For a handheld VHF radio carried as an emergency backup, how should I expect the effective radius of a handheld radio to compare with on-board radios in two-way communications? I figure reception should be the same. But transmission would probably be different, dependent on the power. Would the difference be significant? What factors would change the useful radius of a handheld radio?
This ICOM hand held has a max output of 5W compared with their panel mount radio which outputs at 8W. Unfortunately they don't quote effective ranges on their website (and its highly dependent on many factors anyway). In any case you are limited by line of sight which depending on the plane you are flying (and how high up you are) may be more of an issue. There is some discussion here on the matter and generally people say the work pretty well. Keep in mind also that the aircrafts installed radios have their antenna that are mounted on the exterior of the plane which has an effect on the reception and transmission ability. Using a hand held inside a plane may in practice yield different results even if it has the same transmission specs as an installed radio.