Possible ways to achieve Wireless BCI.
1. Have of the wavelengths small enough penetrate through the tissues and interact with the nerves through the 1mm myelin sheaths gaps. 300ghz or higher
2. Have a frequency that builds 20 cm in diameter or thereabouts 60cm sphere cavities in the EM Field form so that the human brain could fit within the forming a sphere. With that, a Sphear within a Sphere would create a tension frequency. That frequency would penetrate to the brain tissue or interact via bone conduction and muscle signals. ~1.5ghz
3. Have a frequency that builds spheres big enough to create spheres that have within them a tension frequency of 20-50 Hertz to resonate with the brain waves. The size of the average human head is less than 20 cm in diameter. The Schumann resonance principal treating the human head as the Earth and electric sphere that gets built around it like the ionosphere around Earth. Find that ratio. The size of the sphere that would make a 20Hz to 50Hz tension resonance frequency within it, having the brain frequency match waves with that. Then information could be exchanged at each level through resonance frequencies. Still working on the math
*Not BCI just one way
4. For a one-way broadcast-type interaction You could put a frequency either RF in nature or audio that excites the atmosphere right at the perfect distance apart from each other to put a microsound or micro-vibration in the atmosphere right where the average ears would be apart from each other. You would not achieve a BCI interaction but you would rather just have a one-way broadcast out voice to skull-like type broadcast where you just actually stimulate ears with a micro-vibration in the atmosphere finding the perfect frequency to make the atmosphere jingle just enough to create enough of a compression wave at the length of the ears apart from each other so that people would hear things but it wouldn't interact with brain tissue. You would leave no Microwave interaction with nerves and thus no physical evidence.
To see the formulas I am referring to go to this link. https://docs.google.com/document/d/1HwVaQx2CEr1J9s1nORSUQPAruJbupP7LfiHLlV6wSME/edit?usp=sharing