Monopole impedance and gain measurements on finite ground planes

The possibility of making acceptably accurate monopole input impedance and gain measurements on a reduced-size ground plane is discussed. Existing measured and calculated data show that the diameter of a highly conducting ground plane should be at least 4λ (where λ is the wavelength) for measuring the input impedance of quarter-wavelength monopoles. At 25 MHz, the lowest frequency considered here, such a ground plane would require a space at least 48 m in diameter. The model impedance measurements and calculations presented imply that a reduced space of only 10 m by 11 m is required if 16 resistively loaded wire radials are connected to a 3.66 by 4.88-m rectangular aluminum ground plane, thus effectively `extending´ the ground plane by the resistive loading. Measured insertion-loss data acquired using a 1:5 scale-model ground plane with resistively loaded radials indicate that the plane is sufficiently large for gain measurements as well. Measured and calculated monopole standing-wave ratio and insertion loss on a full-scale ground plane verify the results of the model measurements