On Fri, 23 Aug 2019 14:11:25 +0000 (UTC), David Lesher
wrote:

Ran across this:

https://www.doncio.navy.mil/chips/ArticleDetails.aspx?ID=12681

"Dynamic tuning works by physically changing the antenna tuning
network so that it is consistently tuned to the frequency being
"transmitted at that instant, allowing for peak efficiency."

I'm guessing this means they are sending FSK and retuning for
every bit.

Yep. When working at VLF or ELF (300 Hz to 3 KHz) frequencies, the
antennas are very very very very long. If the transmitter or receiver
is moving (submarine or airplane) the antenna tuning will change with
changes in altitude, depth, water salinity, proximity to mountains,
direction of travel, etc. The narrow bandwidths and slow data rates
necessitate some manner of continuous transmission communications
protocol. Here's an example of 1 character every 5 minutes:
"OPERATING ON ELF FREQUENCIES"
http://jproc.ca/radiostor/vlfelf.html
Just think of ELF as your worst communications nightmare.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558