Sorry, I used the wrong terms. It is actually called an FM converter. Here in Australia we can buy an FM converter which is for use with Japanese import cars. This is because these cars come with Japanese FM tuners and the Japanese FM band is unique in the world, covering 76-90MHz. The converter subtracts a fixed frequency from the local station to enable the FM section of the radio to work. It allows AM stations to go straight through unaffected. I thought there may be a similar device available elsewhere to do a similar thing.