How can a vertical antenna, 2 metres in length, be made to resonate in the 80-metre band for mobile use?

A naturally resonant quarter-wave vertical antenna for the 80-metre band needs to be approximately 20 metres long. A 2-metre mobile whip is drastically shorter than this physical requirement. Because it is physically too short, the antenna exhibits a highly capacitive reactance at the feedpoint. To make this short antenna resonant on 80 metres, you must electrically lengthen it. This is done by installing an inductor (often called a loading coil) in series with the antenna. The inductive reactance of the coil cancels out the antenna's capacitive reactance. Adding a capacitor would have the opposite effect, making the antenna electrically shorter.