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SLAC has developed a compact portable transmitter that uses very low frequency (VLF) radiation:
A new type of pocket-sized antenna, developed at the Department of Energy's SLAC National Accelerator Laboratory, could enable mobile communication in situations where conventional radios don't work, such as under water, through the ground and over very long distances through air.
The device emits very low frequency (VLF) radiation with wavelengths of tens to hundreds of miles. These waves travel long distances beyond the horizon and can penetrate environments that would block radio waves with shorter wavelengths. While today's most powerful VLF technology requires gigantic emitters, this antenna is only four inches tall, so it could potentially be used for tasks that demand high mobility, including rescue and defense missions.
"Our device is also hundreds of times more efficient and can transmit data faster than previous devices of comparable size," said SLAC's Mark Kemp, the project's principal investigator. "Its performance pushes the limits of what's technologically possible and puts portable VLF applications, like sending short text messages in challenging situations, within reach."
The SLAC-led team reported their results today [open, DOI: 10.1038/s41467-019-09680-2] [DX] in Nature Communications.
SLAC was originally named Stanford Linear Accelerator Center. Now it's just SLAC or SLAC National Accelerator Laboratory.
Also at Engadget.
(Score: 5, Informative) by VLM on Tuesday April 16 2019, @11:56AM (1 child)
The article contains several interesting points.
1) I've never considered electronically dynamically retuning an antenna while it transmits analog FM or simple FSK. That's a very interesting idea. I'm familiar with Bode-Fano limit more at microwave freqs and they're kinda cheating by talking about an instantaneous BF limit being exceeded as a system. That's like implying a BBC shortwave transmitter can transmit at any instant from 3-30 mhz whereas the reality is you gotta move mechanical tuning parts when shifting more than a couple KHz so at any instant the TX has a bandwidth of like 6 KHz not 27 MHz over a very long term (hours) timeframe. Now the linked article make a strange crystal that can be tuned at electronic speeds like one would "pull" a ham radio crystal oscillator; on one hand a brilliant idea on the other hand kinda cheaty. They violated the B-F limit in the same sense that if I transmit a LIDAR pulse of light then redefine the length of the meter while its in flight I could arrange things such that the light beam "exceeded the speed of light" in a certain sense, but not really. I'm just saying the innovation is dynamically retuning an antenna while transmitting FSK while at every instant being within the constraints of B-F, not violating some basic physical constraints of antenna matching.
A fantastic SN automobile analogy would be experimental fully active auto suspensions could be described to noobs as being "kinda like a magic suspension coil over shocks spring of controllably magic variable Hookes law constant" but that's not how those suckers work at all and the implications will honestly confuse more people than simply describing an active suspension as essentially being a really fast robot arm with a wheel on the end and a shitload of sensors to tell the robot arm how to move.
Piggy backing on the SN analogy above, they haven't made a magic antenna thats 83 times wider bandwidth than physical law dictates is possible; they made a really freak'n fast robot arm that adjusts the antenna tuner controls incredibly fast in time with the frequency shift.
Its a pretty cool idea.
2) The most impressive/cool part of the paper is the methods section; very interesting. Wouldn't mind trying something "inspired by" along these lines in the lower ham bands and getting a QEX journal article out of it, maybe. In my infinite spare time... It seems just a tiny bit too complicated for ham radio operators to implement... or ... is it?
3) "Develop" implies its a little further toward production than the article implies. Its way beyond theoretical, and some lab fooling around worked, but its a long way from generic end users sending text messages per the quote. That's where it might end up some day, of course, but its got a LONG way to go. Some of the article text implying product size for example is a tiny little bit premature.
4) The first thing that came to mind after F-ing around with NFC stuff a couple years ago is this technology would seem to be interesting for NFC base station use. The second thing was wondering if anyone has a continuous auto-tuner for broadcast FM that doesn't tune at a couple Hz rate but swings at modulation speed; I'm guessing that would not make financial sense on the other hand in the cutthroat world of broadcasting if you save 0.1% electrical power that's pure profit and enough to push everyone out who doesn't do that. I'm guessing it wouldn't scale to be useful. The mental concept of a perfectly matched transmitter at all instants is intriguing.
(Score: 1) by Coward, Anonymous on Tuesday April 16 2019, @09:30PM
Can't say I fully agree with that. The more basic message is that LTI (linear time-invariant) systems are nice from an analytical point of view, but not so great for performance. That is also true in many other domains.