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I recently cracked open an Xbox One controller, the impetus for doing so was two-fold:

  1. First and foremost the engineer in me likes to see how things work under the hood as I learn a great deal that way
  2. 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.

Front side with Antenna

Back Side with cut trace

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 SMA connector as well as AC-coupling cap in series.

Zoomed in picture of trace cut

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  • \$\begingroup\$ Since I don't want to take my controller apart... :) Maybe you could remove or short Y1 on the radio board? It won't do much transmitting without a clock. Suppose you could also gnd the input to U3 I would guess that might be an external amplifier. Remove enough parts and it'll stop. \$\endgroup\$
    – Some Hardware Guy
    Commented Jan 15, 2014 at 1:28
  • \$\begingroup\$ Oh and people make things that radiate unintentionally all the time, you don't need "everything in the right position" like you say unless you want to design an efficient system with good range and other qualities. So you cut the antenna in half now it doesn't work as good but it still kind of works. \$\endgroup\$
    – Some Hardware Guy
    Commented Jan 15, 2014 at 1:33
  • \$\begingroup\$ @SomeHardwareGuy So by cutting the trace that leads to the antenna and physically removing the antenna, where exactly is this RF radiating from? The only part of the circuit that appears to be still in tact is the trace leading from U3 to the cut. Can that really be enough to continue working from a shorter distance? I just don't get the physics behind that. \$\endgroup\$
    – SiegeX
    Commented Jan 15, 2014 at 1:49
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    \$\begingroup\$ Yes it is enough to keep working as the other guys mention below. You may find that if you remove U3 it will still work! You need to get back to the source which is probably U4 to really make sure it's dead (Since your goal is no radiation that could interfere with the other guy). That's why I suggested removing the oscillator to see if that works. \$\endgroup\$
    – Some Hardware Guy
    Commented Jan 15, 2014 at 2:58
  • \$\begingroup\$ "Besides putting it into a Faraday cage" . . . . .Although it's a much less elegant solution, you already have your PCB in a faraday-cage-in-waiting! Just put some copper tape on the walls of the inside of your plastic gamepad! I would certainly do all of the other suggestions first, but a little copper taping is always fun! \$\endgroup\$
    – scld
    Commented Jan 15, 2014 at 13:58

2 Answers 2

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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?

The red line you cut IS the antenna path. But not it's radiation source. The reason it now only connects at 1 foot instead of its usual range is because you crippled it. Two ways to completely (for practical purposes) remove the signal is depower the entire antenna/wifi section, which the sub-board is, or connect the antenna trace to ground.

For theory, Antennas should be a full or 3/4th or 1/2, or 1/4th of the wavelength for optimum propagation. Just because it is not that size does not stop it from working, it just stops it from working well. Your antenna is now a few millimeters long instead of a few inches.

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  • \$\begingroup\$ Thank you for your comment. Questions for you, I've definitely heard of using a 1/4 wavelength antenna but not the other quarter fractions your listed. When would one want to use something other than 1/4 wavelength? Secondly, do you think this antenna is a dual-band? The part of the antenna that goes off to the right at a 20* angle doesn't look like anything I've seen from an IFA or slot antenna. \$\endgroup\$
    – SiegeX
    Commented Jan 15, 2014 at 7:02
  • \$\begingroup\$ No idea, it's a custom designed Microsoft wifi chip. You could check the fcc id of the control to find any documents on the wireless properties. As for the fractional wavelength, 3/4th works like 1/4th does, because of the shape of a wave, sinusoidal. It's just as good as 1/4th for practical purposes, so it's a bit redundant, through there are special uses for it. \$\endgroup\$
    – Passerby
    Commented Jan 16, 2014 at 4:19
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Yes, you are correct, it is a variant of an inverted F antenna. The RF is in fact radiating from the short trace. At 2.4 GHz the free space wavelength is about 13 cm. In FR4 material used in PCBs it is about 1/2 of the free space one. So, even a short trace will radiate enough for a very short range.

According to Friis equation, the receive power goes down as a square of the distance between two antennas. So, if with a good IF antenna (gain about 0 dBi) your range is about 50 feet, to get the same receive power at a range of 1 foot your antenna can be 2500 times worse (or have a gain of about -34 dBi). It is quite possible that a short trace would have a gain of ~-30 dBi.

I think the best way to disable WiFi is to find a radio/WiFi chip and unsolder either the VDD or TR pins. TR pin is easier to find since it is soldered to the RF trace.

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  • \$\begingroup\$ Thank you for this answer. I did a bit more research on IFA's and I'm wondering if this is a dual-band antenna? The part that goes off to the right at ~20* makes me think this is for another band. Thoughts? \$\endgroup\$
    – SiegeX
    Commented Jan 15, 2014 at 7:00
  • \$\begingroup\$ It is hard to tell from the picture but it is likely that it is a dual 2.4GHz and 5.8GHz antenna. \$\endgroup\$
    – Yuriy
    Commented Jan 15, 2014 at 23:54

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