Ham Amateur Radio Technician Practice Exam

In free space, how does the gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single three-element Yagi?

1. Approximately 3 dB higher

2. Approximately equal

3. Approximately 2 dB lower

4. Approximately 5 dB higher

The correct answer is: Approximately 3 dB higher

The gain of two three-element, horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart is typically approximately 3 dB higher than that of a single three-element Yagi. This increase in gain can be attributed to the combined effects of the two antennas working together as an array. When the antennas are spaced at a half-wavelength apart and oriented in the same polarization, they reinstate the strong signal reception and transmission characteristics of a Yagi design. The spacing allows for constructive interference, which means that when both antennas are fed with the same signal, they can enhance the overall radiation pattern and directivity. The resultant effect of this configuration allows the antenna system to capture or transmit more power in the desired direction, effectively doubling the power as perceived in terms of gain, resulting in an approximate increase of 3 dB when compared to just a single antenna. This principle is key in antenna design, where combining multiple antennas can significantly improve performance, especially in applications where directional gain is essential.