Please, I need the reference for the wifi antenna connector in Core Plus. I thought it was the same than in Up Core, but it isn´t.
You should use the antenna connector like the one from the UP Shop specific for UP Core Plus:
MHF4 connectors are needed to attach WiFi antennas. These are smaller than the (more common) U.FL antenna connectors. However, it is fairly easy to find "internal" stick-on MHF4 antennas supporting both 2.4GHz and 5GHz bands with the MHF4 connector on Amazon. If you want SMA pigtails to support "external" antennas, these can also be had but require a little more shopping around. I am using both internal and external antennas on two different boards without trouble. I expect (I'm guessing, since I don't work for Aaeon...) the reason for the smaller connectors is that they had to use a smaller WiFi module to squeeze onto the limited space on the UP Core Plus.
Of course you could just order the ones from Aaeon . Although it would be nice if Aaeon simply documented on their spec sheets that MHF4 antennas are needed.
I guess the official WiFi kit has the SMA pigtail connectors (technically, RP-SMA). It says there the connectors are IPEX 4. However, IPEX is actually a brand name (https://www.i-pex.com/Products/Micro-RF-Coax-Connector), while MHF4 is the standard.
You may also be interested in the following: a 3D printed base I designed for mounting antennas, either internal or external, that also provides a nicer place to mount the RTC battery: https://github.com/mmccool/up-core-plus-antenna-base/blob/master/README.md
Thank you very much, Michael, for your explanation. I agree that everything would be easier if Aaeon simply documented on their spec sheets that MHF4 antennas are needed. So MHF4 is the connector for the internal wifi antenna, which is bigger than the U.FL one that the UP CORE has.