The Inverted V
An inverted v antenna is a type of antenna similar to a horizontal dipole, but with the two sides angled towards the ground, typically creating a 120 or 90 degree angle between the dipole legs. It is mostly used in areas of limited space as it can significantly reduce the ground foot print of the antenna without to much impact on the performance of the antenna.
In theory, the gain of an inverted v is similar to that of a dipole at the same elevation as most of the radiation is from the high-current portion of the antenna near the center. Since the center of both antennas are the same height, there is little difference in performance. Antenna modeling software shows this for free-space models, predicting maximum gain of 2.15 dBi for the dipole and 1.9 dBi for the inverted vee.
In reality though, ground proximity and ground conductivity as well as end effects reduce the efficiency of the inverted v considerably compared to the dipole: In a 40-foot example, considering a useful take-off angle of 40 degrees above the horizon, the inverted v produces a maximum gain of 1 dBi in a circular pattern, whereas the dipole produces an oval pattern ranging from 6 dBi toward the sides down to 1.2 dBi toward the ends.
Raising the antenna higher above ground somewhat resolves the disparity, but considering the practical, legal and financial limits which influence most antenna installations, the inverted v will be observably inferior in performance to a dipole by 1 to 2 S-units. However, if space is limited, an inverted v may permit operation on frequencies that would not be possible with a full-sized dipole.