Suggestions for a 10 Meter Underwater Comm Link using Magnetic Induction

Thread Starter

Zebryk

Joined Nov 15, 2012
3
I am looking for suggestions for a 10 Meter Underwater Communication Link using magnetic antennas.
(Being magnetic, it should not make a big difference if freshwater or saltwater.)
The antennas could be for either 125KHz or 13.56MHz.
The coil diameter should be less than 6".
The data rate should be at least 9600BPS.
The transmitter power supply needs to be a maximum of 1 Amp. (In burst mode.)

Any suggestions are appreciated.
 

Papabravo

Joined Feb 24, 2006
16,938
I am looking for suggestions for a 10 Meter Underwater Communication Link using magnetic antennas.
(Being magnetic, it should not make a big difference if freshwater or saltwater.)
The antennas could be for either 125KHz or 13.56MHz.
The coil diameter should be less than 6".
The data rate should be at least 9600BPS.
The transmitter power supply needs to be a maximum of 1 Amp. (In burst mode.)

Any suggestions are appreciated.
Do you have a reason to believe that this is feasible?
 

Papabravo

Joined Feb 24, 2006
16,938
Using near-field magnetic coupling (a communications loosely coupled transformer instead of an antenna) it is possible. Think, short range secure underwater communications. ;)

Magnetic Induction wireless communication system
https://par.nsf.gov/servlets/purl/10082515
10 meters would be in the vicinity of half a wavelength at 13.56 MHz., so is that a working definition of near field?
What about the VLF frequency of 125kHz. Does that have a different near field radius?
 

nsaspook

Joined Aug 27, 2009
8,770

Papabravo

Joined Feb 24, 2006
16,938
So 10M @ 13.56 MHz. would be a heavy lift, but 10M 125 KHz. could be feasible, assuming an appropriate data modulation technique. At 125 KHz., what modulation technique would get you 9600 bits/second? None of the digital modes, I'm currently aware of, that we use in amateur radio communications would make it to that level.
 

nsaspook

Joined Aug 27, 2009
8,770
So 10M @ 13.56 MHz. would be a heavy lift, but 10M 125 KHz. could be feasible, assuming an appropriate data modulation technique. At 125 KHz., what modulation technique would get you 9600 bits/second? None of the digital modes, I'm currently aware of, that we use in amateur radio communications would make it to that level.
The max speed obviously depends on the quality of the link and the given bandwidth with each type of data modulation technique. We had 56Kbs links using 3Khz bandwidth lines with phone modems.

https://www.eenewseurope.com/news/125khz-rfid-transponder-support-data-rates-64kbs-1kbs
The device is a stand-alone, fully functional tag that includes an antenna coil trimmed to an operating frequency of 125kHz. It provides the highest flexibility for universal use, supporting a broad range of data rates from RF/2 to RF/128 (64kBit/s to 1kBit/s at 125kHz), modulation and a variety of coding schemes including ASK, FSK, PSK, Manchester, Bi-phase and NRZ.
 
Last edited:

Papabravo

Joined Feb 24, 2006
16,938
The max speed obviously depends on the quality of the link and the given bandwidth. We had 56Kbs links using 3Khz bandwidth lines with phone modems.
...
So the 56K modems used PCM (V.90 and V.92) and required a digital interface at one end. Does the TS even know what that is and how to do it?
I was thinking that some version of QAM might be required.
 

nsaspook

Joined Aug 27, 2009
8,770
So the 56K modems used PCM (V.90 and V.92) and required a digital interface at one end. Does the TS even know what that is and how to do it?
I was thinking that some version of QAM might be required.
I'm just saying what's possible, not what's probable.
 

Deleted member 115935

Joined Dec 31, 1969
0
For my own interest,

the OP wants to use a max diameter 150 mm coil,
and they want to use max 1 Amp Tx power,

implication is its hand held,

Lets assume its 12 V powered device,

My RF friends here, what sort of field strength at 13.56 MHz would be possible from a coil as described given the power available,

As a mainly digital guy, driving a big coil at 13 Mhz, with an Amp , seems amazing that such an inductive load could be driven.
 

DickCappels

Joined Aug 21, 2008
7,836
For my own interest,

the OP wants to use a max diameter 150 mm coil,
and they want to use max 1 Amp Tx power,

implication is its hand held,

Lets assume its 12 V powered device,

My RF friends here, what sort of field strength at 13.56 MHz would be possible from a coil as described given the power available,

As a mainly digital guy, driving a big coil at 13 Mhz, with an Amp , seems amazing that such an inductive load could be driven.
You just need enough voltage. 12V doesn't feel like enough.
 

nsaspook

Joined Aug 27, 2009
8,770
For my own interest,

the OP wants to use a max diameter 150 mm coil,
and they want to use max 1 Amp Tx power,

implication is its hand held,

Lets assume its 12 V powered device,

My RF friends here, what sort of field strength at 13.56 MHz would be possible from a coil as described given the power available,

As a mainly digital guy, driving a big coil at 13 Mhz, with an Amp , seems amazing that such an inductive load could be driven.
Those coils can be made to be self resonant at 13.56MHz with a simple matching network to couple power. I routinely pump several thousand watts into 13.56MHz water cooled tank cans but that's in a vacuum chamber, not underwater.
https://forum.allaboutcircuits.com/threads/build-cb-transiever.57663/post-1498563
 

timm27

Joined Dec 11, 2020
7
I think 13.56 MHz in salt water would be difficult - have you looked for any resonances? You certainly would not want to start looking to design some sort of equalization. Having said that I think I might try a combination of direct sequence modulation on top of a Manchester code which would give you the opportunity to get coding gain out of a moderately wide-band signal. Another approach for a 10 metre range might be easier and more to look at an acoustic carrier signal. Transducers might be a challenge, but 'fishing sonar' devices have been around for many years and have much more than a 10 metre range and are pretty reliable.
 
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