I might have to look in to this sometime soon. My property is maybe 10 miles outside of Crescent
City. I'd imagine a car or truck antenna from the junkyard would work halfway decent if you got it off the ground far
enough. I know with the shape of the
land at my property that my cellphone doesn't get any signal if I'm down between the hills. I'm hoping to spend a lot more time there when the weather cools off and one goal is to build some sort of antenna tower out of salvaged stuff.
You could always test how strong the signal is with a vehicle radio. That would give you a baseline, and from there try getting an antenna off the ground a fair distance. Having too long of a cable going to the antenna can also be detrimental to the signal. Perhaps finding a spot that naturally has less radio wave obstruction would be easy enough to do with a portable radio. As mentioned above, some antenna designs are directional. They tend to perform better once aimed, but will perform more poorly on signals it isn't aimed at vs an omnidirectional antenna. I don't know about new cars, but older ones tended to have an AM antenna built in to the windshield. If you want a really good signal for both then it might require separate ones for each band.