SLAC has developed a compact portable transmitter [stanford.edu] 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 [wikipedia.org] 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."
SLAC [wikipedia.org] was originally named Stanford Linear Accelerator Center. Now it's just SLAC or SLAC National Accelerator Laboratory.
Also at Engadget [engadget.com].