Which of the following most limits an HF mobile installation?

HF wavelengths are very long, ranging from 10 meters to 160 meters, making it physically impossible to mount a full-size resonant antenna on a standard vehicle. To operate on these bands, mobile antennas must be made electrically short using components like loading coils. The use of these loading coils inherently introduces resistance, meaning a significant portion of the transmitter's power is lost as heat rather than being radiated as an RF signal. Because of this, the overall radiation efficiency of the electrically short antenna becomes the primary limiting factor for an HF mobile station's performance. Distractors like wire gauge can be easily solved by upgrading the installation's cables, and there are no special FCC rules restricting mobile output power on 75 meters. "Picket fencing" is a VHF/UHF multipath phenomenon, not an HF limitation.