RadCom Basics May 2022, Edition 28

24 RadCom Basics before selecting an antenna to use, as these are used to measure the gain of a given antenna. So, what do these terms mean? dBi = Decibels relative to isotropic - measures the gain of an antenna compared to an isotropic radiator (a theoretical antenna that disperses incom- ing energy evenly over the surface of an imaginary sphere). dBd = Decibels relative to a Di- pole - compares the gain of an antenna to the gain of a stand- ard dipole antenna (defined as 2.15 dBi gain). Enough of the technical stuff, what this means is that antenna manufacturers can choose which one to use, which means they inev- itably choose the one which gives the highest gain number. Essentially dBi measurements are always high- Figure 1: This 19 element Yagi for 70cm offers 16.2 dBi (or 14.05 dBd) gain. Here I was using it during a 70cm UKAC contest – the gain and number of elements means it’s got a narrow beam, if you don’t point it directly at the other station, you can’t hear them!”]