Proxmark3 community

KernelJay

Member

Registered: 2015-06-20

Posts: 9

Antenna Tuning

Hi board,
I am preparing some research on how 3D printing can help facilitate DIY/custom antenna fabrication.

I just printed/tested my first design yesterday and I am very pleased with the results.

I get the following data from 'hw tune':
30.80 V @   125.00 kHz
17.05 V @   134.00 kHz
30.80 V @   125.00 kHz <-- Optimal

The antenna works great and seems a bit more tolerant on positioning and distance when reading a particular AWID tag.  (I am using the $60 LF PCB antenna from Rysc as a point of comparison.)

The commercial antenna is tuned a little differently:
15.12 V @   125.00 kHz
26.68 V @   134.00 kHz
29.29 V @   131.87 kHz <-- Optimal

My question for the board is with regards to how important it is where within the 125-134 kHz range my antenna is tuned.  Clearly my antenna is not getting close to the 26.68V @ 134k but my 134k measurement is 12% better than the commercial antenna's 125k measurement.

Any thoughts on optimal voltage/frequency are appreciated!

-Craig

marshmellow

Contributor

From: US

Registered: 2013-06-10

Posts: 2,302

Re: Antenna Tuning

Depends on the tags you want to read.  125khz tags or 134khz tags.
Since most of the tags I read are 125khz I tend to let it be like your results (stronger at 125 kHz)
but if you want an overall antenna you attempt to split the difference. Which your other attempted, but in my opinion failed at, because if you are going to make one side stronger it should be the 125khz range in my experience.

KernelJay

Member

Registered: 2015-06-20

Posts: 9

Re: Antenna Tuning

Thanks for the quick response!  That's pretty much what I was thinking but just wanted a sanity check.

Omikron

Contributor

Registered: 2010-02-12

Posts: 78

Re: Antenna Tuning

I also prefer the "splitting the difference approach".

Here's a LF antenna that I built the other day:

KernelJay

Member

Registered: 2015-06-20

Posts: 9

Re: Antenna Tuning

Nice voltages! What are the rough dimensions? I'm looking at this design partially because of how close it is to an actual badge but I'm generally exploring RFID antenna technology for the first time.

How does your build do for read range? Is it picky about card orientation?

marshmellow

Contributor

From: US

Registered: 2013-06-10

Posts: 2,302

Re: Antenna Tuning

i split the difference, just a hair leaning towards the 125khz range. 

proxmark3> hw tune

Measuring antenna characteristics, please wait...#db# DownloadFPGA(len: 42096)
....#db# DownloadFPGA(len: 42096)

# LF antenna: 43.17 V @   125.00 kHz
# LF antenna: 32.45 V @   134.00 kHz
# LF optimal: 46.20 V @   126.32 kHz
# HF antenna:  0.14 V @    13.56 MHz
# Your HF antenna is unusable.
Displaying LF tuning graph. Divisor 89 is 134khz, 95 is 125khz.

I have the above with 3 antennas, one that is 1 inch square, one that is 1.5 inches square, and one that is about 2.8 inches x 1.6 inches (about the size of a prox card's antenna).

the larger rectangle i use most of the time, occasionally there is a very small tag that requires a smaller antenna to get clean reads..  however, with a strong antenna like this it is sometimes necessary to space the tag at least 1cm from the antenna to get a clean read.  As far as distance, you still can't get more than a few inches (3-5) depending on modulation/chip.  you'd need amplifiers and more power for longer distance. 

the 1.5" square is probably the most versatile antenna though, picking up the smallest tags while not suffering read range/strength yet.

Last edited by marshmellow (2015-07-01 02:42:44)

Omikron

Contributor

Registered: 2010-02-12

Posts: 78

Re: Antenna Tuning

Great construction, marshmellow.

I noticed that all of your antennas are either square or rectangular.  Have you observed any real-world differences between circular and square antennas?  I've played with both in my own designs and sometimes I can't tell if there's a notable difference.

marshmellow

Contributor

From: US

Registered: 2013-06-10

Posts: 2,302

Re: Antenna Tuning

I haven't noticed any differences based on shape, and there really shouldn't be as long as they are tuned properly.  It just easier to cut a square

Last edited by marshmellow (2015-07-01 02:40:33)

KernelJay

Member

Registered: 2015-06-20

Posts: 9

Re: Antenna Tuning

Thanks for the info.  Very helpful stuff! 

If anyone is interested in 3D printing an antenna form, I have posted two models on GitHub for anyone who wants to give them a go.  First is the one pictured at the start of this thread and the other is intended to look like a badge complete on a lanyard for use in tag replay:

The form in the picture was printed by Shapeways for $12.68 but it is only roughly $4 of plastic using more expensive proprietary filaments and closer to $1 on printers using generic filament.

Here is a link to the commit including these models:

https://github.com/VERTCraig/proxmark3/commit/73d336ae1dc311e8a8a8d9e893ce4c5ad8c31f49