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Registered Member #9614
Joined: Mon Jan 14 2013, 10:00PM
Location:
Posts: 44
Are there certain transmitter coil geometries that are better in increasing the range of LF RFID? Most coils are flat/pancake coils but that is most likely for fitting inside a flat card, but are solenoids or inverted cones potentially better? I am planning to model each of these in femm, but I'm not sure how to objectively compare the different geometries, I suppose I could shoot for the same inductance in all geometries?
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Hi, I accidentally discovered a way to do this. If you add a graphene sheet (sellotape + zrzxg graphite) to the coil it boosts range a bit like a ferrite rod.
EDIT: Works best around 13.56 MHz, 125 kHz does work but not as well.
You can just use conventional ferrite rods and this is a well established technique. The main reason why the rods don't have wire directly on them is that it needs the air gap. Also if you move the wire form it gives you better gain, essentially you tune it for the correct band in this way. Length of rod is a factor as is composition and temperature(!) which is why those LF77 clock modules are hard to make due to matching capacitor performance.
Incidentally I came up with an idea for a better scanner, its a pair of ferrite rods with half the tuned circuit on each and a precise offset air gap between them so that RF energy is emitted like a disk drive head. Should work but unable to test at the moment as my LCR is MIA. Best to use two identical rods but tune on the fly using previously discussed method.
Adding iron magnetic end couplers *may* boost range as the magnetic circuit would then be complete. Look at design of induction cookers for ideas!
EDIT: actually need to build one of these to find a missing passport.
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Increasing LF RFID range
