Ham Radio General Class Practice Test

What is the approximate maximum theoretical forward gain of a three element, single-band Yagi antenna?

1. 9.7 dBi

2. 9.7 dBd

3. 5.4 times the gain of a dipole

4. All of these choices are correct

The correct answer is: 9.7 dBi

The maximum theoretical forward gain of a three-element Yagi antenna is often cited as approximately 9.7 dBi. This value denotes the gain as compared to an isotropic radiator, which is a theoretical antenna that radiates power uniformly in all directions. Yagi antennas are specifically designed to focus and direct the radio waves in a particular direction, which results in enhanced gain. The designation of 9.7 dBi reflects the antenna's ability to concentrate power in its main lobe and is derived from the principle of adding gain to the basic dipole. Since a standard dipole antenna typically has a gain of about 2.15 dBi, a three-element Yagi, which includes a driven element and typically two other elements (a reflector and a director), is optimized to achieve this higher level of gain. The calculations for the gain can be complex, involving the design of the elements, spacing, and the operating frequency. However, it's generally accepted that a well-constructed three-element Yagi can achieve around 9.7 dBi under ideal conditions. While the other choices may seem plausible, they either do not accurately express the gain in the correct context (such as dBd, which refers to gain relative to a dipole),