Interference between ground plane and bilog antenna and its effect on EMI measurement uncertainty

In contrast to half-wave dipoles, Bilog antennas are more popular among test engineers due to their broadband properties - it is not necessary to change them during the measurement. On the other hand they bring additional errors into measurement, so there are more uncertainty sources and the entire uncertainty is larger. The effect of ground plane on Bilog antenna directivity pattern and consequently the effect of their interference to measurement uncertainty are not measurable. To examine this influence the numerical simulation has been used. The uncertainty contribution is expressed as maximal difference between antenna factors of Bilog and half-wave dipole antenna respectively at same angle of incidence. From the analysis it is evident that though ground plane presence influences the directivity pattern of Bilog, its entire effect is not so significant on the uncertainty. It is mainly influenced by the Bilog directivity pattern errors (the difference from dipole one) by itself. As it results from the analysis, using Bilog antenna is more suitable for larger measuring distance to get lower uncertainties.