If the characteristic impedance of the transmission line does not match the antenna input impedance then:

When the characteristic impedance of a transmission line differs from the antenna's input impedance, an impedance mismatch occurs. This mismatch prevents the antenna from absorbing all the RF power sent to it, causing some of that energy to reflect back down the feedline toward the transmitter. As the forward-traveling waves collide with the reflected waves, they combine to form a pattern of stationary voltage and current peaks and troughs, known as standing waves. While this mismatch reduces efficiency, it does not mean the antenna will stop radiating entirely; it simply means the SWR will rise above a perfect 1:1 ratio.