Ok, so the idea is't new. It's baked in cars, you mount the antenna on the top of your car, with a wire running to your phone. They work great by the way.
The problem is these antennas can only be used if you're in your car. Well, what if you want to carry your phone with you while hiking? Settle
for bad reception?
Most phones have an external antenna attachment either on the top or right behind the display. Design a mount to clip on your phone, use this as a base. This base will have your external antenna connection hook up when you clip it on. On the top of the base, a connector for a external antenna is placed on top and all wires are connected etc. External antennas generally use a standard connection<link>, so swapping different lengths would be as easy as screwing one off and putting the other on. I played around with the idea, hooked up an antenna, walked around with it, put it up to the phone, etc. and decided although it was a bit bulky, it was a definite solution.
I've googled the idea, and nothing came back but car antennas, boosters, internal stick-on antennas, and one clip-on antenna that really doesn't do anything but redirect waves away from your head. <link>
Best of all, any type of antenna you want to use can be adapted. Just screw off the old antenna and replace it with whatever you want.