Measured antenna factors in the range of 30 to 80 MHz of some tunable dipoles adjusted to their 80 MHz length

The long length of tunable dipoles at the lower frequencies results in undesirable mutual coupling effects, nonuniform illumination of the antenna at shorter measurement distances, and limitations on the antenna height-scan range for vertical polarization. In order to avoid these limitations and undesirable effects, some standards bodies recommend that for frequencies below 80 MHz, the length of the dipole should be fixed at the 80-MHz tuned length. Measured antenna factors of three tunable dipoles adjusted to their 80-MHz tuned length are compared with the antenna factor of a broadband biconical. The antenna factors of the shortened dipoles are shown to exhibit a strong frequency dependence below 80 MHz. The antenna factor of the broadband biconical, by contrast, is shown to be lower and exhibit less frequency dependence