Sorry to drag up an old thread but I would be tempted to make it 3/4 of a wave lenght. Converting to 1/2 wave will throw out the impedence, so to get the full performance you would need to add a matching system, all though this does not matter too much for recieving you should get even better performance by going to a 3/4 wave. Ideally a matched 5/8th wave would be even better.