FAQ
TL;DR: A dry dummy load for testing can cost less than $100, and “This is the proper way to test transmitting equipment.” [Elektroda, Anonymous, post #21681438]
Why it matters: You want omnidirectional, short‑range fields without violating FCC rules or wasting kilowatts; this FAQ shows safer, testable paths for low‑frequency coils.
Quick Facts
- At 200–3000 Hz, small coils are far below 1/4‑wavelength and radiate very inefficiently; near‑field effects dominate. [Elektroda, Anonymous, post #21681434]
- A vortex/toroidal coil behaves like a magnetic dipole at distance; most input power becomes heat. [Elektroda, Anonymous, post #21681436]
- Dry dummy loads leak only small RF and help keep tests compliant in typical lab setups. [Elektroda, Anonymous, post #21681438]
- Quasi‑static field modeling: represent the coil as many straight segments; compute B‑field (see Chapter 7 method). [Elektroda, Anonymous, post #21681449]
- For uniform, known fields around samples, use a solenoid or Helmholtz coil arrangement. [Elektroda, Anonymous, post #21681448]
Does a toroidal or “vortex” coil radiate like a normal antenna at 200–3000 Hz?
No. At these frequencies the coil is tiny relative to wavelength, so it lives in the quasi‑static regime. It couples mainly via near‑field magnetic flux and radiates very poorly. Farther away, the pattern trends toward a weak magnetic dipole. “At these frequencies, a vortex coil makes a good dummy load.” [Elektroda, Anonymous, post #21681436]
How far will 500–1000 W at 200–3000 Hz propagate from a small toroidal coil?
Not far as radiated power. Because the coil is electrically small, most power turns into heat. Any detectable field falls off rapidly with distance in the near field (∝1/r^3 for a magnetic dipole). Expect minimal far‑field radiation despite high wattage. [Elektroda, Anonymous, post #21681436]
Is the field of a toroidal/vortex coil omnidirectional?
Close to the coil, geometry complicates the pattern. Far away, it approaches a magnetic dipole. One commenter noted these coils are about as close as you get to an isotropic radiator at such low frequencies, emphasizing low gain and broad pattern. [Elektroda, Anonymous, post #21681434]
I want a 15–60 ft exposure radius without long‑range spill—what’s the simplest setup?
Use a low‑power source into a proper dummy load for transmitter checks, and only couple intentionally to a nearby test coil. Dry dummy loads leak small RF but remain safe enough for lab work. Place your specimens near the intentional test coil, not the load. [Elektroda, Anonymous, post #21681438]
How do I calculate the magnetic field from my coil geometry?
Model the winding as many straight line segments and sum their contributions (quasi‑static approximation). This method yields the B‑field in 3D from known current and geometry, and scales to arbitrary coil shapes. Validate results with a field meter. [Elektroda, Anonymous, post #21681449]
What’s a dummy load, and why do RF engineers use it?
A dummy load converts essentially all RF power to heat so the transmitter can be tested without radiating. It protects gear, reduces interference, and simplifies compliance. “A dummy load converts ALL the RF power into heat and does not radiate at all.” [Elektroda, Anonymous, post #21681445]
What coil type gives a strong, known field around samples?
Use a solenoidal coil. It produces a strong, relatively uniform internal field and has open ends for easy insertion and removal of samples. Adjust turns, diameter, and current to hit your target B‑field. [Elektroda, Anonymous, post #21681448]
What is a Helmholtz coil, and when should I use one?
A Helmholtz coil uses two identical coils separated by their radius to create a very uniform magnetic field in the volume between them. It’s ideal for controlled biological or sensor exposure tests where field uniformity is critical. [Elektroda, Anonymous, post #21681450]
Can I push 500–1000 W in the ULF/audible range and stay within FCC limits?
High power is risky without shielding and measurement. Study applicable radio regulations before transmitting any RF, even at low frequencies. Consider screened rooms for controlled experiments and avoid unintentional radiation. Seek qualified regulatory guidance. [Elektroda, Anonymous, post #21681452]
How do I measure what my samples actually experience?
Use an RF or magnetic field strength meter near the samples. Map the field at multiple points and log current, frequency, and geometry. This confirms exposure levels and helps you tune coil placement and drive. [Elektroda, Anonymous, post #21681444]
Is the vortex coil a breakthrough for wireless power or therapy?
Skeptical views dominate the thread. Most power becomes heat at these low frequencies, and claimed benefits lack rigorous data here. One expert labeled many vortex‑coil sites as pseudoscience and urged classical EM study. [Elektroda, Anonymous, post #21681442]
What does real‑world low‑frequency efficiency look like?
One practitioner drove 20 kW audio onto a 100 km suspended cable and radiated only a few watts across the audio band. This highlights the severe inefficiency of low‑frequency radiation from electrically small structures. [Elektroda, Anonymous, post #21681450]
What’s an isotropic radiator?
An isotropic radiator emits equally in all directions and serves as a theoretical reference for antenna patterns. In this context, the tiny coil’s pattern is very broad with negligible gain, resembling that ideal at a distance. [Elektroda, Anonymous, post #21681434]
Any cost reality check before I buy fancy coils?
One poster priced 105 ft of 15 AWG enamel wire at about $30, noting large retail markups on ornamental coils. Build test coils yourself and invest in measurement tools instead. [Elektroda, Anonymous, post #21681445]
How do I quickly estimate fields and verify them in practice?
Three steps:
- Extract coil geometry; slice into straight segments and compute quasi‑static B‑field.
- Drive with known current; simulate the sample region.
- Measure with a field meter; refine geometry and current to match targets.
[Elektroda, Anonymous, post #21681449]
Does coil elevation (1–6 ft) matter here?
Not much for far‑field radiation at these frequencies. The setup operates in the near field, where geometry and distance dominate, and the overall pattern reduces to a weak dipole at range. [Elektroda, Anonymous, post #21681436]
