Keep in mind that a telescope antenna are just a piece of metal that you can adjust to different lenghts. There's no loading coil or matching device that makes it work better without a ground plane. It can be adjusted to a lenght that receives a narrow frequency band well but not other bands. It's not a dual band or tripple band antenna. What you normally do are to adjust for the weakest system you monitor to focus on that and other stronger systems are usually received anyhow due to their signal strenghts. Or just monitor one band, like 800Mhz and adjust for that and then configure your scanner to only scan systems that use VHF and adjust the antenna for that band.
There's no portable wide band or all band antenna available. You have to try and get an antenna that seems to match the frequencies of the systems you think are important. That's why there are so many antennas to choose from as the "best" antenna depends on what signal strenghts the systems have that you are monitoring at your unique location.
If you have access to several scanners you can program them with the same systems, as much as it is possible. When one system are received perfectly with one scanner you can delete or avoid that system in the other scanner. After a while you end up having the systems programmed that best suit a scanner and its antenna.
/Ubbe