RF power limits

[Sai Emrys]

One thing I found very neat about the Toorcamp badge was how it was
powered by RF.

My circuitry / EE knowledge, alas, is quite poor.

I was curious about one thing: how much power could such a device get,
in a radio field that is safe for humans and other devices? (E.g. hard
drives, credit cards, whatever else that might get damaged or
improperly messed with.)

This one was strong enough to power 2 LEDs, which to me is quite
impressive. Would making it bigger or multi-layered or the like
improve its power input?

In previous discussions I've had with more knowledgeable people about
wireless power, my takeaway understanding was that it was essentially
not possible beyond extremely minimal power (eg enough to 'power' an
RFID antenna ack), or very dangerous stuff (e.g. microwave antennas
that'll quick-fry anything in their path). Perhaps that needs
revision.

I'd STFW and answer it myself except I don't even know where to start;
just too much I don't know.

O ye who do: thoughts?

Thanks,

- Sai

[3ric Johanson]

Google Telsa and wireless power.



Sent via BlackBerry from T-Mobile

-----Original Message-----
From: Sai Emrys <sai...@gmail.com>

Date: Mon, 20 Jul 2009 14:20:34
To: <toor...@googlegroups.com>
Subject: ToorCamp RF power limits

[Lokkju Brennr]

Or, more recently, MIT has done some productive work with it.
http://www.mit.edu/~soljacic/wireless_power.html

Lokkju

[David Hulton]

Sai,

I had to do a bit of research when designing this and I still don't
fully understand all of the concepts with it, but here's some info on
what I learned. Basically, RFID readers are designed to provide power
inductively (magnetically more than electrically) to the tags by
essentially producing a wave signal (typically a square wave or AC
signal) on a coil which creates changes in the magnetic field of the
coil/antenna. If you have another coil with a capacitor which creates
an LC circuit that resonates at the same frequency (or close enough)
it will be easy for the electrons to move in the same fashion and
essentially creates a similar AC wave on the other side when it comes
into contact with the reader's changing magnetic field. The next trick
is that you just put an LED in the circuit which will pretty much
light up as long as you at least give it a couple milliamps.

Supposedly the HBL guys had a large RFID antenna and corresponding
multi-kilowatt amp that they were going to use to light up people's
badges from a great distance. Something like this would certainly be
very disruptive if you're close by but I think safety mostly depends
on the frequency. They claimed they have lit up similar circuits with
their setup at up to a hundred feet away or so but I'm not sure how
safe something like this is and it's certainly very wasteful if you're
just lighting an LED.

That being said, the antenna design matters a lot. Ours worked fine
because the transmitting antennas are usually pretty small, but
typically the larger the antenna the better the range. One thing we
also discovered was that the number of loops matters quite a bit. I
think this is mostly because when you start messing with different
amounts of turns in the antenna you begin to step up or step down the
voltage (like a transformer) which makes it less optimal for LEDs but
may make it more optimal for higher voltage or lower voltage
applications.

One other thing is that I believe that you achieve greater range with
higher voltage. We experimented with using a tesla coil to power the
badges but the only problem with this is that an extremely high
voltage is created on the badge and it also requires a return ground
(we burnt out some capacitors and LEDs testing this). We didn't have
enough time to calculate the best transformer design to create a mini
receiving tesla coil in the badges but I think this would have been
quite more efficient for wireless power transfer.

Either way, choosing between a huge rfid antenna \w multi-kilowatt amp
vs a tesla coil, I think both would be considered fairly dangerous....

If you're interested in looking more into this, here's some good resources:

http://en.wikipedia.org/wiki/Wireless_energy_transfer
http://en.wikipedia.org/wiki/Electrical_resonance
http://en.wikipedia.org/wiki/LC_circuit
http://ww1.microchip.com/downloads/en/AppNotes/00710c.pdf
http://www.tfcbooks.com/teslafaq/q&a_052.htm
http://www.hvlabs.com/sstc.html
http://www.instructables.com/id/Wireless-Power/
http://www.instructables.com/id/RFID_Reader_Detector_and_Tilt_Sensitive_RFID_Tag/

-David

[Nick]

RF power beaming is way too large and heavy at high power levels (>1W), though getting a few dozen mW at the other end, provided it was reasonably close, would not be that hard or dangerous to do.

I'm actually working on Laser-based power beaming, and we are able to get power levels of >100W @ 1km of separation (NDA limits me on exact numbers until Aug 8th). The biggest downside is the laser is extremely dangerous at anything above 1W of power output on the other end. Another downside is the system needs to be actively aimed at the receiver to generate power.

Our laser beam cooking hotdogs: 

http://www.lasermotive.com/blog/?p=309

[Mathew Lippincott]

Can't think of a way to transmit power, but a friend of mine just showed me how he uses speaker magnets to temporarily disrupt RFID tags when out "shopping"

--
headfullofair.com

[3ric Johanson]

And with one swift post, you've destroyed your career in infosec.

:)

Cheers,
-3ric

[Mathew Lippincott]

good thing I don't have one...
but for real a shoplifting friend just showed me his supposedly successful "method" over the weekend...  too bad it's bunkum, and good thing I don't steal. 

--
headfullofair.com

[Dominic Spill]

This isn't about power transmission but it is a great explanation of
turning the Toorcamp badge from a detector into a reader:

http://ossmann.blogspot.com/2009/07/toorcamp-badge-hacking.html