Here's what I uncovered last night as I continue to try to figure out what connections and internal antennas do what…. Please, someone correct me if I am wrong.
Both internal antennas are 2.4GHz. Looking down at the PC boards once the back cover is removed, the one inside on the upper right is the telemetry antenna and the one on the upper left is for 2.4 GHz WiFi (probably not used to control the H). tracing back the feed line from each…. The connection for telemetry and the new external antenna should be made to the RF board in the center of the ST16 (after you remove the small internal antenna)…. I believe this board has marking of "SR 24" and this appears to be the same board used in the H (craft) as the control/telemetry transceiver. Yuneec is using a protocol called "Zigbee" to control the H --- which has a standard frequency of 2.4 GHz in the US. So that board has two antennas on 2.4 GHz (perhaps a diversity system).
The other "communications" board in the ST16 is a WiFi board (marked "ST18_Wifi) and used for the internal 2.4GHz antenna (upper left side) and the external 5.8 GHz camera antenna-- It's located to the upper left of the ST16 (when looking from the top).
Yuneec has repositioned the external antennas on the ST16 plus model to have the 5.8 GHz now in the middle and the two 2.4 GHz on either side. The WiFi antenna remains internal.
I've seen various mods where people have moved all four antennas externally --- in the FPVGuy's H antenna mod they show Jorge's controller using a itelite DBS ST10 antenna along with the 5.8 GHz mushroom antenna--- I believe this is wrong as the DBS ST10 antenna covers two 2.4 GHz antenna and one 5.8 GHz….So, he should be using THREE 2.4 GHz antennas externally and ONE 5.8 GHz.
I will also point out in the FCC database of files/photos to get certification for the ST16… It appears the test facility had identified the external antennas incorrectly and mixed them up…Having the 5.8 GHz antenna on the left (when using the controller).
In the end…. I don't believe there is any need to move the 2.4 GHz WiFi antenna externally unless you are using the internet…. I don't think it is used for any features on the H.