Because there are so many different reception situations. An antenna is chosen for a particular situation according to its basic reception “pattern”.
The pattern shows the way the antenna absorbs the signal. Let’s examine the four basic TV antennas.
1. The Single Dipole–A single driven element, a dipole produces this pattern. A dipole is two metal sticks that are physically held together by a plastic terminal block that insulates each arm from the other. . The lead in wire is attached to the block. One side of the wire is attached to the block. One side of the wire touching the right element and the other side of the wire touching the other element involved.
If the TV transmitter is spewing waves anywhere between 120° to 60° or 240° to 300° the antenna will absorb to its maximum. If the TV rays lob in anywhere else in the circumference the antenna will absorb less and less till it responds at an absolute minimum at 0° and 180°, the ends of the antenna.
Obviously then, for best reception you must aim the dipoles broadside at the transmitter. If there is more than one station you must compromise your aiming of the fixed antenna for best reception from the multiple stations.