Two-core plastic-coated cable (e.g., speaker twin, figure-8) works fine for a VLF loop wound on a wooden cross
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.
The following is an extract from the SARA Mailing List entries 20 October 2025, with comments reelvant to above:
On Behalf Of Steve Berl:
That [the numbers below] seems pretty close to the numbers in https://solar-center.stanford.edu/SID/DOC/SuperSID-Manual.pdf (See page 13).
On Behalf Of Nathan:
With the wavelengths so long at VLF frequencies, I would think that a
transmission-line treatment is not necessary. Instead, regard the loop
as a lumped inductance with a resistance. The voltage induced by the
transmitter’s magnetic field generates a current that must pass through
the loop inductance and wire resistance to reach the receiver input.
Note that both the loop’s induced voltage and the wire resistance scale
with turn count, while the loop inductance scales with turns squared.
One can then ask how the received signal intensity varies with turn
count and solve via circuit analysis.
When I modelled the loop resistance as 0.01 ohm per turn, the inductance
as 4 μH times turns squared, the receiver input impedance as 600 ohm
resistive, and frequency as 25 kHz, I get the attached plot for a
relative gain as a function of turn count.
Perhaps the greatest uncertainty of this calculation is that there may
be resonances in the loop from capacitive coupling among turns.