Question T9A08
From subelement T9 - T9A
What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz?
Why is this correct?
The correct answer is C (19 inches). For 146 MHz, the wavelength is approximately 2 meters. A quarter-wave is 0.5 meters, which equals 19.7 inches when converted (0.5m × 39.4 in/m). Practical antennas are about 5% shorter than calculated values due to end effects, giving approximately 19 inches. Answer A (112 inches) is a half-wave 6-meter dipole length. Answer B (50 inches) doesn't match any standard calculation. Answer D (12 inches) is too short for this frequency.
Memory tip
Remember the wavelength formula: 300/frequency(MHz) = wavelength in meters. Then convert to inches and take the fraction needed (quarter-wave, half-wave, etc.). The 5% shortening factor consistently appears in practical antenna construction across all bands.
Learn more
Quarter-wave verticals are fundamental monopole antennas that rely on a ground plane to complete the radiation pattern. At VHF frequencies like 146 MHz (2-meter band), these antennas are practical sizes for mobile and handheld applications. The 19-inch length represents the physical radiating element, while the radio's case or vehicle body serves as the ground plane. Understanding this relationship helps explain why handheld radios perform better when held away from your body—you're part of the ground plane system affecting the antenna's radiation pattern and efficiency.
Think about it
Why do you think a quarter-wave vertical antenna needs a ground plane to function effectively, while a half-wave dipole doesn't require one?