If you are thinking about aerials for the 2 metre and 70 centimetre amateur radio bands then 30 cms isn't that far off the required dimensions. For instance an efficient transmitting aerial for 2 mtrs, commonly known as a quarter-wave whip would be 19 inches long (48.26 cms) and the loading coil would then appear to lengthen the whip from 30 cms to 48 cms therefore maintaining the tuning. The position of the coil is also arranged to be approximately a quarter wavelength at 70cms above the groundplane which is about 17.5 cms.
The electrical value of the coil, or the inductance is designed to present a high impedence to the 70 cms RF which stops it going further along the radiating element.
It sounds a bit confusing, being able to stop a signal travelling along a conductor with nothing more than a coil, but it is RF we're talking about which doesn't behave in quite the same way as you would expect DC to behave.
But all of that is somewhat acedemic since the aerial isn't a 2 mtr/70 cms dual band aerial, it's just a car radio AM/FM aerial with a spring in the middle doing a job you could quite easily do with a bent coathanger.
