Oh, you're gonna love this!
A bazooka is a sort of 'multi-element' antenna, meaning that there are really two parts to it, the whole center conductor is the radiating part of the antenna, then there's the two tuned sections that do the feeding of the antenna (outer shield that the feed line connects to. To tune for resonance you'd do it like you would any antenna, make the radiating part's length (center conductor) right. Doing that on each end isw the simplest way. Shorten the thing to raise the resonant point, lengthen it to lower it.
Normal so far, right? Well, here's the 'catch'.
Those two tuned circuits, the shielding the feed line attaches to, need to be lengthed or shortened too. Otherwise, they don't stay "centered" at a particular frequency. If you raise/lower the resonant freq of the center conductor, you ought to raise lower the shiled's resonance point to match it. Otherwise, you won't be able to go as far in one direction (frequency) as you will in the other direction. How do you do that? You change where the 'short' is at the outer ends of those shield sections. Raising the resonance point is no really big deal, just means shorting that shield closerto the center of the antenna. Not all -that- easy, but not all that difficult either. What if you want to lower the resonant frequency? That get's a bit harder to do. How do you 'un-short' the shield to make it longer? I don't know either. So, it's a pretty good idea to start with the thing resonant at the lowest frequency of use, so you can raise that that resonant point later.
Ain't that a bummer?
There are 'tricks' that can make that a bit easier if you build your own. They aren't the 'best' way of doing it, but they do work. Ever think that "pinning coax" would have a useful purpose?? 
It works. It also works better if the whole antenna is made from coax, instead of there being a 'solid' wire/ladder line/whatever on the ends.
Having said all that...
A bazooka antenna sometimes doesn't have to be 're-tuned'. It's sort of 'broad' to start with, right? If that 'broadness' fits where you want it to fit, then it isn't absolutely required that you re-tune it. Will it be as efficient as it possibly could be? Nope. But a bazooka antenna isn't exactly the most efficient antenna in the world to start with. (You'll get arguments about that, take whichever side you'd like.) How much efficiency is lost? I haven't a clue.
- 'Doc
