That last place you want to buy coax from is Radio Shack. As mentioned, cable with a braided shield such as RG-58 is only about half there. Good cable has the shield braid wires snugged next to each other. RS's cable shields are more like the lattice around your porch. Once you have conductive Mylar shield around the cable (and usually a drain wire) such as the typical RG-59 or RG-6, you have 100% shielding. Before the foil shields, you could get very expensive double-shielded cable that typically had two good close-packed shields around the center conductor, some with the shields insulated from each other, others with the shields in intimate contact. And the wire was silver-plated when the military bought it.
Dean