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DARPA is seeking to build very low frequency/ultra low frequency transmitters that are also very small:
Wireless transmitters that operate at very or ultra low frequencies (0.3‐30 kHz) typically require some big antenna complexes to handle their communications. Scientists at the Defense Advanced Research Projects Agency (DARPA) said they are interested looking to eliminate that issue and develop smaller physical structures that could handle new long-distance communication applications.
In a Request for Information, DARPA wrote: "At these frequencies, free‐space electromagnetic (EM) field wavelengths are measured in tens of kilometers, resulting in very large transmitter structures when employing conventional antenna approaches. Electrically‐small antennas are defined as having dimensions much smaller than the EM wavelength, with examples in the literature of antenna‐sizes as small as 1/10th of the EM wavelength. DARPA is seeking innovation to bring that size below 1/10,000 of the EM wavelength or by at least a factor of 103 smaller than the current state of the art (SOA)."
Such a tremendous reduction in size is impossible to achieve through traditional antenna design so DARPA said it is looking to gather information "in the areas of materials, mechanical actuation, and overall transmitter architectures to address impedance matching, power handling, signal modulation, scalability, and other system level considerations."
The article mentions other wireless research that DARPA is involved in, including the XPRIZE-like Spectrum Collaboration Challenge.
(Score: 1) by butthurt on Thursday August 18 2016, @09:48AM
The lowest frequency they mention is 300 Hz. The speed of light is 3 X 108 m/s in vacuo so the corresponding wavelength is 106 m and 1/10,000th of that is 102 m. I searched the Web and the first ELF antenna I read about is a square less than 1 m across, intended for use at the same frequencies mentioned in the article:
The L-600S is an effective H-Field receiving system for use in EMI noisy areas, or within E-field attenuated areas. The directional loop design in combination with the receiver's high pass filtering, greatly reduces unwanted noise and hum. Frequency response is 300 Hz to 30 kHz (broadband). The 24 inch square PVC loop may be broken down into components for storage.
-- https://www.lfengineering.com/products.cfm [lfengineering.com]
I don't understand fractal antennas but I understand that they can be significantly smaller than the wavelength of the radiation.
https://en.wikipedia.org/wiki/Fractal_antenna [wikipedia.org]
The ferrite loop antennas used in medium-wave receivers are, I believe, smaller than 1/10,000th of the wavelength of the radiation. I don't understand why they're not used for transmitting.
(Score: 2, Informative) by Anonymous Coward on Thursday August 18 2016, @12:59PM
Tiny transmitting antennas are very inefficient, as the efficiency is proportional to the square of the antenna size in wavelengths. For a crude example, an antenna 1/10,000 of a wavelength would have an efficiency on the order of 1/100,000,000. To radiate one milliwatt (0.001 watt) would require putting 100,000 watts into the antenna. The 100,000 watts is reactive power and goes back and forth between the transmitter and the space around the antenna, so in principle could be reused. Conductive losses in the antenna structure are another huge problem, and any conductors near the antenna will light up with sparks, as in Tesla coils and Tesla's Wardenclyffe wireless power tower.