Telescopic Antenna's
Some people have a misconception that a telescopic antenna needs to be fully extended to receive the best reception. This is not so.
Basically every frequency has a resonant point. However there is no need to have a antenna for each point just as long as the antenna is the right length for the band. Example being the centre of the civil air band is 127.000mhz so using the formula 234 divided by 127 (the optimum freq) you get 1.8 feet. Times that by 12 and it gives you 22.1 inch's. There for a vertical antenna for civil air band needs to be 22 inch's roughly.
So if you are using a telescopic antenna and you want to it optimised for Civil air it needs to be around 22 inchs.
Like wise if you wanted to use the same antenna to now listen to the Mil air band at 22 inch's it wouldn't be as efficient and so that is why they are expandable ! so you can make it shorter so going on the same principle middle of the Mil air band is say 325mhz (225 to 400mhz is the band) so 234 divided by 325 gives you 0.72 feet now times that by 12 and you get 8.64 inch's so to listen to the Mil band with the same ant you need to reduce it to around 9 inch's.
So not trying to teach anyone to suck eggs but if you are using a telescopic (which work great by the way) use the formula to ensure the antenna is optimised for the freq your listening to.
Sadly the manufactures fail to explain this as it would mean them putting a scale on the antenna i guess and that costs money.