The Rubber Ducky antenna (or Rubber Duck aerial) is an electrically short

Electrically short

An electrically short antenna is an antenna of length 2h, such that2 \pi h\over\lambda \scriptstyle\ll 1.An electrically short transmission line is a transmission line in which all of its effects will complete within one cycle of an impressed alternating current....

 monopole antenna

Monopole antenna

A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...

 which functions somewhat like a base-loaded whip antenna

Whip antenna

A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...

 and is sealed in a rubber or plastic jacket to protect the antenna. Electrically short antennas are often used in portable equipment because a one-quarter wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 element, necessary for resonance of a linear element over a ground-plane, is often too long for convenient portable operation.