Whip antenna

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A whip antenna is the most common example of a monopole antenna, an antenna with a single driven element and a ground plane.

The whip antenna is a stiff but flexible wire mounted, usually vertically, with one end adjacent to a ground plane. The whip antenna can also be called a half-dipole antenna, and as such, has a toroidal radiation pattern where the axis of the toroid centers about the whip. The length of the whip determines its wavelength, although it may be shortened with a loading coil anywhere along the antenna. Whips are generally a fraction of their actual operating wavelength, with half-wave and quarter-wave whips being very common. These antennas are widely used, especially for mobile applications and hand-held radios. They are usually attached to a vehicle and designed to be flexible, so that they don't break when struck; their name is derived from their whip-like motion when disturbed.

Being vertically mounted causes the whip antenna to have vertical polarization. Whips are thought of as omnidirectional, because they radiate equally in all directions in a horizontal plane, although they have a conical blind zone directly above them.

Only if the radio is electrically large is a true "ground plane" formed. In general, the whip and the radio are half elements to one another, forming an asymmetrical dipole. For instance, if the whip and the radio are about the same size a dipole is formed.

Main variations:

The whip is made physically shorter by either an inductor (coil) between the whip and the radio (base loading). If the coil is not too large and heavy, it can be 1/3, 1/2 or 2/3 along the whip to improve apparent gain by changing the radiation pattern.

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 create the effect of stacked dipoles at a higher band (usually x2 or x3 frequency).

At higher frequencies (2.4 GHz, but Military whips for 50 MHz to 80 MHz band exist), 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.