As Paul said, the cable quality (signal loss) is critical. It is VERY easy to buy an external antenna with "just a few yards" of cable, and wind
up with a weaker signal than the internal antenna (with zero cable losses) provides.
Any reputable vendor should be able to tell you "this antenna has a gain of ## decibels (db) and this cable has a loss of ## decibels per hundred feet."
Figure out the loss for the cable length you will need, subtract that from the antenna gain, and you'll know if it will accomplish anything. And again, a good vendor (as opposed to a discount clerk) should be offering to do that for you.