Radio reception Audi 80 tdi avant bj 94 B4

Hello, can someone help me ? I have already used the 2nd new radio (last Aldi ) and the reception is just bad. Roof antenna near tailgate ; there is no rust etc. Question: is it due to the inferior quality of the radios or can it be a techn. Problem with the antenna – and what could I do then ? Would be nice if someone could answer me cleverly ;-)) Greeting smila