The last criteria that must be addressed is the orientation, or polarization of the antenna. TV antennas use horizontal polar- ization and the type of stations scanner enthusiasts listen to have vertical polarization. Horizontal polarization is where the antenna elements are parallel with the surface of the earth and vertical polarization is where the elements are perpendicular to the surface. TV antennas are designed for horizontal polarization and must be modified for scanner use. Fortunately, the modification is very simple.
Most TV antennas use square tubing as the boom.
Be sure to keep the same center to center spacing so the clamp will pass through the holes comfortably. Depending on the antenna you chose to modify, you might have to move the location of the mounting holes forward or backwards one or two inches to clear an element or the feed system. The balun transformer that usually comes with the antenna will still be used. You can use 75 ohm cable without significant degradation of performance. So this gives us a single antenna that offers a reasonable cost to performance trade-off for the scanner listener.