Mobile operation means using an amateur station from a moving vehicle, whether a car, truck or boat. VHF and UHF FM from a car, through repeaters, is the everyday form. HF mobile is more of a challenge, but a well-installed station can work the world. Maritime mobile does the same on the water.
Safety first
Driving is the job; the radio is not. Distracted driving kills. Many places restrict handheld and even hands-free communications equipment while driving, and the rules differ by state and country. Check yours.
Park to dial, tune and log. Use memories, speaker mics and voice announcements so you rarely need to look.
Do not log or text while driving. If you cannot do it by touch, wait until you stop.
Use a passenger. A second person can run the radio, log and read the map.
Secure everything. A mic, handheld or radio that comes loose in a crash becomes a projectile. Mount with brackets, keep cables out of pedals and airbag paths.
RF exposure: keep antennas as far from occupants as is practical. See the RF section below.
The installation
Illustrative. Follow the radio's manual and keep cables clear of airbags.
Power. Run heavy-gauge wire straight to the battery with a fuse close to the battery on the positive lead. A 12 V accessory socket is not designed for a radio's current draw. Pass the cable through a grommet in the firewall. See Power output and SWR protection and Wire gauge and fusing.
Antenna. A vehicle's metal body is the ground plane; a mount on the roof or trunk usually works well. A short HF whip is a compromise: it is inefficient and narrow, which is the main limit on HF mobile; see Mobile whips and loading coils. For VHF and UHF a quarter-wave or longer whip works very well; see VHF/UHF mobile and base radios.
Noise. Alternators, ignition, fuel pumps and electronics add receive noise. Short, clean power and ground connections, and ferrites on leads, usually help; see Common-mode chokes.
RF exposure in a vehicle
A mobile transmitter puts an antenna a few feet from the people inside. VHF and UHF mobile power is modest, but HF mobile at 100 W is another matter, and a poorly placed antenna can put a strong field at head height.
Place the antenna on the roof or trunk, not on a glass mount near the passengers.
Use the lowest power that makes the contact.
Keep handhelds and their antennas away from the head; use a speaker mic or external antenna.
Follow your radio's and antenna's instructions; see RF exposure.
Maritime mobile
On a boat the principles are the same, with extra considerations: a salt-water environment, vibration, and a different ground. An amateur station on a vessel needs the master's permission. For a US-documented vessel in international waters, that and your US license are what the rules ask; inside another country's waters, that country's rules apply.
Amateur radio is not a marine safety service. For distress, safety and weather, use proper marine radio equipment. Do not rely on a ham station for emergencies.
Corrosion-proof the installation and keep the power wiring fused and waterproofed.
Avoid interfering with the vessel's navigation and communications gear.
Operators often add "maritime mobile" to their signoff, and are popular targets for HF chasers.