I'm just getting set up in gmrs. I just got my first mobile set up as a base. I have a Radioddity DB20-g with a Sirio CX455 base antenna on a 20' mast. I have the radio programmed and can hit a local repeater. I trying to hit a linked repeater that is 51 driving miles from me with no luck. Is that to far away or do I need a better radio or my antenna to be higher?