During my experiment with several MAXON WTA-2 hand held antennas (AKA Rubber Ducks) I still don't get the relationship between a short helical wound antenna and a duck Any way after codifying the center loading coils I started experimenting with the length of the top section and found that you could easily tune the entire antenna just like the big boys do on 75 meters mobile, course you can't talk on 75 much anymore due to the CB invasion and the foul language but that's the way it goes.  
       The helical wound material is very hard steel and covered with copper, very hard to extend and compress the windings so I chopped off about one inch from the top and soldered on some "buss" wire that I formed into a coil by winding around a drill bit. If you do not know what buss wire is, don't ask. Anyway you could tune the top section by extending and compressing this new coil section. You could stop here and cover it with heat shrink and let rip.

   Then I decided to try brass tubing as the top section. Make a telescope section as I had done on previous antennas for DFing. The small 1/8 inch tube fits inside the larger 5/32 tube.