A whip antenna is the most common example of a monopole radio antenna. Technically, this means that instead of two antennae working together, either side-by-side, or forming a loop, one antenna is replaced. Whip antennas are used frequently in devices such as hand-held radios and mobile phones. Their name is derived from the flexible, whip-like motion they exhibit when struck.
The length of the whip determines its potential wavelength. It is also possible to shorten the whip with a loading coil anywhere along the length of the antenna. This allows the inductance to be increased without increasing the size of the whip. Half-wave and quarter-wave whips are also very common.
A whip antenna is almost always vertically mounted onto its base vehicle, resulting in vertical polarization. Because they radiate in every direction on a horizontal plane, whips are often referred to as omnidirectional. This is not strictly true, however, since all whip antennas have a conical blind spot directly above them.
The ideal length of the whip antenna is determined by the wavelength of the radio waves it is used with. The most common type is the quarter-wave whip, which is approximately 1 /4 wavelength long, but they can be either longer or shorter by design, varying from compact electrically short antennas 1/ 10 wavelength long, up to 5 /8 wavelength to improve directivity.
Whips are the most common type of monopole antenna, and are used in the higher frequency HF, VHF and UHF radio bands. They are widely used as the antennas for hand-held radios, cordless phones, walkie-talkies, FM radios, boom boxes, and Wi-Fi enabled devices, and are attached to vehicles as the antennas for car radios and two-way radios for wheeled vehicles and for aircraft. Larger versions mounted on roofs, balconies and radio masts are used as base station antennas for amateur radio and police, fire, ambulance, taxi, and other vehicle dispatchers.
In a whip antenna not mounted on a conductive surface, such as one mounted on a mast, the lack of reflected radio waves from the ground plane causes the lobe of the radiation pattern to be tilted up toward the sky so less power is radiated in horizontal directions, undesireable for terrestrial communication. Also the unbalanced impedance of the monopole element causes RF currents in the supporting mast and on the outside of the ground shield conductor of the coaxial feedline, causing these structures to radiate radio waves, which usually has a deleterious effect on the radiation pattern.
To prevent this, with stationary whips mounted on structures, an artificial "ground plane" consisting of three or four rods a quarter-wavelength long connected to the opposite side of the feedline, extending horizontally from the base of the whip, is often used. This is called a ground plane antenna. These few short wire elements serve to receive the displacement current from the driven element and return it to the ground conductor of the transmission line, making the antenna behave somewhat as if it has a continuous conducting plane under it.
The radiation resistance of a quarter wave ground plane antenna with horizontal ground wires is around 22 Ω, a poor match to coaxial cable feedline, and the main lobe of the radiation pattern is still tilted up toward the sky. Often (see pictures) the ground plane rods are sloped downward at a 45-degree angle, which has the effect of lowering the main lobe of the radiation pattern so more of the power is radiated in horizontal directions, and increases the input impedance for a good match to standard 50-ohm coaxial cable. To match 75-ohm coaxial cable, the ends of the ground plane can be turned downward or a folded monopole driven element can be used.