Replacement accesory scanner antennas are not that expensive. You can get a tuned 800mhz antenna for cheap. Smiley Antenna Co. offers an extensive line of tuned 5/8 duckies for just about any frequency and any connector for around $20.00
http://www.smileyantenna.com
There are many other brands and umpteen places to buy handheld antennas. They're silly all over the place.
Keep your telescopic for uses on other frequencies. You never know when you may want to listen to something else or go on a trip ect....
I find out exactly where my telescopics are tuned at by using an Antenna Analyzer such as an MFJ 269. Then when the meter reveals the optimum lengths for a desired frequency range, I mark the length adjustment with a fine line of enamel paint. Your chick's fingernail polish will do the same.....or you can use your own polish
ect... Or a piece of tape can be used if it doesn't interfere with the sections closing.
good luck