# Whip antenna

Whip antenna on car

Three large fiberglass whips mounted on a mast.

The whip antenna is a monopole antenna, and like a vertical dipole has an omnidirectional radiation pattern, radiating equal radio power in all azimuthal directions (perpendicular to the antenna's axis), with the radiated power falling off with elevation angle to zero on the antenna's axis. Whip antennas 1/4 wavelength long or less (the most common type) have a single main lobe, with field strength maximum in horizontal directions, falling monotonically to zero on the axis. Antennas longer than a quarter wavelength have patterns consisting of several conical "lobes"; with radiation maxima at several elevation angles; the longer the electrical length of the antenna, the more lobes the pattern has.

Vertical whip antennas are widely used for nondirectional radio communication on the surface of the Earth, where the direction to the transmitter (or the receiver) is unknown or constantly changing, for example in portable FM radio receivers, walkie-talkies, and two-way radios in vehicles. This is because they transmit (or receive) equally well in all horizontal directions, while radiating little radio energy up into the sky where it is wasted.

## Length

Whip antennas are normally designed as resonant antennas; the rod acts as a resonator for radio waves, with standing waves of voltage and current reflected back and forth from its ends. Therefore, the length of the antenna rod is determined by the wavelength of the radio waves used. The most common length is approximately one-quarter of the wavelength, called a "quarter-wave whip" (although often shortened by the use of a loading coil; see Electrically short whips below). For example, the common quarter-wave whip antennas used on FM radios in the USA are approximately 75 cm long, which is roughly one-quarter the length of radio waves in the FM radio band, which are 2.78 to 3.41 meters long. Half-wave antennas are also common.

A quarter wave vertical antenna working against a perfect infinite ground will have a gain of 5.19 dBi and about 36.8 ohms of radiation resistance. Whips mounted on vehicles use the metal skin of the vehicle as a ground plane. In hand-held devices usually no explicit ground plane is provided, and the ground side of the antenna's feed line is just connected to the ground on the device's circuit board.[1] Therefore, the radio itself, and possibly the user's hand, serves as a rudimentary ground plane. Since these are no larger than the size of the antenna itself, the combination of whip and radio often functions more as an asymmetrical dipole antenna than as a monopole antenna. The gain will suffer somewhat compared to a half wave metallic diople or a whip with a well defined ground plane.

## Ground plane antenna

With stationary whips mounted on structures, an artificial "ground plane" consisting of three or four rods a quarter-wavelength long extending horizontally from the base of the whip is often used. This provides a stable input impedance and pattern by helping prevent RF currents in the supporting mast and along the outside of the feed line.[2] This type of antenna is called a ground plane antenna.[3] Often the ground plane rods are sloped downward toward the ground, which lowers the main lobe of the radiation pattern and increases the normal 36.8 ohm radiation resistance closer to 50 ohms to provide a better impedance match with standard 50 ohm coaxial cable feedline.

## Electrically short whips

A rubber ducky antenna, a common type of electrically short whip, on a handheld UHF CB transceiver. With rubber sheath (left) removed.

Multi-band operation is possible with coils at about one-half or one-third and two-thirds that do not affect the aerial much at the lowest band, but it creates the effect of stacked dipoles at a higher band (usually ×2 or ×3 frequency).

At higher frequencies (2.4 GHz, but military whips for 50 MHz to 80 MHz band exist, and are standard issue for the SINCGARS radio in the 30–88 MHz range), the feed coax can go up the centre of a tube. The insulated junction of the tube and whip is fed from the coax and the lower tube end where coax cable enters has an insulated mount. This kind of vertical whip is a full dipole and thus needs no ground plane. It generally works better several wavelengths above ground, hence the limitation normally to microwave bands.

## Vehicle antenna damage

Whip antennas on vehicles can be damaged by automatic car wash equipment, especially those that use spinning brushes to abrasively rub dirt off the exterior of the vehicle body.[4] Because the brushes must make contact with the vehicle surface, they can bend or completely break off whip antennas. These antennas are generally recommended to be removed or retracted so that the brushes do not make contact, or the vehicle owner should only use a "touchless" spray jet automatic car wash.