On 3/3/13 10:47 AM, Chris Albertson wrote:
It's not an e-field antenna. The goal is the sense the current in the LCD.
My bet is no much of anything leaks out of that RSA device or a wrist
watch either. You have to figure that tiny battery lasts for over a
year and even if ALL the energy in the battery went to making RF you'd
divide the battery energy by the battery life time to get power.
What's have some micro watts at most. Then you figure most of the
bettery power really goes into heat not RF.
detecting femtowatts isn't really an issue for RF.. 1 fW is -120dBm,
which is a "strong" signal in a lot of applications. Typical FM
receivers have sensitivities around 0.2 microvolt into 50ohms, which is
about a femtowatt. -150dBm is getting tougher, but is still 24 dB
above the kTB noise floor in a 1 Hz BW.
However, what we're looking for here is most likely a changing magnetic
field. It's not being "radiated" away, the energy stays in the circuit
(in the near field) for the most part. (question, does putting a RSA
fob in a lossy magnetic medium make the battery go dead faster?)
So the question really comes down to how small a repetitive change in
magnetic field can be detected?
Or if you're using an efield probe, it's sort of the same thing. You're
not concerned about far field radiation, which will be very small (the
"antenna" is a tiny fraction of a wavelength.
Here's a similar thing.. Say you have a twisted pair carrying a signal:
very little radiates away. But if you have a small probe (E or H) you
can put it closer to one wire of the pair than the other, and
(potentially) detect the E or H field from the wire.
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