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.
Though usually considered a form of monopole antenna, a whip antenna is actually only a monopole if the vehicle on which it is mounted is considerably larger than the whip itself. If the antenna and the vehicle are similar in size, an asymmetrical dipole is formed. Multi-band operation is possible if an inductor coil is placed either at halfway along the length of the antenna or at one third and two thirds of the way along.
There are several assets and advantages to the use of a whip antenna. One example is the electrical and mechanical simplicity. There is little to no installation necessary and they are relatively easy to operate. However, because they lack a stable electrical ground system, whip antennas are somewhat inefficient. In addition, they are often collapseable or telescoping, so that the antenna is easy to move and store when not in use.
Also, any whip directly connected to a transmitter radiates radio-frequency energy into the surrounding environment. This can be cause for concern, especially in cases involving cell phones and hand-held radios where the hands and head of the user are being subjected to powerful electromagnetic fields. These fields have also been known to cause nearby electronic devices, including medical equipment, to malfunction.
When dealing with high-powered or long range wireless communication, an outdoor antenna with well-engineered and stable feed system tends to work much better than a whip antenna. The use of an outdoor antenna also removes some of the risk of exposure to high levels of electromagnetic energy, largely because the antenna itself is located at a safe distance from humans and other electronic equipment.