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The art of making a great AM/FM antenna.
Mobile One makes AM/FM antennas for Cars,Trains, Boats, Caravans & Buildings.

1. The most important aspect of any low frequency antenna is the antenna impedance, Car radios have an antenna impedance of 75 to 100 Ohms which make it easy to use external antennas, Desktop radios and Hi Fi sound systems (Panasonic, Kenwood, etc) have a 300 Ohm or higher AM antenna input impedance or they may use an internal ferrite core antenna and have no simple way of including an external antenna. 
Note: Kenwood and Panasonic desktop brand Hi Fi systems or radios are notorious for having high impedance antennas which make it impossible to have external antennas, these type of radios usually come with a twisted wire external antenna in the shape of a small square using plastic covered twisted wire. You cannot easily improve the reception without the use of a balun.

2. The next most important aspect is the antenna feed line or cable cable capacitance, It is important that when including an external antenna you use Low Capacitance coax such as RG62, Do NOT use CB coax RG58 as this will attenuate the AM signal, It will not however, attenuate your FM signal.

If impedance and capacitance are not right then the AM signal will be greatly reduced

IN A NUTSHELL - A $25 old second hand AM/FM car radio bought at a trash & treasure can out perform a $500 indoor desktop stereo system radio & cd player on AM anytime.

The Longer The Antenna The Better The Performance

Look at where you plan to put the antenna then measure the maximum length you can use, then get the longest antenna that will match your requirements, by far the best antenna is the M2W for FM and the M1W for AM 

Cable Lengths Are Not Critical

Unlike UHF CB antennas, the length of cable is not critical but should still remain as short as required, excess cable should never be coiled.

There Are Different Types Of Coatings For AM/FM Antennas

                         PVC - A  thick coat of PVC plastic and is suitable for most applications, available in Black or White
                         Polyolefin - A  thick coat of Polyolefin rubber, for high quality & very long lasting finish, available in all colours.

Some Do's & Don't with AM/FM Antennas

 

DO - Test for shorts or open circuit if possible with an multi meter
DO - Mount the antenna high as possible - This can improve the signal and reduce any background noise.
DO - Use a spring even if mounted on a bull bar to absorb vibration from the engine
DO - Use a wire strap to earth the base if necessary - but make sure it is secured
DON'T - Mount the antenna close along side any metal object or antenna

 
The Main Causes For Poor Performance With AM/FM Antennas
Poor ground plane, Antenna too short, Antenna too low, Antenna to close to another.

(www.mobileone.com.au)