Which of the following increases the resonant frequency of a dipole antenna?

The resonant frequency of a dipole antenna is inversely related to its length. As the length of the antenna decreases, the resonant frequency increases. This is due to the relationship defined in antenna theory, where the resonant frequency is determined by the physical dimensions of the antenna. A shorter dipole antenna will resonate at a higher frequency because it has a shorter electrical length, allowing it to effectively transmit and receive signals at those higher frequencies. Shortening the antenna reduces the overall electrical path length, leading to a resonance shift upwards in frequency.

In contrast, lengthening the antenna would lower the resonant frequency, while the insertion of coils or incorporation of capacitive loading typically serves to lower the resonant frequency or adjust the matching of the antenna rather than increase it. These concepts are fundamental in antenna design and tuning.