As an example, here is an antenna load from Radio Shack that requires a 57" whip and is normally bottom loaded. Putting it on the riser required a shorter whip, but not a direct trade off. The over all length of the antenna with the riser under it is 80". According to the MFJ-209, I have a 1.5:1 bandwidth from 27.815 to 26.515 with it up there. And it seems to work well with both local and DX contacts.
Someone asked me this:
Any thoughts how i can put a whip adjuster on the top of the riser to raise and lower a 102?
Maybe I would if I had some idea of why you'd want to do that. To change the resonance so it can be used on other bands, or just to clear obstacles like a low garage, etc.?
I've been drilling a hole in the top nut on the bushing and threading it with a tap in order to put in a set screw for whip length adjustment.
If that is the idea, you do that and slip the bushing over the whip before you screw it into the top of the extension. Before that is done, a rubber seat for a faucet is placed around the whip in order to keep it centered as it is moved up and down the tube. When you've found your tune, tightened the set screw for the frequency range you are working.