I added an external antenna for my Raspberry Pi because the enclosed aluminum wiring tray seriously degrades the wifi signal.
Disable onboard wifi
I found this discussed and documented here.
nano /boot/config.txt
dtoverlay=pi3-disable-bt
dtoverlay=pi3-disable-wifi
systemctl disable hciuart
nano /etc/modprobe.d/raspi-blacklist.conf
#wifi
blacklist brcmfmac
blacklist brcmutil
#bt
blacklist btbcm
blacklist hci_uart
reboot
Performance
