It’s pretty straightforward… you just connect the two wires from the antenna coil to the correct pins on the USB port… however, the problem you will undoubtedly run into is tuning. This is not normal RF stuff where you just take a length of wire of the correct length for the frequency involved and attach it… this is a resonant L/C circuit, and when you are fiddling with the antenna part, you are just fiddling with the L part of the L/C circuit.
Basically the frequency of a resonant circuit is determined by the combination of L (inductance) and C (capacitance)… and these work together to produce a resonance at a specific frequency. To make it simple, let’s say you have an L/C circuit where L is 10 and C is 10 and the frequency is 125kHz… and now you come along with a new antenna that has an L value of 15… well that’s going to drop the frequency down to like 100kHz… so to adjust things, you need to drop your C value to 5 to get you back to 125kHz… these are just bullshit numbers, but the example is correct. If you bring in an antenna that has a higher inductance you need to drop your capacitance to hit that frequency target, and vice versa… if your antenna is lower inductance you need more capacitance to maintain the target frequency.
The problem is, you can’t easily change the capacitance of the proxmark3 because it would involve removing and replacing SMT caps on the board… not fun. However, there is one possibility here… and that would be if your new antenna was lower inductance, then you could add parallel capacitance to the new antenna to make your adjustments.
So you might be wondering why some people have been able to just use an xEM Access Controller antenna with their proxmark3 without much fuss, and others are having a hard time… well that has to do with a few things… one being the fact that the proxmark3 is open source, so you are going to get a load of different designs and models and even massive parts changes and terrible tolerance values within a single part SKU… so even if you buy a “proxmark3 wingding edition” there is little chance that there will be uniformity for that product line from one batch to the next… whole cap values could change (as would the antenna inductance values) and the caps they use might have huge tolerances of like 10% which would swing the target frequency quite a bit, especially if they also had shit tolerance on their inductors… you might end up with an L/C circuit “antenna” that is 20% off target!
So, with that out of the way, to answer your specific questions;
yeah… the two wires from the access controller antenna must be connected to the correct pins on the USB jack… there are 4 pins in the jack and 2 are used for HF and 2 are used for LF and I really don’t remember which, but you can test with a simple meter and a working LF antenna… the actual two wires themselves don’t matter… this is not DC and there is no polarity.
nope, just make sure you use the hw tune command to check your antenna quality.