A vertical antenna is a single upright wire or tube, and it is the natural choice when space is tight or when you want signals in all directions. Unlike a dipole, which needs two high supports, a vertical needs one base point and a set of ground radials. This vertical antenna length calculator gives the radiator length for the three common vertical designs: quarter wave, half wave, and 5/8 wave, in feet and metres, plus the radial length that a quarter-wave design needs.
The formulas follow the same family as the dipole. A quarter-wave vertical is 234 divided by frequency in megahertz, in feet, exactly one leg of a dipole stood upright. A half-wave vertical is 468 divided by frequency, the full dipole stood upright. A 5/8-wave vertical is 585 divided by frequency, which is 468 times 1.25. For example, at 14.2 megahertz the quarter wave is about 16.5 feet, the half wave about 33 feet, and the 5/8 wave about 41 feet. The tool gives all of them in metres too.
The quarter-wave ground plane is the most popular vertical, and it is really half an antenna. The upright radiator is one quarter wavelength, and the ground plane, a set of radials or a metal sheet, acts as the missing half. Radials should be about the same quarter-wave length as the radiator, and more radials give a better, more stable ground. Four radials are a minimum, sixteen or more are much better, and elevated radials need fewer than buried ones because they lose less energy to the soil.
The half-wave vertical is taller but simpler at the base. Because it is a complete half wavelength, it does not need a ground plane to resonate, and it feeds at a high impedance point, so it needs a matching network at the base. Its radiation angle is low, which suits long distance work, and it is a common choice for portable masts and for bands where a ground plane is impractical. The cost is height and the matching unit, which must handle the full transmitter power.
The 5/8-wave vertical is a favourite for VHF and for single-band HF work because it squeezes a little extra low-angle gain from its longer length. At 5/8 of a wavelength the feed impedance is not naturally 50 ohms, so it needs a matching coil at the base, and the extra length past half a wavelength is what shapes the pattern toward the horizon. That is why 5/8-wave antennas are common on vehicles and rooftops: the base coil is a small price for a flatter, longer-reaching pattern.
Ground and height decide how well any vertical works. A vertical over poor, dry soil with few radials loses much of its power warming the ground, while the same antenna over a good radial field or near salt water performs far better. Raising the base above ground, with elevated radials sloping slightly down, often beats adding more buried wire. Keep the radiator clear of gutters, metal roofs, and overhead wires, and remember that the base of a transmitting vertical carries high voltage, so fence it off from people and pets.
Build practice is the same as for any wire antenna: cut long, measure, trim. Cut the radiator a few percent longer than the calculator says, install it at its final height with its radials in place, and measure with an antenna analyzer. Trim in small steps and remeasure until the lowest standing wave ratio sits on your chosen frequency. If resonance will not move to the target, suspect the radial system or nearby metal before cutting more, because a poor ground shifts resonance and wastes power.
In short, this tool gives you the three vertical lengths from frequency: quarter wave from 234 divided by frequency, half wave from 468, and 5/8 wave from 585, all in feet and metres, plus the quarter-wave radial length. Add a proper radial field, keep the base safe and clear, trim while measuring, and respect your licence rules. One upright wire, well grounded, is an honest antenna for small spaces.