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.