 3.2 nF

⚙️ Key components

1. Loop

• Multi-turn insulated wire (e.g. enamelled copper)
• Keep resistance low → improves Q

2. Capacitor

• High voltage (even on receive, Q can generate kV)
• Air variable or vacuum variable ideal
• Or switched polypropylene caps

3. Coupling (important)

• Small 1–2 turn link coil
• Loose coupling → preserves Q

🎯 Performance characteristics

• Bandwidth: often 100–500 Hz
• Very sharp tuning (you must retune frequently)
• Excellent rejection of broadband noise

🧲 Option 2 — Short vertical + loading coil

This mimics a “traditional antenna + tuner”, but the tuner becomes huge.

🔧 Layout

     Short vertical wire (3–10 m)
              │
        Large loading coil (mH–H!)
              │
        Variable capacitor
              │
           Ground

⚠️ Reality check

• Coil can be physically large (ferrite rod or air-core drum)
• Efficiency is extremely low
• Very sensitive to ground losses

👉 This is closer to how VLF transmitters work, but for receive it’s usually inferior to a loop.

🔊 Front-end (this matters more than the antenna)

At 20 kHz, your limiting factor is usually noise + impedance mismatch, not radiation efficiency.

Best practice:

• High impedance preamp (JFET or op-amp front end)
• Place right at the antenna
• Differential input if possible

Common approach:

• Loop → link coil → high-Z buffer → SDR / soundcard

🧠 Practical build suggestion (tailored to you)

Given your interferometry work:

I’d suggest:

• Two identical tuned loops
• Spaced apart (baseline)
• Each with:
  • fixed capacitor bank
  • fine tuning (small variable cap or varactor)

Then:

• Feed into SDR (your SDRplay RSPduo setup fits perfectly)
• Do phase comparison in GNU Radio

⚡ A more experimental idea (you might like this)

Instead of high-Q tuning:

👉 Use:

• untuned loop
• very low-noise preamp
• digital filtering

Pros:

• no constant retuning
• wider bandwidth for transient work

Cons:

• lower SNR than resonant loop

🧾 Bottom line

• There’s no practical “box tuner” at 20 kHz
• The loop + capacitor is your tuner
• For your application:
  👉 tuned magnetic loop + high-Z preamp = best performance