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How can I hack my "RFID Vehicle Gate Pass" to give it more antenna power? Answered

This gate pass is used to get me into my community.  I am pretty sure its whats referred to as a "Passive RFID Vehicle Transponder".  It doesn't have batteries.. therefor passive.  But the read range of the transponder is very POOR so I wanted to figure out a way to make the PASSIVE tag.. ACTIVE.  Lets say with a battery and new antenna.

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AcidCow (author)2011-04-07

also just realised that ... you'd probably need to produce a sine-wave of sorts - but I suppose a square wave would suffice as well... so it's not just a matter of coil + battery... it'd be more like coil + battery + ac circuit or timer or some such. guys at a radio shack should be able to give you a good idea of what you could use. wish we had radio shacks (or a fair equivalent) here in South Africa!

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AcidCow (author)2011-04-07

I would suspect that this device is powered by some sort of induction.... I'm no electrical engineer - but I'd say that by creating a coil of copper wire and replacing the triangular (fractal antenna I assume - this will use a different calculation to that supplied above - as it works on a similar principal to harmonics etc... like a cellphone antenna) you could induce a greater current from further away - however, since magnetic fields deteriorate at an exponential rate, you'd need an exponentially bigger coil to get further and further away.
perhaps just induce the current by having some sort of elctromagnetic field generate near the device. if that works... then presto. if not then you'd have wasted your time winding a simple elctro magnetic coild and attaching it to a standard battery.
once again - I'm no electrician... but steve seems to have a firm understanding - perhaps he could debunk my logic of confirm it?

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lemonie (author)2010-03-20

You could possibly change the antenna, how is it constructed (inside)?

L

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mmrogge (author)lemonie2010-03-21

Here are some photos.  The antenna is triangular.  What throws me is that its not a loop of any shape.  Its more like a figure eight in a sense.  You can see where I tried soldering into the loop.  I temporarily attached an ext cell phone car antenna I had laying around.  I have figured out a work around for now.. but would love to amplify this device somehow.

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steveastrouk (author)mmrogge2010-03-21

Well that's a 915Mhz RFID tag from Amtech, that antenna is probably optimal for the circuit, to hack it, you're going to have to experiment with a 915 Mhz antenna with some gain, like a can-tenna or something, but that means that the thing will have to be pointed accurately at the tag sensor at the gate.

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mmrogge (author)steveastrouk2010-03-22

What about using a 900 mhz cell phone booster?

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steveastrouk (author)mmrogge2010-03-22

Its worth a try. You don't need much of an antenna at 915Mhz, it's only about 3" long.

Steve

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mmrogge (author)steveastrouk2010-03-25

I looked up cell phone boosters on ebay.  The good ones are like $300, which is clearly not going to happen.  But there are cheap ones for a buck or two that supposidely STICK ONTO your cell phone.  What exactly did you may by 3 inches long?? and do you think the stick on type would work?  I cant imagine it would.

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steveastrouk (author)mmrogge2010-03-25

The  maths of antenna theory say that it should ideally be 1/4 wavelength long.

Since c= frequency x wavelength

Wavelength = c/ frequency = 300 E6/915E6 metres

...so Wavelength /4 ~ 3"

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mmrogge (author)steveastrouk2010-03-25

Sorry... that's way over my head. Do you think that one of those stick on cell phone boosters.. selling for a buck on ebay would do anything whatsoever.

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steveastrouk (author)mmrogge2010-03-26

Its vaguely possible it might help - and what have you lost ? A buck.

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lemonie (author)mmrogge2010-03-22

Nice pictures, I see some capacitance built into the board - the metal towards to middle in 1 & 2.
Does orientation make a difference? I'm thinking what steve said might be an option.

L

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mmrogge (author)lemonie2010-03-22

Yes, orientation matters.  The pass is generally mounted at 90 to 45 degrees for the antenna to read it.  I have suspected for years that the gain on the antenna is turned way down.. but as a result the pass only reads 2-4 yards from the antenna.. and there is a pretty narrow sweet spot.   Thanks for your help.

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steveastrouk (author)2010-03-20

If its passive, you're trying to tackle it from the wrong side - the gate needs modifying.

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mmrogge (author)steveastrouk2010-03-21

Thanks for your help.  Check out the photos I posted.  I cant access electronics of the gate.  But there is nothing keeping me from modifying my pass. 

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