near-field magnetic comm technolgy

[Carey Fisher - NCS]

http://www.theinquirer.net/?article=11653

Interesting short range headset, also in EE Times (Nov 3, p79). Anyone have
an idea where to start in designing something like this? Would you treat
the "antennas" like (large) air-core transformers? This is the only link
I've found by Googling - anyone know of others?

 

[John Woodgate]

I read in sci.electronics.design that Carey Fisher - NCS
south.net>) about 'near-field magnetic comm technolgy', on Sun, 9 Nov
2003:

http://www.theinquirer.net/?article=11653

Interesting short range headset, also in EE Times (Nov 3, p79). Anyone have
an idea where to start in designing something like this? Would you treat
the "antennas" like (large) air-core transformers? This is the only link
I've found by Googling - anyone know of others?

The above URL seems to give conflicting information.

"The foneGEAR chord free headset "near field" magnetic communications –
induction - supports 25 hours of talk time on one AA battery and runs on
the low frequency 13.5MHz band."

13.5 MHz isn't 'the low frequency band', and its wavelength is 22 m, so
the near field radius is about 3.5 m. So the transmission is not purely
by induction unless the range is really short.

"The firm told the INQUIRER that you would have to get really close to
someone using a similar device before you'd experience interference."

Well, maybe. It depends on whether different systems operate on
sufficiently different carrier frequencies.

"The Department of Defense (DOD) and NASA are both interested in the
technology, which is non-progagating."

It can't really be non-propagating, AFAICS.

"The firm said that unlike radio frequency (RF) technologies, the power
generated is about 100 nanowatts. The range of the headset is about four
feet."

It IS radio-frequency technology. 13.5 MHz isn't audio, even for bats.
(;-)

Of course, the URL text may have been subjected to journalistic
'improvement'.

I'll be surprised if 4 feet is enough range to be practically useful. I
have radio-frequency headphones for assisted hearing of TV and radio
without disturbing the neighbours, and I find the range of around 10 m
is about right.

Furthermore, *baseband audio* induction communication systems have been
around for many decades.

 

[Carey Fisher - NCS]

The corresponding article in EETimes is:
http://www.eetimes.com/in_focus/communications/OEG20031031S0049

Seems to me they don't REALLY mean RF vs Magnetic Field since, as you point
out, 13+ MHz is Radio Frequency. They must mean Electromagnetic vs
Magnetic. So they are using the magnetic field alone with no electric field
component. Why do editors of a magazine directed at EE professionals make
mistakes like this?

This is still interesting as it has applications for headset use in a RF
(EM!) rich environment with reduced EMI issues. So I'm still interested in
any info anyone has about designing stuff like this. I'll try some more
Googling also.

 

[onestone]

This sounds like the Libertylink Chip from Aura. It is magnetic
Induction for the comms. Range is rated at 1.5mtr in the spec sheet I
have. There are very definite though obscure applications in military roles.

Al

 

[Clifford Heath]

This article teaches a pragmatic approach to calculating the
near-field (magnetic) coupling:
<http://www.hi-point.com/rfid made easy.pdf>.
It's by EM Microelectronic-Marin SA, <http://www.emmarin.ch>

 

[Carey Fisher - NCS]

Good link. Thanks!!!

 

[John Woodgate]

I read in sci.electronics.design that Carey Fisher - NCS
lsouth.net>) about 'near-field magnetic comm technolgy', on Sun, 9 Nov
2003:

Seems to me they don't REALLY mean RF vs Magnetic Field since, as you
point out, 13+ MHz is Radio Frequency. They must mean Electromagnetic
vs Magnetic. So they are using the magnetic field alone with no
electric field component. Why do editors of a magazine directed at EE
professionals make mistakes like this?

Because they are editors, not electronic engineers. Even editors who
have some technical knowledge often have big gaps and unusual opinions.

This is still interesting as it has applications for headset use in a RF
(EM!) rich environment with reduced EMI issues. So I'm still interested
in any info anyone has about designing stuff like this. I'll try some
more Googling also.

The sending antenna is typically an air-cored coil of fairly large
dimensions e.g. 150 mm square. The receiving antenna may well be on a
ferrite rod. Both antennas, of course, have figure-of-eight directional
responses. It requires heroic methods (angular diversity reception) to
overcome this.

You can do this with baseband audio, as well, and a lot of design
information is available. But the r.f. solution probably requires much
less power, especially if you use one of the high-efficiency versions of
amplitude modulation. OTOH, spectrum management authorities may not let
you use the 13.5 MHz band (or any other ISM band) for speech
communication.