575Colorado,
You know enough that you got it right. Coils introduce losses. How much loss is more a matter of what you are willing to put up with, or that you have to put up with. If the length of a usable antenna on a mobile is such that you can't move 10 feet without hitting something, then a coil to shorten the thing is worth the loss it will produce.
'Shortened' antennas do not work as well as un-shortened ones, depending on where they are. A shortened antenna for a fixed position is not going to be as 'good' as the 'full-sized' antenna would be. Why? Cuz like it or not, the length of an antenna determines it's radiation pattern. Not talking about 1/4w, 1/2w, 5/8w, or whatever, but a shortened version of any of those lengths as opposed to the full sized one. With mobiles you have a potential problem, the @#$ things are just too long, right? So, a little 'shortening' in the mix is okay under those circumstances. You can also go a little too far very quickly. A 6" CB antenna just ain't gonna cut it, I don't care who makes it.
Which antenna to use? Whatever fits the situation, and your wallet. It's your wallet, treat it like you want.
One last thing. Power handling ability, as in "I'll never need it but a 10,000 watt capability is nice to have!", is a misconception. If it will handle whatever power you use then it's just as good as that 10,000 watt monster (all things being equal). That 'monster' has losses too. At 10,000 watts those losses don't amount to much. But at 5 watts, they certainly can!
This is all a sort of generalization, nothing specific about any particular antenna, even the 'miraculous' ones.
- 'Doc