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How can you disable antenna that the Xbox One controller use

 
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vivitern
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PostPosted: Mon Nov 27, 2017 9:18 am    Post subject: How can you disable antenna that the Xbox One controller use Reply with quote

I recently cracked open an Xbox One controller, the impetus for doing so was two-fold:
First and foremost the engineer in me likes to see how things work under the hood as I learn a great deal that way
Secondly, I wanted to mod my existing Xbox 360 fight-stick controller to work with the Xbox One by splicing into the Xbox One's PCB for button presses; this is colloquially called "pad hacking."
I was able to successfully mod my controller to utilize the Xbox One's PCB and now my attention is aimed at electrically disabling the wireless functionality. The reason this is necessary is that wireless communication is forbidden in fighting game tournaments due to the possibility of your controller affecting the inputs of your opponent (or vice versa.)
It's been a loooong time since I studied RF but I'm fairly certain that the antenna circled in blue in the picture below is an Inverted-F Antenna (IFA), is this correct?
I had thought that physically de-soldering the antenna would prevent the controller from wirelessly connecting, but I was wrong! It still connected at a distance of about 6-8 ft.
Next I thought that cutting the trace on the bottom side of the board leading to the antenna (shown in red) would surely do the trick but again I was wrong; it would still connect if I was within a foot or two of the Xbox One. As an aside, what exactly is that trace's functionality? Continuity tests shows to be at ground potential (this board has a common ground). Originally I thought this was the feed line but now I'm thinking this is the shorting trace which acts like a parallel inductor. Any ideas?
So what kind of voodoo is going on here, I had thought that to get a device to properly radiate in a functional manner requires all the right parts in all the right places. This board, however, seems to be built like the terminator; it just keeps working. Besides putting it into a Faraday cage, can anybody offer up suggestions on how to electrically disable this PCB from radiating?
As a side note, I'm not 100% sure what frequency band the Xbox One controller uses but if I had to guess it would be 2.4Ghz as that is what its 360 predecessor used.



EDIT
Here is zoomed in picture of the trace that I cut. I followed the trace with blue dots so you can see how it connects to the antenna on the other side of the board. The trace appears to have an Fiber Optic Connector 6754482-1(datasheet:http://www.kynix.com/uploadfiles/pdf/6754482-1.pdf) as well as AC-coupling cap in series.
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PostPosted: Mon Nov 27, 2017 9:53 pm    Post subject: Reply with quote

Yes, you found the antenna. Looks to me to be a modified pifa antenna, a bit different than an IFA, but they are somewhat similar anyway: http://www.antenna-theory.com/antennas/patches/pifa.php

The antenna you desoldered probably had an antenna efficiency of -3 dB (50%). When you desoldered it, you probably dropped the efficiency down to -20 dB (1%). When you cut the trace, the remaining stub might have had -26 dB efficeincy left. For RF link budgets, typical output power for BT is 5 dBm, with received signal sensitivity of -85 dBm. So there is about 90 dB of link budget for pathloss, and it becomes harder and harder to kill more dB without removing the radio.

You can look up the exact freuqency of what you are transmitting at via the FCC doc, it is probably 2.4GHz.

On your smaller pic, the little connector by the label CR1 is an RF test connector, a jack plugs in there to test the radio and discconnects the antenna. Tot he right of that is the antenna impedance matching and the antenna. Where you cut is the transmission line back to the radio.

If you want to further reduce the range you can copper tape or shield the radio and cut the trace length even shorter.
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