Question 3-65J4

To increase the resonant frequency of a 1/4 wavelength antenna, you need to make it electrically shorter. Here's why: An antenna's resonant frequency is determined by its electrical length. For a 1/4 wavelength antenna, resonance occurs when its length is effectively one-quarter of the operating wavelength. If you want to increase the resonant frequency, you need to effectively shorten the antenna's electrical length. * **A) Add a capacitor in series:** If you want to operate at a higher frequency ($f_{new}$), the original antenna (designed for a lower frequency, $f_{old}$) is now physically longer than the required 1/4 wavelength for $f_{new}$. An antenna that is electrically longer than 1/4 wavelength (but less than 1/2 wavelength) exhibits inductive reactance. To cancel this inductive reactance and restore resonance at the higher frequency, you must add an equal amount of series capacitive reactance. Therefore, adding a capacitor in series effectively makes the antenna electrically shorter and increases its resonant frequency. * **B) Lower capacitor value:** This option is vague without context. If an antenna is already loaded with a capacitor to lower its resonant frequency, then reducing that capacitor's value would indeed increase the resonant frequency. However, it's not a general method of increasing resonance by *adding* a component from scratch. * **C) Cut antenna:** Physically cutting the antenna makes it shorter. A shorter antenna will inherently resonate at a higher frequency. This is a direct and effective method, but option A describes an electrical loading technique using a component rather than a physical modification. In the context of "adding" a component, A is the more specific answer. * **D) Add an inductor:** Adding an inductor in series increases the total inductive reactance, making the antenna appear electrically *longer*. This would *decrease* the resonant frequency, not increase it.