Ham Amateur Radio Technician Practice Exam

Which antenna type is most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day?

1. A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground

2. A vertically polarized dipole at 1/2 wavelength above the ground

3. A loop antenna at ground level

4. A Yagi antenna at a high angle

The correct answer is: A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground

The choice of a horizontal dipole placed between 1/10 and 1/4 wavelength above the ground is particularly effective for near vertical incidence skywave (NVIS) communication on the 40-meter band during the day due to its ability to radiate signals in a way that is favorable for short-skip propagation. When an antenna is set at a low height, specifically within that 1/10 to 1/4 wavelength range above the ground, it facilitates the horizontal polarization of the RF signals, allowing for effective radiation at high angles. This is crucial for NVIS, as this mode of propagation relies on signals being directed upward at near-vertical angles, which then reflect off the ionosphere and back down to cover short distances. This low placement allows for decreased ground losses and better coupling with the ground, ensuring that a significant portion of the energy is efficiently transmitted skyward instead of being lost to the ground. The resulting high-angle radiation pattern is perfect for establishing reliable communications over short distances, especially under the conditions typical of daytime propagation on the 40-meter band. In contrast, other antenna options may not achieve the same level of effectiveness for NVIS communication. For example, a vertically polarized dipole significantly above the ground