Hi,
this is my experience on using an external antenna instead of the PCB trace for the DLP-7970ABP.
Start by removing C24 and C23. Afterwards pick a U-FL cable and connect it to the SMA of a board in order to have a board where to adapt the impedances. This is my "design", it tooks less than 20 minutes to manufacture and it is a board which GND plane I solder to the big GND pad in the DLP-7970ABP :) For the impedance matching network, I just put 0805 footprints everywhere and use the big pad to solder one ending of the coil and the GND plane for the other.
I started with a ~3uH coil. This is a bit higher than the recommended value but the antenna geometry was fixed so I had no choice. This Z is ~ 2.88 + 256.25j.
After an R ~ 2K and a ~47pF serial capacitor, I got something quite acceptable. I know I could have adjusted the components a little more tightly but I was in a hurry and I did not have a lot of choices in my lab :) VSWR ~1.5. BW should be narrower too :P Whatever, first test.
If your requirements are not very demanding, with this configuration, I'm able to read in varios positions all kind of tags, 15693 and 14443 with no problem at all. See video. The antenna is around that crappy white box :) The first card is 15693 and it can be read from > 10cm. The second one is a DESFIRE card 14443 and obviously range is shorter.
Hope this information is useful to somebody, have a nice day! :)
PS: I'm a very bad cameraman, I know.
