Rubber ducky antenna

Rubber Ducky antenna on a transceiver

The Rubber Ducky antenna (or Rubber Duck aerial) is an electrically short antenna which functions somewhat like a base-loaded whip or monopole antenna 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 wave-length element, necessary for resonance of a linear element over a ground-plane, is often too long for convenient portable operation. Many years after its invention in 1958, the Rubber Ducky antenna became the antenna of choice for many portable radio devices, including walkie-talkies and other portable transceivers, scanners, GPS receivers, and any other device where safety and robustness take precedence over antenna capabilities.

Early portable antennas

In the time-frame of its development, antennas on portable equipment usually consisted of telescoping rods that were extended for operation and later retracted for storage. Electrically short antennas have considerable capacitive reactance, so to provide an approximate impedance match, it is usual to add an inductor in series with the antenna. Antennas which have these inductors built into their bases are called base loaded antennas.

Entire length used for base loading

Protective rubber removed from the antenna on a UHF CB transceiver. The entire length used for base loading.

It is possible to make an antenna in which the entire length of the driven element is an inductor, configured much like a spring. In fact, if a springy material is used, the antenna becomes flexible and somewhat immune to damage. If the spring antenna is further enclosed in a plastic or rubber-like covering, it is called a Rubber Ducky antenna.

Effective aperture

Because the length of this antenna is significantly smaller than a wavelength the effective aperture is approximately:

$A_{e}={\frac {3\lambda ^{2}}{8\pi }}$

A surprising result is that even though the Rubber Ducky antenna may be small, its effective aperture can be comparable to larger antennas.

Reasonable performance

If care is taken in its design to produce a reasonably high radiation resistance, one can produce a useful antenna. Rubber Ducky antennas have reasonable performance, but they do not have either the gain or the aperture of larger antennas. Therefore, their performance will always be somewhat of a compromise. They are difficult to characterize electrically because the current distribution along the element is not sinusoidal as is the case of a thin linear array. However, there are a few “Rules-of-thumb” that can be used to design these antennas:

Performance trade-offs

• If the coils of the spring are wide (a large diameter), relative to the length of the array, the resulting antenna will have narrow bandwidth.
• Conversely, if the coils of the spring are narrow, relative to the length of the array, the resulting antenna will have its largest possible bandwidth.
• If the antenna is resonant, and the spring has a large diameter, the impedance will be well below 50 ohms, tending towards zero ohms with large inductors as the structure starts to resemble a series-tuned circuit with little radiation resistance.
• If the antenna is resonant, and the spring has a small diameter, the impedance will increase towards 70 ohms.

Therefore, from these rules, one can surmise that it is possible to design a Rubber Ducky antenna that has about 50 ohms impedance at its feed-point but a compromise of bandwidth may be necessary. Modern Rubber Ducky antennas such as those used on cell phones are tapered in such a way that few performance compromises are necessary.

Some are different

Some Rubber Ducky antennas are designed quite differently than the original design. One type uses a spring only for support. The spring is electrically shorted out. The antenna is therefore electrically, a linear element antenna. Some other Rubber Ducky antennas use a spring of non-conducting material for support and comprise a collinear array antenna. Such antennas are still called Rubber Ducky antennas even though they function quite differently (and often better) than the original spring antenna. The Rubber Ducky antenna has recently become known as the Flagelliform antenna as well.

See also

• isotropic antenna
• omnidirectional antenna
• whip antenna
• driven element
• balun
• amateur radio
• shortwave listening

References

2. Raines, J.K., "Simple Formulas for Loaded Antennas," Microwave Journal, January, 2009, electronic edition, http://www.mwjournal.com/Media/pdf/raines2-2.pdf