To lengthen an antenna electrically, which component should be added?

To lengthen an antenna electrically, adding a coil is the correct approach. A coil, which is also referred to as an inductor, introduces inductance to the antenna system. When an inductor is added to an antenna, it increases the total electrical length of the antenna without physically increasing its size. This happens because inductors allow a phase shift in the current flowing through them, effectively altering the resonant frequency of the antenna. As a result, the antenna can resonate at lower frequencies, which is particularly useful in matching the antenna to operational frequencies for improved performance.

In contrast, a capacitor, while it can also be part of tuning or matching circuits, typically serves to shorten the effective electrical length of an antenna rather than lengthening it. A resistor may be used for various purposes, such as power dissipation or impedance matching, but it does not contribute to changing the electrical length of the antenna. Although an inductor is essentially a coil, the terminology might be nuanced in some contexts, but the desired function to lengthen an antenna specifically points to the utility of a coil.