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

• A. Shortening it
• B. Lengthening it
• C. Adding reactance
• D. Using different materials

Answer: (A)

Shortening a dipole antenna increases its resonant frequency because the resonant frequency is inversely proportional to the length of the antenna. The fundamental principle behind this is that a dipole antenna resonates at a frequency where its length is approximately half of the wavelength of the operating frequency.

When you shorten the antenna, you reduce this length, which in turn raises the frequency at which it resonates. The relationship is such that as the physical size of the antenna decreases, the capability of the antenna to effectively radiate or receive radio waves at lower frequencies diminishes, prompting it to be more tuned to higher frequencies instead.

In contrast, lengthening the antenna would lower the resonant frequency, which is not the desired effect when the goal is to increase frequency. Adding reactance typically modifies the impedance rather than directly influencing the physical resonant frequency itself, while using different materials might impact the antenna's performance factors, such as bandwidth or efficiency, but would not inherently change the resonant frequency in the same way that altering the length does.