The antenna connectors on the F43 are Motorola MX connectors. The spec is 1/4-32 screw by 7/32 in. long. I am actually in the market for an aftermarket antenna myself. For an external (I assume that you want to put one on you car or similar) antenna, I would Google for an MX to BNC adapter (which I know exist because I have one), or MX to what ever you have. I've attached a link for an adapter.