Low Frequency Wireless Experimentation

The typical Tesla coil operates at a frequency well below the AM broadcast band. This portion of the spectrum is used for low-frequency broadcasting, maritime communications and navigation, aviation beacons, time standards, and unregulated power line carrier (PLC) communications systems. Regardless of the operating frequency, there is going to be some potential for interference with existing services. The average Tesla coil hobbyist, looking for the biggest possible artificial lightning display, is not likely to be overly concerned with radio-frequency interference, unless a complaint is received. I read once that someone received a “hefty fine” from the FCC for operating a Tesla coil in the vicinity of a local airport, and I’m sure there are other stories. In Yugoslavia individual Tesla coil experimentation is forbidden. This may also be the case for Germany.

The radio-frequency signatures of a sparking and a non-sparking [continuous-wave] Tesla coil are quite different. In the first case the energy is spread out over a wide portion of the band in the vicinity of secondary’s resonant frequency and there is a likelihood for the presence of weak higher frequency spectral components. In the second case, the energy is very much concentrated at the machine’s operating frequency. The nature of the radio-frequency interference that can potentially occur with other RF devices are also of a differing nature. In the first case it is mainly the higher frequency components that create a potential for interference with nearby receivers. In the second case the less-well characterized potential for interference is with other communications services operating on the same frequency.

When contemplating Tesla coil experimentation with the specific purpose of wireless propagation investigations, the potential for interference conflicts with other services has to be the foremost consideration. In 2003 the FCC declined to make the 135.7-137.8 kHz sliver band available to the amateur service in light of the potential for interference conflicts with power line carrier (PLC) operations associated with the national power grid. This demonstrates how seriously they view the potential interference issue. However, there is still an opportunity to conduct propagation research under the FCC’s experimental license program [see http://www.fcc.gov/oet/faqs/elbfaqs.html]. Also, the already existing 160-190 kHz band under Part 15 rules is available for license-free low power investigations [FCC Part 15 limits in the 160-190 kHz band limit unlicensed operations to one watt total input power to the final radio frequency stage (exclusive of filament or heater power) with the length of the antenna transmission line, antenna and ground lead not to exceed 15 meters. These limitations should produce EIRPs in the 0.00005-0.0002 Watt range. See also http://www.part15.org/mwa/rules/rule15_217.txt]. From the May 14, 2003 FCC Report and Order related to proposed allocation:

Accordingly, we decline to make an allocation to the amateur service in the LF spectrum at this time. As indicated above, we do believe there is potential for some limited operation in these bands under individual experimental licenses. Operations at LF under our experimental license program [see http://www.fcc.gov/oet/faqs/elbfaqs.html] will allow amateur use to be coordinated with utility companies on a case-by-case basis, and allow empirical data to be developed on the sharing possibilities in this band for future consideration. In addition, amateurs may still make use of the 160-190 kHz band under our Part 15 rules [see http://www.part15.org/mwa/rules/rule15_217.txt], which are much more restrictive, and therefore more protective of PLCs, than the limits proposed in the Notice.

While 1 watt to the final stage is a significant restriction, it provides an opportunity to develop all the circuitry needed for higher power operation. Frequency-stable rf power supplies can be developed, and also techniques for real-time trimming of the transmitting element. Spread spectrum techniques can also be investigated.