Subelement J: Antennas— Topic 65: Frequency and Bandwidth
Question 3-65J5
Element 3 (GROL)What happens to the bandwidth of an antenna as it is shortened through the use of loading coils?
Explanation
When an antenna is shortened, it becomes capacitively reactive. To restore it to resonance, a loading coil is added to introduce the necessary inductance. While this makes the antenna electrically resonant at the desired frequency, the loading coil significantly increases the "Q" (quality factor) of the antenna system.
Antenna bandwidth is inversely proportional to its Q factor. A higher Q system resonates very sharply at a specific frequency, but its performance drops rapidly as you move away from that frequency. Therefore, increasing the Q with loading coils causes the antenna's usable bandwidth to **decrease**. The SWR curve becomes much sharper, making the antenna very narrowband and sensitive to frequency changes.
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