As it is illegal to monitor the airband, I'm not going to deal specifically with that band.
Transmitters are not necessarily located where you think they maybe so although you have line of site to a particular location, you may not be able to receive transmissions which you think may originate there.
Noise on a receiver can be caused by many electrical devices - central heating, plasma TVs, computers are but three culprits and there is little you can do about it.
Aerials need to be designed for a particular frequency, or a small range of frequencies. Using a UHF TV aerial for VHF will not work! A further consideration is the polarisation of the signal to determine whether the aerial should be horizontal or vertical.
Aerials should be fed with good quality co-axial cable of the same impedance of a) the radio and b) the aerial. Using cheap cable will lose a lot of the signal. Reception will always be better with an aerial as high as possible, preferably outdoors and away from obstructions.
Lastly, the smaller scanner-type receivers are inevitably a compromise so do not expect top quality reception. Many are designed to use the small antenna provided and plugging in an outdoor aerial may overload the receiver. It will not cause damage but you may find that it will receive signals from adjacent frequencies on top of those you are expecting.
Last edited by HEATHROW DIRECTOR; 20th Oct 2011 at 07:18.