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#1 2010-02-24 22:49:26

wbahn
Contributor
Registered: 2010-02-03
Posts: 23

125kHz antenna construction

I've constructed a 125kHz LF antenna and have no idea how to tune it. Here is what I have done so far.

I used 30 AWG magnet wire and wrapped a coil of about 110 turns around a plastic form that measures 3" x 3.9". When I ran the tune command it reported values in the 2V range for both LF frequencies and said that the optimal was 0.05V at 9.42kHz (which I am assuming is the lowest frequency explored). I started removing turns and the voltage started coming up. I have removed somewhere in the vicinity of half the turns and the tune command now reports the following:

> tune

# LF antenna: 32.59 V @   125.00 kHz
# LF antenna: 27.18 V @   134.00 kHz
# LF optimal:  0.09 V @     9.42 kHz
# HF antenna:  0.03 V @    13.56 MHz
# Your LF antenna is unusable.
# Your HF antenna is unusable.

At one point the 125kHz voltage was in the 45V range. The LF optimal frequency has never changed and the voltage has never gotten above what it shows above.

I have no idea how to proceed from here -- any help would be appreciated.

I've looked for designs for RFID antennas on the web and haven't found anything definite (except for 13.56MHz antennas). In addition, I have found many sources with formulas for the impedance of a rectangular coil and they all seem at odds with each other and yield values that vary over nearly an order of magnitude for the same parameters.

But I'm not even sure what inductance I should be shooting for. I found some commercial antennas on the web that seem to have inductances of 680uH, which at least is consistent with another source that said that LF antennas have inductances ranging from several hunderd to a few thousand uH. I have no idea what my present antenna is, but my best guess is that it is in the 500uH to 700uH range.

Oh, and I have installed a 47pF capacitor for C35, in case anyone is wondering.

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#2 2010-02-25 00:01:13

d18c7db
Contributor
Registered: 2008-08-19
Posts: 292

Re: 125kHz antenna construction

Strike a ballance where the 125 and 134 Khz voltages are about equal and you're done, looking at your output you're already there.

I don't know why the LF optimal is not reporting the correct value, can you open the plot window, do a tune and losamples and post the picture of the waveform? It should look line a bell curve with the peak at about 129Khz which is 94 on the x asis, IIRC.

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#3 2010-02-25 08:17:00

wbahn
Contributor
Registered: 2010-02-03
Posts: 23

Re: 125kHz antenna construction

Hopefully this will give a clue to someone as to what may be happening.

When I run losamples, the yellow (D4) and red (D8) LEDs (the one by the button) come on for about 15 seconds. Then the red one becomes significantly brighter for about another fifteen seconds (the yellow appears to stay about the same). Then the thing apparently resets, which kills the prox gui window.

This behavior is identical on all three PM3 boards I built, so it is unlikely to be a bad solder joint, or a component accidentally put in the wrong spot or the wrong way. Of course, it's always possible that I misunderstood something along the way and systematically installed something wrong.

What should the LEDs be doing during sampling? The DC voltage at TP8 is 126mV for the first 15s and 3.22V for the second 15s. I'm assuming that the 126mV is attained by a PWM waveform with a duty cycle of about 4%. Does that ring any bells as far as when, in the code, D8 is driven by a 4% duty cycle PWM signal?

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#4 2010-02-27 07:13:37

wbahn
Contributor
Registered: 2010-02-03
Posts: 23

Re: 125kHz antenna construction

The problem appears to be solved. A couple of times I have asked if the problem might be a mismatch between firmware and client software revisions and how to determine which is compatible with which. No response.

Because I can only download binary images with my programmer (it doesn't work with s19 files), I had to burn my boards with a 2007 version (the ebuller version). But I couldn't figure out how to get a known compatible version of the software.

Because I need to get a working system in the hands of some students next week, I went ahead and spent over $400 to get a board and antenna from ProxMark3.com. Guess what, it behaves almost identically to my boards. Although the manual from their website says that their boards are preloaded with the Westhues code, that does not appear to be the case. I have no idea what version it is, but it doesn't work with either of the client software versions available from their site or the Summer 2009 software.

While I was waiting for my ace-in-the-hole costly commercial solution to arrive (and which doesn't work), I wrote my own program to take the s19 files and build a binary ROM image from them. Upon using it to build a file for the Summer 2009 release I downloaded it into one of my boards and, viola, they appear to work just fine. My antenna reported over 30V at 125kHz and claimed it was tuned for 127kHz. After taping it into a CD case it has shifted and is only 22V at 125kHz and is optimal with 40.8V at 115kHz.

As for the LF antenna that I spent $39 on, well it only returns 4.03V at 125kHz and reports optimal of 4.16V at 47.6kHz. Doesn't strike me as a very good antenna (in fact, the PM3 reports it as marginal).

So this turned out to be a $400 waste of money (and, yes, I'm a bit miffed, but I'll get over it).

The good news is that my boards work and I can now move forward.

For anyone that wants a binary ARM ROM image of the full Summer 2009 firmware, I can give that to you. I am also more than happy to give out my program that converts multiple SREC files  (they are NOT s19 files, BTW) to a binary image. Somewhat surprisingly, it worked perfectly on the first attempt (or at least no errors in the image have made themselves known).

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#5 2010-02-27 10:28:26

d18c7db
Contributor
Registered: 2008-08-19
Posts: 292

Re: 125kHz antenna construction

Great news that you got to the bottom if it and solved your problem. Personally, I treat the firmware and client software as a matching set and I never run an earlier or unknown client with a later firmware.

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#6 2010-02-27 11:13:13

iZsh
Contributor
Registered: 2010-01-02
Posts: 95

Re: 125kHz antenna construction

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#7 2010-02-28 08:50:16

wbahn
Contributor
Registered: 2010-02-03
Posts: 23

Re: 125kHz antenna construction

iZsh: I think I'll pass for now. Good luck, though.

d18c7db: Unfortunately, having them as a matched set wasn't an option for me up to this point. The only full ROM binary image I could find was for 20070827 in the Flash directory of the files site and it doesn't contain the client software. I asked both how to get a ROM image for a release that I could get the client software or how I might get the client software for that version of the firmware, since apparently that version of the firmware, at least when run with later versions of the client software, isn't capable of updating the Flash via the USB.

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#8 2010-02-28 22:23:52

iZsh
Contributor
Registered: 2010-01-02
Posts: 95

Re: 125kHz antenna construction

I was not saying "courageous" because I thought it would be less stable (in fact I think it's probably more stable, given all the fixes committed since the summer release). I was using this word because you already spent quite some time on making it work.

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#9 2010-03-01 01:54:39

d18c7db
Contributor
Registered: 2008-08-19
Posts: 292

Re: 125kHz antenna construction

Yes I'd tend to aggree, when I was last dong some development on the code late last year the code seemed flakey, like when it would crash then simply adding a printf statement would "fix" it. I had very little confidence in it, however in hindsight I believe a lot of that was due to the elf parser issues we recently fixed (thanx marcan) so the current (northern hemisphere) winter release should be a lot more stable.

I haven't had much time to actually test the functionality in terms of reading/writing tags but if anyone else has, please pipe up and let us know in this thread, so that if enough positive testing shows it is working, we can start recommending this firmware release as "the" one to use instead of the Summer 09.

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#10 2010-03-01 07:25:55

wbahn
Contributor
Registered: 2010-02-03
Posts: 23

Re: 125kHz antenna construction

I've just made a new antenna and initial indications are that it is pretty well tuned.

It is a 3"x3" square antenna made from 85 turns of #30 AWG magnet wire and produces the following results from the "tune" command:

====================================
> tune
#db# Measuring antenna characteristics, please wait.

# LF antenna: 40.95 V @   125.00 kHz
# LF antenna: 16.38 V @   134.00 kHz
# LF optimal: 40.95 V @   121.21 kHz
# HF antenna:  0.03 V @    13.56 MHz
# Your HF antenna is unusable.
====================================

Now it is time to start working with some tags.

Last edited by wbahn (2010-03-01 07:26:58)

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#11 2014-01-27 15:16:14

genexis
Contributor
Registered: 2014-01-25
Posts: 77

Re: 125kHz antenna construction

Did you have to add any capacitors to it? It just wire to the USB connectors

wbahn wrote:

I've just made a new antenna and initial indications are that it is pretty well tuned.

It is a 3"x3" square antenna made from 85 turns of #30 AWG magnet wire and produces the following results from the "tune" command:

====================================
> tune
#db# Measuring antenna characteristics, please wait.

# LF antenna: 40.95 V @   125.00 kHz
# LF antenna: 16.38 V @   134.00 kHz
# LF optimal: 40.95 V @   121.21 kHz
# HF antenna:  0.03 V @    13.56 MHz
# Your HF antenna is unusable.
====================================

Now it is time to start working with some tags.

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