Why height matters at VHF
VHF signals travel mostly in a line of sight. Unlike HF, they are not normally reflected back to earth by the ionosphere, so the distance to the horizon largely sets the range. Raising an antenna extends how far it can “see.” Obstacles such as hills, buildings and trees reduce the range further.
The radio horizon formula
A commonly used rule of thumb for the radio horizon, which slightly exceeds the visual horizon because of atmospheric bending, is:
- Distance in statute miles is about 1.4 times the square root of the antenna height in feet.
- Distance in nautical miles is about 1.2 times the square root of the height in feet.
To estimate the range between two stations, calculate the horizon for each antenna and add them together. These are approximations for open, flat terrain or water.
Worked example
Suppose a boat has an antenna 16 feet above the water, and you are talking to a handheld held about 5 feet up.
- Boat antenna: about 1.4 x 4 = roughly 5.6 miles.
- Handheld: about 1.4 x 2.2 = roughly 3.1 miles.
- Combined: roughly 8.7 miles.
Doubling the boat antenna height to 32 feet raises its own horizon to about 7.9 miles, so the combined range grows to roughly 11 miles. Doubling height does not double range, because range follows the square root.
Real-world range will be less (or occasionally more) depending on power, receiver sensitivity, terrain, and antenna gain. A handheld’s own short antenna and low height limit its results.
Height versus gain
A higher-gain antenna concentrates energy toward the horizon, which can help, but it narrows the vertical beam. On a boat that rolls and pitches, very high gain can cause signals to fade as the antenna tilts. Marine antennas are often grouped as lower-gain types for sailboats and higher-gain types for power boats, though you should follow the maker’s guidance for your vessel.
Mounting on a boat
- Mount as high as practical, clear of metal obstructions.
- Keep the antenna away from other antennas and radar where possible.
- Seal and support coax connections; salt water is hard on them.
- Keep coax short enough to limit loss, or use lower-loss cable.
How far apart should VHF antennas be?
Two antennas close together can interfere with each other, particularly when one transmits while the other receives. Manufacturers often recommend minimum spacing; where possible, mount them with vertical or horizontal separation and follow the instructions supplied with your antennas and radios. Specific distances depend on equipment, so check the documentation.
Ham radio tips for 2 meters and 70 cm
For repeater work, a rooftop or mast-mounted antenna usually helps both hearing and reaching the repeater. But height requires cable, and cable has loss on VHF and UHF; compare options in the coax cable loss chart and read the ham radio coax guide. A lower-loss cable at a modest height can beat thin cable at a higher one. For designs, see ham radio antenna designs you can build at home.
Common questions
How far will a VHF radio reach? It depends mainly on antenna heights and terrain. Handhelds may reach a few miles in open terrain; a high base or boat antenna can reach farther. Use the formula above for an estimate.
What is the best VHF antenna for a small boat? A compact, shorter whip suited to the boat’s size and rated for marine use is typical. Pick one that fits your mounting location and radio.
Where should a VHF antenna be mounted on a boat? High, clear and away from other antennas, as described above.