Two-core plastic-coated cable (e.g., speaker twin, figure-8) works fine for a VLF loop wound on a wooden cross.
Short answer: yes—two-core plastic-coated cable (e.g., speaker twin, figure-8) works fine for a VLF loop wound on a wooden cross.
Here’s how to make it work well:
- Wire the pair in parallel (a “folded” loop). Join the two cores together at the top of the loop, and also together at the feed end. That keeps currents in the same direction around the loop, halves the resistance, and preserves the loop area.
- Do NOT wire it as “go out on one core, back on the other” (series in opposite directions) — that cancels the magnetic field and kills sensitivity.
- Turns & spacing. VLF needs lots of turns; the limiting factor is inter-turn capacitance. The thicker jacket on twin cable slightly increases self-capacitance; just leave a few mm between turns (use tape or spacers) to keep the self-resonant frequency comfortably above your tuning range.
- Conductor size. At 3–30 kHz the skin depth in copper is ~0.6–1.2 mm, so ordinary stranded 0.5–1.5 mm² copper is fine. Litz helps more at MF/HF than at VLF. Lower resistance → better Q.
- Dielectric losses. PVC insulation is acceptable at kHz. If you’re chasing maximum Q, polyethylene/PTFE is a bit lower-loss, but the improvement is modest for VLF receiving.
- Electrostatic pickup. A plain loop (twin cable or enamelled wire) is sensitive to E-field noise. If your site is noisy, use a balanced high-Z differential preamp at the loop, keep the feed short, or consider a shielded loop (coaxial “Faraday shield” with the shield opened at the top and grounded at the preamp). Twin cable won’t provide shielding by itself.
- Mechanical & weather. Secure the windings; don’t trap a continuous metal ring (no foil tape bands) near the loop — that makes a shorted turn. For outdoors, pick UV-stable jacket and seal the end connections.
- Tuning & matching. Resonating with a large film capacitor across the feed (or at the top junction) will boost SNR and set bandwidth; VLF often needs tens to hundreds of nF depending on loop L. If you run the two cores in parallel, remember the loop resistance halves while L is ~unchanged, which helps the Q.
So yes: two-core plastic cable is a perfectly viable (and often convenient) substitute for enamelled wire on a VLF loop — just parallel the cores so the currents add, keep turns a bit spaced, and mind E-field noise with good front-end practice.