What is one disadvantage of using a shortened mobile antenna as opposed to a full-size antenna?

To make a physically short antenna resonant on HF frequencies, a loading coil must be used. These loading coils inherently possess a high Q (quality factor). While a high Q helps make the short antenna resonant, it comes with a significant trade-off: a very narrow operating bandwidth. Because the bandwidth is so limited, the antenna will only present an acceptable SWR over a very tiny slice of the band. If the operator changes frequencies by even a few kilohertz, they will often need to manually retune the antenna or adjust a matching network to safely transmit. Shortened antennas do not cause signal distortion or increase harmonic radiation. In fact, they act somewhat like band-pass filters due to their high Q, which actually suppresses harmonics.