Well, put simply, you really want a flat plane area under the aerial - like a car roof. Acts a bit like a mirror in extremely simplistic terms. Not too important what metal it is. At DAB frequencies to work best it wants to be a minimum of little over 0.3 metre radius to be most effective. Difficult to achieve, so just aim to get antenna above bodywork, rather than be screened as it is now on your parcel shelf.
Mobile antennas tend to be a bit of a compromise - you should see some of things we try for short wave bands!
