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Forums
4hv.org :: Forums :: Electromagnetic Radiation
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Increasing LF RFID range

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Alex Yuan
Mon Feb 25 2019, 03:19PM Print
Alex  Yuan 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?
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Sulaiman
Mon Feb 25 2019, 03:42PM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
It is not just antenna efficiency, impedance matching to/from the electronics is also important.
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johnf
Mon Feb 25 2019, 06:22PM
johnf Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
antenna aperture is the most important criterea ie how many field lines does the antenna intercept
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Conundrum
Thu Feb 28 2019, 06:34AM
Conundrum Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
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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