A japan base only has trouble reaching the southern part of south america... that's pretty good coverage.
Once april rolls around, I tend to build a base in central america and one in northern africa, the latter only having interceptors and radars. Then either in april or may, I put a base on the south pole, as an intercept/research base(because eventually my main base can't hold more researchers, I start recruiting them at the south pole when I have 30 in mainbase).
6 radars each at my central america and north africa bases gives me 100% radar coverage. Might be a bit over the top, but who cares.