The best way I know of is to say that an isotropic antenna is the 'ideal' antenna in an 'ideal' location where nothing around it can affect it. It's the 'best' possible for whatever style of antenna it is. It's a 'virtual' antenna, doesn't have to put up with the 'real world' conditions and limitations. The first thing to do with any isotropic gain figure is to knock off about 2 dB for a closer 'real world' gain figure (2.12 dB I think). It's not exact, but that'll get you close to what you might expect if you're lucky and everything works just right. (If you are that lucky please buy me a lottery ticket!!) Isotropic antennas give you an idea of what a particular antenna might do, it shows a trend. It's what you can expect if you do everything exactly right, sort of, but you ain't never ever going to get there for real.
- 'Doc