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#1 2009-11-30 22:23:26

eletari
Member
Registered: 2009-11-30
Posts: 3

Building an HF antenna, but can only get 3V

Hi all,

Admission: I'm not good with electronics, so I'm probably doing something wrong here.  Hence the question.  I'm building this antenna to use with a Proxmark3 that my department bought.

So, I'm following the instructions given at:

http://code.google.com/p/proxmark3/wiki … se_antenna

I've tried twice, getting pretty precise with the measurements (starting high and working my way down), but each time I've only been able to get about 3V out of the antenna.  My real question is whether I'm doing something fundamentally wrong, since I don't quite understand part of the directions. 

Here's what I'm doing.  Once I've stripped the cable and cut out the white and red wires, I've got just the green and black.  I trim them to length and strip the ends.  Then, the instructions say to "Make an antenna coil of 3 windings using the green cable. Connect the green cable with the black one and tape them together so they won’t unwind."  At this point I get confused, because the green wire is only 19cm, clearly too short to make 3 coils.  So, instead, I twisted the ends of the green and black together to make one big loop, taped them to make sure they'd stay together, and then doubled my new big loop three times, so that the antenna looks, as far as I can tell, basically like the one in the picture.

Is this basically what I should be doing?  I'm so ignorant about electronics and all that I'm not even sure that I'm right to connect the black and green wires together in a loop like that, so feel free to tell me if I'm being really stupid here.  As I tried this repeatedly, starting with each wire slightly too long, making the coils, tuning, cutting them shorter, making the coil, tuning, etc. the voltage basically just hovered slightly below 3V, and tune just told me that my antenna was marginal (and it seemed to be missing signals when attempting to communicate with a card).

Am I doing something fundamentally wrong, or should I just keep trying until I get it just right?

Thanks in advance!
Adam

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#2 2009-12-02 22:07:38

rleroy
Contributor
From: Between a REQB and a ATQB
Registered: 2009-03-04
Posts: 61

Re: Building an HF antenna, but can only get 3V

3V only! This is sick, you need to improve it a lot more to be more successful... And yes, you need to connect both ends of the wires, because you need to have a current in this antenna, and then the current will make the RFID field ...

Have a look at the antenna that I built based on Roel's design, here: http://www.proxmark.org/forum/topic/273 … -antennas/

Basically I had the same problem as you. But at the end, I only did three small loops with my wire, and it boosted the V around 12.5V ... which I found is enough for most of the tasks you will have to perform with 13.56mhz...

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#3 2009-12-03 19:07:15

eletari
Member
Registered: 2009-11-30
Posts: 3

Re: Building an HF antenna, but can only get 3V

Cool, I'll try it your way, with the much smaller coil, and see how that works.  Do you know whether the measurements given are taken from the end of the wires to the very end of the Hirose connector or to the base of the plug, where the wires emerge?

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#4 2009-12-08 16:47:20

eletari
Member
Registered: 2009-11-30
Posts: 3

Re: Building an HF antenna, but can only get 3V

And poof, it works, first try yields 12.5V.  Magical.  Thanks rleroy!

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#5 2009-12-08 19:25:12

rleroy
Contributor
From: Between a REQB and a ATQB
Registered: 2009-03-04
Posts: 61

Re: Building an HF antenna, but can only get 3V

smile

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