It's why I suggested the external antenna jack.
There is a circuit in the output - past that EXTERNAL antenna jack that loads - uses - matches - to the EXPECTED antenna it was designed for - so to place a BNC on it - you'd have to open up the unit and see if the BNC can work with the loading circuit still inside. You'd have to remove that loading coil/cap or combo - to obtain the same "ohmic" impedance the External Antenna Jack already provided. But, if it makes it convenient - once the old loading circuit is removed you should be ok to put in the BNC.
If you just put in the BNC and try to use it without removing the originals', there may be a performance hit against you due to the purpose of the loading circuit provided for the original antenna. You may lose out on a more efficient way if you just removed that socket and put in the BNC without looking at the loading/matching network the HT used. Doing it this way, you can possibly even damage the final the HT uses - because of the loading originally used is different to match a different antenna setup than what you're trying to do with the BNC.
There is a circuit in the output - past that EXTERNAL antenna jack that loads - uses - matches - to the EXPECTED antenna it was designed for - so to place a BNC on it - you'd have to open up the unit and see if the BNC can work with the loading circuit still inside. You'd have to remove that loading coil/cap or combo - to obtain the same "ohmic" impedance the External Antenna Jack already provided. But, if it makes it convenient - once the old loading circuit is removed you should be ok to put in the BNC.
If you just put in the BNC and try to use it without removing the originals', there may be a performance hit against you due to the purpose of the loading circuit provided for the original antenna. You may lose out on a more efficient way if you just removed that socket and put in the BNC without looking at the loading/matching network the HT used. Doing it this way, you can possibly even damage the final the HT uses - because of the loading originally used is different to match a different antenna setup than what you're trying to do with the BNC.