Can I get an external antenna for my iPhone?

I travel around the UK on my boat and am often inside the steel hull and frequently am in a bad signal area. Is it possible to plug-in an external antenna? I'd like to connect it to a lead that goes outside. I already have such a cable and mag-mount external antenna which I've used with other phones in the past, and it works a treat.

I guess the question boils down to whether the ihone's connector has access to the internal antenna circuitry, and if so, is there an adaptor available to plug into my antenna lead, which has a pretty standard connector. Thanks.