What is one disadvantage of using a shortened mobile antenna as opposed to a full-size antenna?
Why A physically shortened HF mobile whip is made resonant with a loading coil, which makes the antenna a high-Q resonant circuit. High Q means a sharp, narrow resonance, so SWR climbs quickly as you tune away from the design frequency and you may need to retune or adjust the coil tap for even a small frequency change. Full-size antennas like a quarter-wave have lower Q and cover much more of a band.
Watch out The claim that the Q will be very low has it backwards: shortening and loading raises Q, which is exactly what causes the narrow bandwidth. Distortion and harmonic radiation are transmitter issues, not a function of antenna length.
Short antenna = high Q = narrow bandwidth. Short whip, short tuning range.