Well simply stated, some rx antennas are at best a compromise or tuned to an "average" frequency in the case of a dual band rx only antenna, it is built to the highest frequency in each band (therefor making the antenna shorter and requiring less material, thereby costing less to make) for example a dual band scanner I looked at is 21" long, but the tx capable antenna for the same frequencies is 38" long, this gives you more "capture area" there for it works better.